J. Mass Spectrom. Soc. Jpn.

Vol. 53, No. 4, 2005

INTEGRATED PAPER

Fundamental Aspects of Gas Phase Ion Chemistry
Studied Using the Selected Ion Flow Tube Technique
Shuji K6ID῏
(Received May 6, 2005; Accepted May 17, 2005)
Gas phase ion chemistry using the selected ion ﬂow tube (SIFT) technique is discussed with the focus on
fundamental studies on ion structure, energetics, reactivity, and dynamics: Laser-induced ﬂuorescence (LIF)
kinetics/dynamics of N2῍(v) ions (vibrationally state-selected ion῎molecule reactions; energy transfer processes),mechanisms of prototypical organic ion reactions (SN2 nucleophilic substitution and kinetic isotope e#ects;hydrogen/deuterium exchange; isotope e#ects in termolecular association), and ion῎molecule reactions withbiological implications (SN2 reactions at a nitrogen center; ECO2 elimination reactions of alkyl hydroperoxides).
Anion chemistry of small nitroalkanes is also presented. A powerful combination of SIFT and negative ion
photoelectron spectroscopy is used to study thermochemistry and structure of energetic and exotic ions (alkyl
peroxides; anionic derivatives of diazo and azole compounds, cyclooctatetraene, and cyclopentanone). Finally,some of the new directions in SIFT research are discussed, i.e., reactions of ions with polyatomic organic radicals,
and chemical ionization mass spectrometry with new detection schemes.

1.

Introduction

The selected ion ﬂow tube (SIFT) is an MS῎MS technique to investigate chemical reactions of massselected ions in a ﬂowing afterglow environment.1)
Mass-selected ions are thermalized by collision withbu#er gas molecules (e.g., helium) prior to reaction
with a neutral reagent added. Compared to conventional ion beam techniques, low collision energies with
the SIFT technique allow studies of ion῎molecule reactions under truly thermal conditions (300 K or Ecm ῌ
0.04 eV). The technique was originally developed bySmith and Adams at Birmingham.2), 3) A later review4)
described some additional features to the original
design, e.g., the VT῎SIFDT (variable temperatureselected ion ﬂow drift tube) apparatus to explore di#erent collision-energy regimes. Graul and Squires5) thoroughly and comprehensively reviewed ﬂow reactor
techniques in general, including SIFT. The techniquelater incorporated laser-induced ﬂuorescence6) andtriple quadrupole7) detections for extended capabilities.
This article discusses the SIFT ion chemistry work at
Colorado, with the focus on fundamental studies which
the author has conducted in recent years. Following abrief description of the instrument, studies of vibrationally state-selected ion῎molecule reactions andenergy transfer processes are presented as examples ofthe most fundamental aspects of gas phase ion chemistry. Nucleophilic substitution (SN2) reactions and associated kinetic isotope e#ects, hydrogen/deuterium exchange reactions, and isotope e#ects in termolecularassociation reactions are subsequently discussed, with
an emphasis on how isotope labeling can be used toprobe the structure and dynamics of ion῎neutral inter῏ Department of Chemistry and Biochemistry, University ofColorado (Boulder, CO 80309῎0215, USA)
E-mail: Shuji.Kato῎colorado.edu

mediate complexes. These sections are followed bydiscussions of individual ion chemistry in two categories. First, there are ion῎molecule reactions with strong
biological implications: SN2 reactions at a nitrogencenter and a novel ECO2 type of elimination reactions.
Secondly, the technique has been used extensively todetermine the thermochemistry and structure of energetic and exotic ions. The article concludes by presenting some of the most recent projects and new directions that are being pursued with the SIFT technique,i.e., reactions of ions with polyatomic organic radicals
and new schemes for chemical ionization mass spectrometry.
2.

SIFT Instrument

The selected ion ﬂow tube apparatus used at Colorado is schematically illustrated in Fig. 1. Details of thedesign and operation have been given elsewhere.6), 8)
Ions are produced by using simple electron impact onneutral precursors or sequential ion-molecule reactionsin the source ﬂow tube containing helium (ῌ0.3 Torr).
They are selected by the SIFT quadrupole mass ﬁlter
and injected through an injection oriﬁce into thesecond ﬂow tube containing the helium bu#er gas (0.5Torr,ῌ300 K). The SIFT injection energy (Einj), which
is deﬁned by the potential di#erence between thesource ﬂow tube and the injection oriﬁce, is typically
set to 20 eV in order to cleanly inject the reagent ionswith negligible fragmentation. Injected ions are quickly thermally equilibrated to 300 K by collisions withthe helium bu#er gas. A laminar ﬂow of He is established after the initial turbulent ﬂow regime (30῎40 cm
in length). The ions entrained in He are then allowed toreact with neutral gases added through multiple reactant inlets placed along the reaction ﬂow tube. The
length of the laminar region (ῌ1 m) and the ﬂow velocity (ῌ100 msῌ1) gives a maximum reaction time ofabout 10 ms. Parent and product ions are analyzed

῍ 183 ῍

S. Kato

Fig. 1.

The tandem ﬂowing afterglow SIFT instrument.

with a detection quadrupole mass ﬁlter placed at theend of the ﬂow tube. Absolute reaction rate constants
are measured by monitoring the kinetic decay of theparent ions while varying the reaction distance, i.e., theneutral inlet positions. The detector has a triple quadrupole capability that allows studies of collisioninduced dissociation (CID) of ions. Alternatively, unimolecular dissociation can be e#ected by SIFT injectionof ions at higher Einj ; in this case ion dissociation takes
place near the injection oriﬁce in the high-pressure He
environment. This mode of experiment (“SIFT῏CID”)
involves multiple ion῏He collisions, thus producing
qualitative and yet informative results with high sensitivity. An additional feature with the SIFT instrument
is the drift section in the second ﬂow tube (Fig. 1). Ion
mobilities and reactions at higher kinetic energies (upto Ecmῌ1 eV) are examined in the electric ﬁeld appliedalong the ﬂow tube.9), 10)Compared to polyatomic ions, vibrational relaxationof small diatomic ions is extremely slow in He. Thus,
ion chemistry of vibrationally excited ions can also be
explored. The capability of the SIFT technique has
been extended by incorporating high-resolution spectroscopic detection with laser induced ﬂuorescence (LIF,
Fig. 1).6), 11) The SIFT῏LIF technique allows studies ofvibrationally state selected ion chemistry, e.g., vibrational speciﬁcity and energy transfer in ion-molecule
reactions, at thermal energy (Ecmῌ0.04 eV). The optical detection has a signiﬁcant advantage over the conventional monitor ion technique,12) which chemicallyprobes for vibrationally excited states using reactions
that are fortuitously speciﬁc to certain excited states,e.g., N2ῌ(v῎0)῍Ar῍N2῍Arῌ without isotope speciﬁcity. Reactant ions are generated by electron impact invarious vibrational states, and immediately extracted
into the SIFT quadrupole chamber before collisional
relaxation takes place. The ions are then mass selected
and injected into the second ﬂow tube, where they are
rapidly thermalized translationally and rotationallywhile retaining vibrational excitation. Following areaction with a reagent added through the sample inlet(Fig. 1), the reactant and product ion vibrational states
are monitored using LIF. There are requirements for
SIFT-LIF experiments to be feasible:6), 11) Vibrationallyexcited reagent ions need to be readily generated andstably injected at high densities. Collisional quenchingof vibrationally excited states must be su$ciently slow
with the He bu#er gas (although slow quenching with
He can also be measured; see below) and the radiative
quenching be also negligible within the reaction timescale of about 150 ms before the LIF detection.6) Small,
non-polar N2ῌ(v) ions have thus been studied extensively with this technique using the B῏X electronictransition for LIF.
3.

Vibrationally State-Selected Ion Chemistry

3.1 Vibrational energy transfer and relaxation
Vibrational relaxation of ions in collision with raregas atoms (He, Ne, Ar, Kr, Xe) is slow. The ion῏neutralinteraction is relatively weak because the rare gas
atoms have no permanent dipoles or chemical interactions with ions. Relaxation via vibration-to-vibration(V-V) or vibration-to-rotation (V-R) energy transfer
does not occur with monatomic rare gas molecules, andonly the vibration-to-translation (V-T) process is available for the disposal of ion vibrational energy. Compared to intermolecular potential wells for neutral῏neutral systems, however, charge῏induced dipole interactions facilitate deep potential wells for ion῏neutral systems (typicallyῌ0.2 eV). As the consequence, ion vibrational relaxation is generally more e$cient than thecorresponding neutral relaxation.12) V-T energy transfer/relaxation with rare gas atoms is the simplest caseto examine the e#ects of intermolecular potentials
during ion῏neutral versus neutral῏neutral collisions.
Because of the inherent experimental di$culties, ion
vibrational relaxation has been considerably less explored than is relaxation of neutral molecules.13)῏15)
We used the SIFT῏LIF technique to measure vibrational relaxation of N2ῌ(v) ions with rare gas atoms.16)

῎ 184 ῎Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT Techniquekq(v)N2ῌ(v)M ῍῍῎ N2(v'v)M
kCT(v)῍῍῎ N2Mῌ(1)(2)

The optical detection allows individual vibrational
levels of N2(v) to be monitored. At an injection energyof Einj49 eV, initial vibrational population distributions of N2(v) were determined by LIF to be 46(v0),23(v1), 15(v2), 12(v3), 4(v4), and1(v5).17) Following addition of reagent rare gases, evolution of these states can be measured and analyzed,
and vibrationally speciﬁc quenching rate constants
kq(v) determined (Eq. (1)). In practice, measuring slow
rate constants is challenging because the measurement
can easily be biased by experimental artifacts arisingfrom trace impurities and ion mobility changes upon
addition of large amounts of reagents. Also, there areconcurrent charge transfer reactions (Eq. (2)) with Ar(for v0), Kr, and Xe. Conventional mass spectrometry, on the other hand, is blind to the quenching process in Eq. (1) but can detect the charge transfer products in Eq. (2) as well as impurity reactions. We thus
employed a powerful combination of SIFTῐLIF andmass spectrometry to unambiguously determine theion-rare gas relaxation rate constants. 16)
The measured rate constants kq,10 for vibrational relaxation of N2ῌ(v1) ῎ N2ῌ(v0) are very small; 9.8 (0.7)10῍16, 3.6 ( 0.5)10῍14, and 1.6 ( 0.2)10῍13
cm3 s῍1 for He, Ne, and Xe, respectively.16) Upper limits
were obtained for Ar (1.210῍11 cm3 s῍1)18) and Kr(210῍12 cm3 s῍1).16) These values are one to twoorders of magnitude smaller than previous measurements, which presumably were a#ected by impurities(reviewed in Kato et al.).16) Speciﬁcally, vibrational
relaxation with Ar was measured for the ﬁrst timeusing LIF. This process cannot be studied with theconventional monitor technique because Ar itself is the
only monitor species available to distinguish between
N2ῌ(v0) and Nῌ(v0). It is conﬁrmed that Nῌ(v0) Ar charge transfer is an excellent monitor reaction;
vibrational relaxation is negligibly slow compared to
the fast charge transfer (kCT(v0)410῍10 cm3 s῍1).
For comparison with neutralῐneutral vibration relaxation, the rate constant kq,10 is converted to thedeactivation probability per collision, P ( kq,10/kcol ,where kcol is the collision rate constant). The values ofP are extremely small, ranging from 1.710῍6 (He) to
8.410῍5 (Ne) and 1.610῍4 (Xe) for collisions withN2ῌ(v1).16) The ine$cient relaxation is reasonablebecause the N2ῌ ion (we  2207 cm῍1) has to dissipate
the large vibrational energy solely into translation.
The small probabilities imply that N2(v) relaxation
proceeds via transfer of a single vibrational quantum
per collision, i.e., v'  v 1, in accord with Landauῐ
Teller theory for V-T energy transfer established forneutralῐneutral collisions at thermal energy.13), 14)There is a marked di#erence observed between relaxations of N2ῌ(v) and N2(v) however. Measured or estimated values of P for neutral N2(v) are 1.610῍8(He), 610῍13 (Ne), and 110῍14 (Ar);16) relaxation ismore e$cient for lighter collision partners. This trend
is consistent with LandauῐTeller theory for V-T energy
transfer, which assumes the relaxation to occur at therepulsive part of the intermolecular potential. In contrast, observed N2ῌ(v) ion relaxation is more e$cientfor heavier collision partners and has a positive correlation with the rare gas polarizability. The magnitudesof the deactivation probability are also greater for N2ῌ(v) than for N2(v). The attractive, chargeῐinduced
dipole interaction must play a signiﬁcant role in theN2ῌ(v) ion vibrational relaxation. We discussed theresults in terms of direct versus complex-formation
mechanisms.16)
The SIFTῐLIF technique was extended to studies ofN2ῌ(v) vibrational relaxation/energy transfer with diatomic molecules N2,19)ῐ21) O2,18), 21) CO and NO,22) andH2 .23) In addition to V-T energy transfer, relaxation via
V-V and V-R processes is possible for these diatomics;
the vibrational constants we are 2359 cm῍1 (N2), 1580cm῍1 (O2), 2170 cm῍1 (CO), 1904 cm῍1 (NO), and 4395
cm῍1 (H2) in comparison to 2207 cm῍1 (N2ῌ). The attractive potential wells are especially deeper ( 1 eV)with N2, O2, and NO due to the electronῐexchangeinteractions.24), 25) E$cient relaxation of N2ῌ(v) is thus
expected for these reactants.
Rate constants kq and probabilities P were determined for N2ῌ(v) vibrational relaxation, in competition
with concurrent charge transfer (N2 , O2 , CO, NO) andhydrogen atom transfer (N2ῌH2 ῎ N2HῌH). As
expected, vibrational relaxation is generally more e$cient with the diatomic molecules by orders of magnitude than with rare gas atoms, but there are interesting
variations found among the di#erent diatomics. Vibrational relaxation with H2 is noticeably slow and only
an upper limit of 0.06 was obtained for P , on average
for all vibrational levels of N2ῌ(v0).23) This may bereasonable in light of the poor vibrational energy
match between N2ῌ and H2 and also the small polarizability of H2.
An intriguing comparison is made between N2ῌ(v) NO and N2ῌ(v)  CO.22) On average for v0, vibrational quenching is e$cient for NO with P of 0.22
whereas the deactivation probability is only 0.04 forCO, even though CO and N2ῌ has a nearly perfectvibrational energy match for resonant V-V energy
transfer. This suggests that ion vibrational relaxation
does not always follow the resonance principles thatare well known for neutrals.15) Instead, the lifetime ofenergized adducts is a more important factor than is
the V-V energy gap. The N2ῌ(v)NO system is likely
to have a deep attractive well due to the near energyῐ
resonant, electronῐexchange interaction between N2ῌ
and electronically excited NOῌ(a3Sῌ).25) E$cient vibrational energy transfer may occur during the long complex lifetime. Relaxation of N2ῌ(v) with O2 is also
e$cient and vibrational-level dependent, the value ofP increasing from 0.15 (v1) to 0.30 (v4).21) A
similar electronῐexchange interaction with the excited
TachibanaO2ῌ(a4Pu) state has been suggested.25)
et al.26) proposed vibrational relaxation within thestable quartet [N2ῌO2] complex, which is computationally shown to be formed from N2ῌ and O2 andcorrelate with N2  O2ῌ(a4Pu).
The SIFTῐLIF technique revealed another novel
aspect in ion-molecule interactions: multiquantum vibrational transfer/deactivation in collision of N2ῌ(v)῏ 185 ῏

S. Katowith N2 and O2.11),19)21) The [N2N2]ῌ adduct formed
from N2ῌN2 is strongly bound by about 1.1 eV.27), 28)
In collision of vibrationally excited 15N2ῌ(v2) ionswith 14N2, the reactant and product ion vibrational
states were distinguished by high-resolution LIF withisotope speciﬁcity. We observed simultaneous transfer
of charge and vibration in this near-energy resonant
charge transfer reaction (Eq. (3)).19) Most notably, twovibrational quanta are transferred e$ciently in a single
collision.
15

N2(v2)14N2(v'0) ῍
15
N2(v0)14N2(v'2)(3)

A detailed kinetic analysis including all reactant vibrational states (v0ῐ4) indicates that approximately
10% of the initial 15N2ῌ(v2) forms 14N2ῌ(v'2) (Eq. (3))
and, by symmetry, another 10% forms 14N2(v'2) neutral (plus 15N2ῌ(v0)).21) The observation that the N2ῌ(v2) ion relaxes via Dv2 transitions with such ahigh e$ciency (Pῌ0.2) is a marked result. For neutralῐneutral collisions, only the Dv1 transition isallowed at thermal energy to ﬁrst order within theframework of LandauῐTeller theory.13), 14)
A possible mechanism is direct V-V energy transfer
between the high-frequency moieties in the long-lived[15N2ῐ14N2] complex, as inferred from the similar vibrational constants for N2ῌ and N2. Alternatively, multiquantum vibrational transfer may result from nearresonant, intracomplex chargeῐexchange transitions
Extensive quantum[15N2ῌ14N2]῎῍[15N214N2ῌ].
mechanical calculations of the state-to-state crosssections29) support the importance of Dv2 transitions in thermal energy N2ῌ(v)N2 collisions, alongwith facile single-quantum V-V and V-T processes
which were also conﬁrmed from analysis of the LIF
kinetics.21)
Multiquantum vibrational transfer/relaxation was
also indicated in the N2ῌ(v)O2 reaction from SIFTῐLIF rate kinetics for N2ῌ(v0 ῐ 4).21) As noted above, adeep potential well facilitated by the intermediacy ofelectronically excited O2ῌ(a4Pu) must be responsible forthe observation. By comparison, ion deactivation proceeds exclusively via the Dv1 transition for NOῌ(v)  M systems,30) which have shallow potential wells.
In this vein, deactivation of N2ῌ(v)  NO may also
proceed via multiquantum vibrational energy transfer.
Unfortunately, for this system the fast charge transfer(Eq. (2)) precludes detailed analysis of the v2 ῍ v0versus v2 ῍ v1 components in vibrational deactivation (Eq. (1)).
3.2 Vibrational speciﬁcity in chemical reactions
A dramatic enhancement in charge-transfer rates
was observed by vibrational excitation of the N2ῌ ion
in the N2ῌ(v0 ῐ 4)  Kr reactions at thermal energy17);
the rate constant increases by a factor of ῌ80 in goingfrom v0 to v4. The observed increase is almost
entirely due to enhanced charge transfer since thevibrational relaxation has been shown to be negligibly
slow.16) This is a particularly interesting system because charge transfer is highly exothermic for vibrational ground state N2ῌ and yet the v0 charge transfer is surprising slow (k1.0 10῍12 cm3 s῍1), occurring
once in 800 collision events.

N2ῌ(v0)Kr ῍ N2Krῌ(2P3/2)1.580 eV(4)

῍ N2Krῌ(2P1/2)0.915 eV(5)

If energy mismatch is the problem, one may expectthat charge transfer would be suppressed even moreheavily for higher vibrational excitation in N2ῌ as theapparent energy gap becomes greater. Based on thisenergy-gap argument, an early ﬂow tube kinetics
study31) assigned the more reactive component to N2ῌ(v0) whereas the less reactive to N2(v 0). The
state-speciﬁc detection with SIFT-LIF, however, unambiguously determined that the rates are enhancedby vibrational excitation of N2(v).
The observed trend of vibrational enhancement qualitatively agrees with calculations using modiﬁed Rapp
ῐFrancis theory,17) which formulates the state-selective
reaction cross section s(v) as the product of an energyresonance term and the associated FranckῐCondon
factor qvv' , summed over all ﬁnal v' states involved.
This treatment has found considerable success in predicting state-selective reaction cross sections at highcollision energies.32) At 300 K (Ecmῌ 0.04 eV), however,the calculated rate constants for N2(v)  Kr are several orders of magnitude smaller than the experimental
values.17)
The experimental result is quantitatively explainedby a model17), 33) that assumes, in reality, that the associated FranckῐCondon factors for the N2(v) ῐ N2(v')
transitions are slightly perturbed during a close approach of the (N2 ῐ Kr)ῌ pair (Fig. 2). When the ion andneutral approach at slow velocity, the wave functions
for N2ῌ and N2 would have su$cient time to adjust toeach other.11) To model such distortion of Franckῐ῏ 186 ῏Fig. 2.

Schematic energy diagram for N2 and N2ῌ
representing the “distorted FranckῐCondon”
model. The wave functions and associatedFranckῐCondon factors are perturbed by theclose approach of a reactant. In this example
the N2 wave function is simply shifted fromthe equilibrium internuclear distance, shownby the dotted line. The displacement has been
exaggerated for the sake of presentation. The
FranckῐCondon overlaps are consequently
altered.

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT TechniqueCondon factors within the transient (N2῎Kr)ῌ pair, thevibrational wave functions of N2ῌ and N2 are displaced
relative to each other. In this instance the N2 wave
function is simply shifted from the equilibrium internuclear distance (re) of 1.0977 Å by a short displacement of dῐ0.02 Å (Eq. (6)),17), 33) while the N2ῌ wave
function is ﬁxed at reῐ1.1164 Å. The FranckῑCondon
overlap is consequently altered.
qv,v'ῐ

ῒΐ Y (r) Y (r῍d)drῒ
N2ῌ

2

N2

(6)

ῌ

The rate constants for N2 (v)῍Kr were calculated fromthe most energy-resonant product channels (DEῑ100meV), in this case those producing N2 (v῍3)῍Krῌ(2P1/2)(Eqs. (7) ῑ (11)). This is a reasonable approximation in
the spirit of Rapp-Francis theory, as discussed in Katoet al.,17) that chargeῑtransfer cross sections become extremely sensitive to the energy gap at low collisionenergies.
῍

N2 (vῐ0)῍Kr ῏N2(v'ῐ3)῍Kr῍(2P1/2)῍59 meV(7)N2῍(vῐ1)῍Kr ῏N2(v'ῐ4)῍Kr῍(2P1/2)῍51 meV(8)N2῍(vῐ2)῍Kr ῏N2(v'ῐ5)῍Kr῍(2P1/2)῍41 meV(9)N2῍(vῐ3)῍Kr ῏N2(v'ῐ6)῍Kr῍(2P1/2)῍32 meV(10)

N2῍(vῐ4)῍Kr ῏N2(v'ῐ7)῍Kr῍(2P1/2)῍22 meV(11)

The “distorted FranckῑCondon” model was originally developed by Lipeles to explain anomalous productvibrational state distributions in slow ion-molecule collisions.34) We extended his approach to predict stateselective reaction cross sections at low Ecm, where theRappῑFrancis treatment starts to fail. We also applied
this model to successfully reproduce the N2ῌ(vῐ 1 ῑ 4 )῍Ar charge transfer rates.33) The measured rate constants are nearly unchanged over the exothermic channels N2ῌ(vῐ 1 ῑ 4 )῍Ar῏N2῍Arῌ, a result unexplainedby standard FranckῑCondon arguments.
Vibrational speciﬁcity in reactions of N2ῌ(v) with
diatomic reagents was also studied. Upon vibrational
excitation, the chargeῑtransfer component for the N2ῌ(v)῍CO reaction22) increases considerably whereas theN2ῌ(v)῍NO22) and N2ῌ(v)῍O2 charge transfer rates18), 21)
show only moderate increases. These observations are
intriguing since, for both the NO and O2 reactions,electronically excited states of the ion products become
energetically accessible, i.e., NO῍(a3S῍) from N2ῌ(v῎1)
and O2ῌ(a4Pu) from N2ῌ(v῎2). In contrast, for the N2ῌ(v)῍CO reaction, the lowest electronically excited product ion state CO῍(A2P) is not thermodynamically accessible. The moderate increase in the N2ῌ(v῎2)῍O2charge transfer rate has been explained theoreticallyby state coupling via the quartet [N2ῌO2]῍ complex26)
and also by wavepacket dynamics.35) A guided ion
beam experiment reported an anomalously small
chargeῑtransfer reactivity for N2ῌ(vῐ1)῍O2.36) However, such a decrease is not observed in other studies

using mass spectrometric kinetics (reviewed in Kato etal.18), 21)) and photoelectronῑphotoion coincidence,37)
and the discrepancy remains unresolved.
We also studied vibrational speciﬁcity in the hydrogen-atom transfer reaction N2ῌ(v)῍H2 ῏ N2H῍῍H.23)
The reaction rates are large and nearly unchanged
with respect to the reactant vibrational excitation. The
fact that the rates signiﬁcantly exceed the Langevincollision limit suggests that the reaction proceeds via
initial charge transfer at large intermolecular distances
producing N2῍H2ῌ, followed by proton transfer at ashorter range. The Langevin rate is greater forN2῍H2ῌ than for N2ῌ῍H2 . Vibrational speciﬁcity may
well be smeared out during these sequential processes
in a single collision event.
4.

Dynamics of Organic Ion-Molecule Reactions

4.1 Nucleophilic substitution (SN2) reactions
Nucleophilic substitution is one of the most fundamental chemical transformations in solution, and there
have been numerous gas phase studies as well.38) In theabsence of solvent molecules, gas phase SN2 studies
reveal intrinsic features of this reaction (mechanismand energetics, transition state structure, energy transfer/randomization, product dynamics) as well as provide key calibration for theory. This part discusses
mechanisms and dynamic e#ects in SN2 reactions insome detail to provide a background for later sections.
SN2 reactions of methyl halides with halide anions,X῍῍CH3Y ῏ CH3X῍Y῍, represent the simplest
system in this category. The SIFT technique was used
to detect the occurrence of a slow, symmetric SN2reaction (XῐYῐCl) in the gas phase.39)37

Cl῍῍CH335Cl ῏ CH337Cl῍35Cl῍(12)The reaction is extremely slow at room temperature
with the measured rate coe$cient of k῍3.5῏10῍14 cm3s῍1 (one reaction in ῍50000 collisions). The remarkable slowness of the thermoneutral SN2 reaction is in
sharp contrast to the general rapidity of protonῑtransfer (PT) reactions, which occur at nearly every collision
for many systems that are thermoneutral or nearthermoneutral.40)
Figure 3 schematically shows potential energy
curves for SN2 and PT processes, for which the reactions are assumed to be similarly exothermic by DErxn.
The transition state (TS) for proton transfer is locateddeep in the ion-molecule attractive potential well(Fig. 3a); the TS barrier is as low as a few kcal mol῍1 orvirtually nonexistent for typical PT systems, e.g., those
involving oxo acids.41) In contrast, for SN2 transformation involving Walden inversion (Fig. 3b), the centralenergy barrier (DEῌcent) is considerably high and comparable with the well depth (complexation energy
DEcomp῍10 ῑ 25 kcal mol῍1).42) The double-well scheme
was ﬁrst proposed by Olmstead and Brauman43) for
solvent-free SN2 reactions in the gas phase. Even when
the transition state is somewhat lower in energy than
the reactants (i.e., the overall energy barrier DEῌovrῑ0),
back-dissociation of the reactant complex [X῍ῌCH3Y]
is favored over Walden inversion to form the productcomplex [XCH3ῌY῍], based on a simple density-ofstates argument. The reaction e$ciency can thus beῐ 187 ῐ

S. Kato

Fig. 3.

Schematic potential energy curves for (a) proton transfer reaction of base anion Bῌ with acid HA, and (b) SN2
reaction of nucleophile Xῌ with methyl halide CH3Y, in the gas phase. The diagram (a) represents typical proton῎transfer processes involving oxo- and sulfo- acids. Zero-point energies have been omitted for simplicity.

considerably small.
The slow SN2 reaction (Eq. (12)) is dramatically enhanced as the ion῎molecule collision energy is increased in the SIFT-drift ﬁeld,39) stimulating fundamental interest in the mechanism of gas phase nucleophilic
substitution. A high-level ab initio theoretical study44)
suggested this behavior as evidence for the occurrenceof a novel “front-side” SN2 mechanism, in which 37Clῌ
approaches the carbon atom from the same side as
35
Clῌ leaves (Fig. 4b). At the G2(῏) level of theory44)
the energy threshold for the front-side attack was predicted to be 46.3 kcal molῌ1. However, guided ion
beam experiments under single collision conditions45)
measured the threshold for the translational activation
to be 11 ῐ 4 kcal molῌ1, signiﬁcantly lower than thetheoretical front-side threshold. Therefore, the frontside mechanism must not be reasonable. The observed
enhancement can then be attributed to translational
activation of the conventional back-side SN2 reaction(Fig. 4a), which must be dynamically restricted (seebelow). A more recent, ab initio direct dynamics
trajectory simulation supports the back-side SN2
mechanism46); substantial vibrational excitation in the
CH3Cl reactant is required to access the front-side transition state, and even so, the front-side pathway will
still be dominated by the back-side reaction.
For the thermoneutral Clῌ ῏ CH3Cl system, the transition state for the back-side SN2 reaction is actuallycalculated to be slightly higher in energy than thereactants at the G2(῏) level of theory (DEῌovrῑ῏2.7
kcal molῌ1).47) This is consistent with the fact that thereaction is very slow at 300 K and accelerated bytranslational excitation. However, why is the translational threshold (ῌ11 kcal molῌ1)45) signiﬁcantly greater than the overall energy barrier? As it turned out, the“simple” SN2 reaction (Eq. (12)) is far from simple. The
Clῌ῏CH3Cl reaction is a member of methyl halide SN2

Fig. 4.

Various types of nucleophilic substitution
reactions. (a) conventional back-side SN2, (b)“front-side” SN2, and (c) SN2 at nitrogen.

reactions that display nonstatistical kinetics anddynamics (see Sun, Song, and Hase48), 49) and references
therein). The short lifetime of the initial [Clῌ῎CH3Cl]
complex (tῌ4 ps) hampers e$cient coupling of the Clῌ῎CH3Cl intermolecular mode (i.e., translational energy)
and CH3Cl intramolecular mode, resulting in the failure
to utilize the translational energy to surmount thecentral SN2 barrier.48) Orientational and angularmomentum constraints are also suggested to explain
the ine$cient translational activation observed.45), 50)
While statistical behaviors have been demonstrated forlarge SN2 systems with complex lifetimes tῒ10 ns,51)
SN2 reactions of methyl halides are nonstatistical inmany cases primarily because of the ine$cient intramolecular vibrational relaxation (IVR) within thecomplex lifetimes of ῍10 ps or shorter. Such incom-

῍ 188 ῍Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT Techniqueplete energy randomization violates a basic assumption in statistical rate theories, e.g., canonical, transition state theory (TST) and microcanonical, Riceῐ
RamspergerῐKasselῐMarcus (RRKM) theory.52), 53)
In discussing “nonstatistical” behaviors, it should beclariﬁed in what region of the potential energy surface
IVR is impeded and how it a#ects the experimental
observables. Statistical rate theories assume complete
randomization of energy (i.e., statistical population ofphase space) all the way from the reactants via theinitial complex up to the transition state (A in Fig. 3b).
Thus, nonstatistical behaviors show up as deviations inrate kinetics (e.g., kinetic isotope e#ects, see below) andas di#erential e#ects of reactant excitation (translation, rotation, vibration) on the reaction rates.54), 55)
When energy randomization is hampered after thetransition state all the way down to the products (C inFig. 3b), nonstatistical dynamics are observed asanomalous product energy distributions,56), 57) while therate kinetics may behave normally. It has also been
predicted that the SN2 reaction of HOῌ῍CH3F avoids
the deep potential energy minimum in the exit
channel49); it is caused by ine$cient IVR in the post TS
region within the time scale of the product complex
dissociation.
A more subtle case is ine$cient translation-tovibration (T-V) energy coupling in the TS region (B in
Fig. 3b). As shown by simulations for the Clῌ῍CH3Br
and Clῌ῍CH3Cl systems,48), 57) this traps the complex in
the central barrier region and allows multiple TS recrossings to occur, another violation of a basic assumption in statistical rate theories.52), 53) The reaction probability per ion-molecule collision will consequently
decrease. Direct techniques such as transition-state
spectroscopy may be used to probe the transient species trapped in this region. Recently, femtosecond
time-resolved photoelectron spectroscopy has been
employed58) to monitor the dissociation dynamics of anenergized SN2 complex IῌῌCH3I. Because of the experimental constraints58) the complex cannot have su$cient energy to surmount the SN2 barrier. In this symmetric system the SN2 process is not distinguishable
from complex dissociation, either. Nonetheless, a future
breakthrough may be expected with these approaches.
4.2 Kinetic isotope e#ects in SN2 reactions
Kinetic isotope e#ects (KIEs)59) have been used as asomewhat indirect but sensitive probe for the mechanism and transition state structure of SN2 reactions.
For methyl halides reacting with nucleophile anions(Nuῌ), the secondary a-deuterium KIE is deﬁned as theratio of the rate constants for unlabeled (kH) to deuterated (kD) SN2 substrates (KIE ῑ kH/kD).
kH

CH3X῍Nuῌ ῍῎ CH3Nu῍XῌkD

CD3X῍Nuῌ ῍῎ CD3Nu῍Xῌ

up to the transition state. The assumption allows forprediction of KIEs based on conventional TST and theassociated partition function relationship between thereactants and transition state. Deviations from statistical dynamics are noticed as the di#erence between
experimental and theoretical KIEs, either in the magnitude for a speciﬁc reaction, or in the trend for a series ofsystematic SN2 reactions.
To illustrate the latter, Poirier et al.60) devised a transition-state “looseness” parameter, RTS (Fig. 5a). For agiven SN2 substrate reacting with di#erent nucleophiles, they demonstrated computationally that KIE
values are greater for nucleophiles that have larger
RTS. As RTS increases and the transition state becomeslooser, more “space” becomes available around the Caῐ
H(D) bonds at the transition state. Thus, at the transition state, the out-of-plane CῐH bending frequencies arelowered more dramatically than are the correspondingCῐD frequencies, leading to an increase in KIEs frominverse toward normal (kH/kD῏1). This e#ect is contrasted by that of methyl CῐH stretching frequencies,
which are the major contributor to the overall inverse
KIE but are nearly unchanged across di#erent nucleophiles.60)
Under well-deﬁned thermal energy conditions with
the SIFT technique, we determined a-deuterium KIEs61)
for SN2 reactions of methyl halides CH3Cl, CH3Br, andCH3I and compared them with theoretical predictions(Kato et al. 61) and references cited therein). Experimental KIEs for methyl chloride are found to increase frominverse toward normal as RTS increases (Fig. 6), in
excellent agreement with TST predictions in both thetrend and magnitude. This suggests that SN2 reactionsof CH3Cl proceed statistically. There is a caveat, however, that consistent KIEs do not necessarily warrant

(13)
(14)

The inverse KIE (kH/kD῎1) is generally consistent
with a tight SN2 transition state, for which the overall
energy barrier DEῌovr is slightly higher for the protonated reactant than for the deuterated reactant.52) A
working hypothesis in quantitatively discussing KIEs
is that the reaction is statistical, i.e., there is complete
energy randomization all the way from the reactants
῏ 189 ῏Fig. 5.

(a) The transition-state “looseness” parameter(RTS), deﬁned as the sum of the lengths of thenucleophile atom (Nu)-methyl carbon (Ca) andmethyl carbon-leaving group atom (LG).
Schematically shown is the out-of-plane CaῐH(D) umbrella mode at the transition state.
Schematic SN2 transition states for methyl
halides: (b) Fῌ῍CH3X, (c) Fῌ(H2O)῍CH3X, and(d) Fῌ(CH3OH)῍CH3X. The reagents can be
speciﬁcally deuterated at the methyl halide (H(1)), the hydroxyl group (H(2)), or the methylgroup in methanol (H(3)).

S. Kato

plained by these adjustments.
SIFT studies have been extended to microsolvated
SN2 reactions, which have attracted considerable attention recently.38), 68), 69) Reactions of Fῌ(H2O)ῌCH3X (X῎Cl, Br, I) and their deuterated analogs display substantial inverse KIEs by deuteration of the water solvent as well as of the methyl group in methyl halides.70)
For unlabeled reactants, the reaction proceeds via twochannels (Eqs. (15) and (16)).

Fig. 6.

Experimental (open symbols) and theoretical(closed symbols) KIEs (ῐkH/kD) as a function of
the transition-state looseness parameter (RTS)
for the SN2 reactions of methyl halides with avariety of nucleophiles in the gas phase. Datataken from Kato et al.61) The lines are intended
only to guide the eye.

statistical dynamics in every aspect of the reaction.
Among the methyl chloride series, the CH3ClῌFῌ reaction shows a nonstatistical behavior such that the rateis independent of the reactant internal energy.54) A
direct mechanism without formation of a long-lived
complex has been suggested from the observed deviation of reaction cross sections at high collisionenergies62) and by ab initio direct dynamics calculations(collision energy not speciﬁed).63) The CH3ClῌClῌ reaction proceeds via a similar direct (nonstatistical)
mechanism associated with favored reactant orientation,45), 48), 50) and nonstatistical barrier-recrossing is
also predicted.48) As such, KIEs are only sensitive to
nonstatistical attributes that do not cancel in the rate
ratio kH/kD. By deﬁnition, they are also blind to dynamics after the transition state region.
For SN2 reactions of heavier methyl halides, observed a-deuterium KIEs are distinctly anomalous.61)
Experimental KIE values for CH3Br and CH3I decrease(i.e., become more inverse) as RTS increases (Fig. 6).
Interestingly, computed partition functions for CH3I
still predict a normal, positive correlation between the
KIE value and the looseness parameter. Althoughmagnitudes of computational KIE values for individual
reactions may vary depending on the theoretical methods employed,64), 65) the opposite trend between experiments and statistical theory clearly indicates that SN2
dynamics of the heavier methyl halides is nonstatistical. Other kinetics observations also suggest incomplete IVR for these systems.54) The transition-statelooseness model has been modiﬁed by incorporatingsteric e#ects in more reﬁned manners, e.g., consideration of atomic radii for the nucleophile and leavinggroups66) and three dimensional “crowdedness” imposed by polyatomic nucleophiles.67) However, the observed discrepancy in the KIE trend cannot be ex-Fῌ(H2O)ῌCH3X ῌXῌῌCH3FῌH2O(15)

ῌXῌ(H2O)ῌCH3F(16)

The measured KIE values are in excellent agreementwith TST predictions, which have been made for Fῌ
(H2O)ῌCH3Cl.71) This may not be surprising since IVR
is expected to be very e$cient for these large systems.
The product branching, however, shows intriguing dynamical e#ects. For this reaction, the measured product yields for Eq. (15) (including potential formation of
CH3F(H2O)ῌClῌ)72) and Eq. (16) are approximately 70%
and 30%, respectively, despite the fact that the latter issigniﬁcantly more favorable energetically. Tachikawa
performed direct ab initio dynamics calculations at
higher collision energies (Ecm῎4῍25 kcal molῌ1) andpredicted that formation of Clῌ(H2O) (Eq. (16)) is ine$cient because of a geometrical/dynamical constraint
after the SN2 transformation.72), 73)
While the measured isotope e#ects are consistent
with TST, a factor analysis dissecting the overall KIEs
provides useful clues to the structure and molecularinteraction at the SN2 transition state.67), 71) The transition-state structure for the Fῌ(H2O)ῌCH3X reactions
is schematically shown in Fig. 5c, in comparison to thebare ion system FῌῌCH3X (Fig. 5b). In addition to theinverse methyl halide KIE as explained by a mechanism similar to the bare ion case, the strong inverse
KIE upon deuteration of the water solvent is due primarily to the stretching vibration between the oxygen
atom and the hydrogen atom H(2) that is hydrogenbonded to ﬂuorine (Fig. 5c).67), 71) The O῎H(2) stretchingfrequency increases at the transition state compared tothat in free Fῌ(H2O), giving rise to the strong inverse
isotope e#ect.67), 71)
We further studied KIEs for the SN2 reactions of
CH3X with Fῌ(methanol), Fῌ(isopropyl alcohol), and Fῌ
(hydrogen ﬂuoride).74) Substantial inverse KIEs wereobserved in all these reactions by deuteration of thehydroxyl group of the solvent (or deuteration of hydrogen ﬂuoride). This is totally consistent with the Fῌ
(H2O)ῌCH3X results70); the kinetic isotope e#ects could
similarly arise from the increase in the O ῎ H(2) stretching frequency at the transition state, as schematicallyillustrated for Fῌ(methanol)ῌCH3X (Fig. 5d) although
no structure and vibrational data are available. The
inverse KIE observed for Fῌ(HF) versus Fῌ(DF) is explained in essentially the same manner. Moderate inverse KIEs were also measured for deuteration of theremote methyl group in methanol (H(3) in Fig. 5d). At
the transition state for Fῌ(H2O)ῌCH3Cl (Fig. 5c) the
terminal O῎H(2) bond interacts with the methyl groupin CH3Cl, giving rise to low-frequency fundamental
modes and contributing to the inverse isotope e#ects

῍ 190 ῍Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT Techniqueobserved.71) We speculate that a similar mechanism isoperative for the KIE upon the deuteration of H(3) inmethanol.74)
4.3 Hydrogen/deuterium exchange and scrambling
In highly exothermic ion-molecule reactions, lifetimeof the intermediate complex can be substantially shortthat rapid complex dissociation may e#ectively compete with other chemical processes that are to occur
therein. The reaction of CH3ῌ with CH4 is exothermicby nearly 30 kcal mol῍1 and produces the ethyl cationand H2 via the protonated ethane intermediate complex[C2H7ῌ]. Hiraoka and Kebarle75) found evidence for the
existence of two structures for the C2H7ῌ intermediate,both classical and non-classical. The SIFT technique
was used to probe the hydrogen/deuterium (H/D)
scrambling process during this reaction,76) by startingfrom di#erently labeled reactants, e.g., CD3ῌ῍CH4(Scheme 1), CHD2ῌ῍CH3D, and CH3ῌ῍CHD3 and observing variations in the losses of H2, HD, and D2.
Isotope scrambling occurs via interconversion between
the non-classical 1 and classical 2 structures. The
reaction is all the way downhill relative to the reactants, since the relative energies of those intermediates
as well as of the connecting transition states are signiﬁcantly lower than the reactant energy by at least 20
kcal mol῍1. 75), 77) If the complex is su$ciently longlived, equilibrium between the two structures is established and the losses of hydrogen will be statistical andthe same, regardless of the di#erent deuteration in thereactants. The experiments indicate that the scrambl-

Scheme 1.

ing is incomplete and there are subtle di#erencesamong the individual reactant systems. This is causedby rapid dissociation of the complex; a modeling showsthat approximately seven H/D scramblings occur
before the complex dissociates to products.76)
Hydrogen/deuterium exchange of ions with neutral
reagents is a thermoneutral process besides the small
isotope e#ects. In anion H/D exchange, the reaction isconsidered to proceed via a key intermediate complex
3, in which all hydrogen atoms are identical(Scheme 2).78) Abstraction of a proton by B῍ followedby complex dissociation results in one H/D exchange.
By repeating collisions with BD, all hydrogen atoms in
AHn῍ may exchange. We observed anomalous H/D
exchange for mono- and diﬂuorophenyl anions reacting with D2O and CH3OD.79) The observed behavior
reveals the dynamical motion within the ion῍dipole
complex. For example in the p-diﬂuorophenyl anion,
all three hydrogen atoms are thermodynamically identical after intracomplex deuteron transfer to form 5 or 6,
neglecting any small isotope e#ects (Scheme 3). Thus
one would expect that the three hydrogen atoms in theanion will exchange with either reagent. In fact,
CH3OD does exchange all three, but D2O exchanges
only one. This indicates that DO῍ is trapped on one
side of the benzene ring and cannot access the otherside bearing two hydrogen atoms, whereas CH3O῍ canmigrate freely within the complex.
The kinetically restricted exchange with D2O can beunderstood in terms of the calculated energy
diagram.79) Ab initio computations indicate that theDO῍ species can migrate along the side of the benzene
ring where it is formed (5); however, it cannot migrate
across the barrier caused by the ﬂuorine atoms or by

H/D scrambling following reactive collisionof CD3ῌ and CH4.

Scheme 3.

Scheme 2.

H/D exchange reaction of an anion AHnῌwith a deuterated neutral reagent BD.

Kinetically restricted H/D exchange for the p-diﬂuorophenyl anion with D2O and CH3OD.
ῌ 191 ῌ

S. Kato

the p electrons on the top and bottom of the aromaticring. For H/D exchange with methanol the CH3Oῌ
species can cross over to the other side of the ring bysurmounting the p electron barrier (6). The asterisks in
6 indicate the location of the methoxide anion at the“crossover” transition state. A similar transition state
exists for hydroxide but the energy is higher than thereactants so the crossover does not take place. The
number of exchangeable hydrogens thus provides information about the dynamics of ion῏dipole intermediate complexes. On the other hand, these results caution
against a simple interpretation of ion structure based
solely on the extent of H/D exchange.78), 79)
4.4 Termolecular association
Mechanisms of gas phase ion-molecule termolecularassociation reactions can be probed by species- andsite-speciﬁc deuteration of the reagents. For example,
in the association reaction of methoxide and watermolecules that is assisted by collisional stabilization
with the helium bu#er gas, how are the association
rates a#ected by deuterium substitution of the bridging hydrogen (Eq. (18)) or the methyl group (Eq. (19))
relative to the perprotio system (Eq. (17))?
He

CH3Oῌ῍H2O ῌ῍ CH3OῌῌH2O
He

CH3Oῌ῍D2O ῌ῍ CH3OῌῌD2O
He

CD3Oῌ῍H2O ῌ῍ CD3OῌῌH2O

(17)
(18)
(19)

We systematically studied 14 association reactionsof hydroxide῏water, methoxide῏water, methoxide῏
methanol, and ethoxide῏ethanol and their deuterated
analogs.80) Apparent bimolecular rates are measured asa function of helium pressure to derive the rate constants for termolecular association. Association rates
are signiﬁcantly enhanced by deuteration of the alkyl
groups (by factors of 1.4῏3.0) whereas deuteration of
the bridging hydrogen does not a#ect the association
rates.
The observed isotope e#ects can be understood interms of the structure and vibrational modes for the
association complex.80) For example, in association of
methoxide with water, the intermediate complex 7
formed from CH3Oῌ and H2O (rate constant kf) is likely
to have a hydrogen-bonded structure (Scheme 4).81), 82)
The energized complex either back-dissociates to thereactants (rate constant kb) or is collisionally stabilized
to the adduct CH3OῌῌH2O (rate constant ks , neglecting slow radiative stabilization). If the increase in thecomplex density of states overwhelms the increase in
the reactant density of states upon deuterium substitution, the complex lifetime increases (i.e., kb decreases)

Scheme 4.

and more e$cient adduct formation results. The observed lifetime increase upon deuteration of the methylgroup is dominated by the increase of the density ofstates provided by the low frequency modes (ῐ200cmῌ1), which are due to methyl tortions and internalrotation.81) On the other hand, deuterium substitution
on the bridging position does not a#ect the complex
lifetime because the high-frequency O῏H and O῏D vibrations 83) do not substantially increase the density ofstates. Detailed analysis of the magnitude and pressure
dependence of the association rates has also provided
insights into the potential energy surface (singleversus double-well) and dynamical bottleneck for ion῏molecule association.80)
5.

Fundamental Ion Chemistry of Biological Interest

5.1 SN2 reaction at nitrogen center
Nucleophilic substitution at a nitrogen center is aconvenient synthetic path to the formation of C῏Nbonds.84) The reactions have also been recognized toplay a signiﬁcant role in carcinogenesis.85), 86) The existence of a classical SN2 transition state at a nitrogen
substrate has been inferred using double labeling experiments in the liquid phase,87) and theoretically supported for a simple system Xῌ῍H2NY῍XNH2῍
Yῌ.88), 89) However, no experimental data were available for the gas-phase kinetics of SN2 reactions at nitrogen centers.
We ﬁrst investigated the SN2 reactions of chloramine(NH2Cl) with several anionic nucleophiles (Fig. 4c).90)
The unstable chloramine substrate was synthesized insitu and allowed to react with several nucleophiles inthe ﬂow tube (Nuῌ῏HOῌ, CH3Oῌ, C2H5Oῌ, C3H7Oῌ, Fῌ,C6H5CH2Oῌ , CF3CH2Oῌ, HSῌ, and Clῌ, in the decreasing order of gas phase basicity). The most stable
ion-molecule complex has the structure 8, which directly correlates with the proton῏transfer products NuH῍
NHClῌ. In fact, proton transfer was observed for
C3H7Oῌ and more basic anions, leading to the assignment of the deprotonation enthalpy of chloramine90) as
374.4῎3.0 kcal molῌ1. Nucleophilic substitution wasobserved for all reactant anions via the transition state
9 producing NuNH2 ῍ Clῌ. The SN2 reactions are
faster than the corresponding SN2 reactions of methyl
chloride; the chloramine reactions take place at nearly
every collision when the reaction is exothermic. Most
dramatically, the symmetric SN2 reaction of 37Clῌ῍
H2N35Cl is found to be more than two orders of magnitude faster than the carbon-center analog of 37Clῌ῍
CH335Cl (Eq. (12)). There has been controversy on theSN2 barrier height DEῌovr at nitrogen relative to that at
carbon. The signiﬁcantly enhanced SN2 reactivity of

Termolecular association of methoxide and water assisted by collision with He.
῎ 192 ῎Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT Technique

Scheme 5.

Competitive proton transfer and SN2 reactions of chloramine with a nucleophile Nuῌ.

NH2Cl supports a high-level ab initio theoreticalprediction91) that overall barrier heights for identity
SN2 reactions Xῌ῍NH2X are negative and lower thanfor carbonῐcenter SN2 reactions Xῌ῍CH3X.
5.2 ECO2 elimination reactions
For larger reactants such as propyl halides reactingwith nucleophile anions, elimination at the b-carbon(E2) can compete with nucleophilic substitution at the
a-carbon (Eqs. (20) versus (21)).
SN2

῍῎CH3CH2CH2Nu῍XῌScheme 6.

(20)

CH3CH2CH2X῍Nuῌ

Competitive proton transfer and ECO2
reactions of methyl hydroperoxide withﬂuoride.

E2

῍῎CH3CH῎CH2῍NuH῍Xῌ (21)These two pathways yield the same halide anion Xῌ
and hence cannot be distinguished directly by production analysis. Relative importance of SN2 versus E2channels was deduced from systematic measurementsof rate constants for a-branched alkyl halides.92) It was
concluded that the SN2 channel dominates with sulfur
anions, whereas the reaction proceeds mainly via theE2 channel with oxo anions.92) Deuterium KIEs havealso been measured using the SIFT technique to di#erentiate the two mechanisms.93) More recently an ingenious method has been devised that uses dianions as
attacking nuculeophiles,94) thereby leaving a negative
charge on the (otherwise) neutral product and allowing
for its mass spectrometric detection.
We recently used the SIFT technique to study adi#erent type of elimination reactions (“ECO2”), in
which alkyl hydroperoxides react with gas phaseanions to form C῎O double bonds in the product.95) In
the rapid reaction of methyl hydroperoxide with ﬂuoride (49῏ e$ciency with respect to the collision rate,
Scheme 6), formation of hydroxide via complex 11 is
the major pathway (ῌ85῏) whereas slightly endothermic proton transfer via complex 10 is minor (ῌ10῏). Nucleophilic substitution at the methyl group toform HOOῌ is endothermic and is not observed. The
ECO2 reaction proceeds via anionic attack on an a-hydrogen followed by concerted elimination of formaldehyde and HOῌ. The mechanism is substantiated byobserving HOῌ as the major product from deuterated
reactant CD3OOH. The importance of the a-hydrogenis demonstrated by the formation of hydroxide from
CH3OOH and CH3CH2OOH and not from (CH3)3COOH.95)
Energy and structure calculations support the mechanism and high e$ciency for the ECO2 reaction (Fig. 7).

The CH3OOH῍Fῌ reaction proceeds via initial formation of complex 11 with the ﬂuoride associated with
one of the a-hydrogens, then surmounting the key
transition state TS1 which is nearly 15 kcal molῌ1lower in energy than the reactants. Consistent withthe concerted nature of the reaction, the ECO2 transition state is an antiperiplanar structure with CῐH andOῐO bonds elongated by about 0.2 Å relative to free
CH3OOH while the CῐO bond is contracting.95) The
reverse reaction, CH3OOῌ ῍ HF, is found to proceed atnearly every collision and yields HOῌ as the major
primary reaction product (ῌ80῏), despite the possibility for exothermic proton transfer. This is also totallyconsistent with the computation that the reactants canfollow the same pathway as above after the initial
complex 10 has surmounted the transition state TS2(Fig. 7).
The gas phase ion study has unambiguously revealed the mechanism and importance of base-mediateddecomposition of alkyl hydroperoxides, which has
been controversial in solution.96), 97) These results also
bear biological implications associated with degradation of lipid hydroperoxides producing genotoxic aldehydes and ketones.98), 99) In contrast to ﬂuoride, aweaker base such as NCῌ cannot induce the ECO2 reaction with CH3OOH.95) However, our preliminary calculations suggest that as the acidity of the a-hydrogen
increases, weak bases such as cyanide and chloride
may facilitate ECO2 decomposition. Those peroxides
include allyl hydroperoxide (CH2῎CHCH2OOH) andconjugated diene hydroperoxides, good models for endogenous biological lipid hydroperoxides.

῏ 193 ῏

S. Kato

Fig. 7.

Relative energies (kcal molῌ1) on the [Fῌ, CH3OOH] potential energy surface (electronic ῍ ZPE). Data taken from
Blanksby et al. 95) The inset shows the structure of the ECO2 transition state, TS1.

We also studied collision-induced dissociation of the[M῎H]ῌ anions of alkyl hydroperoxides, CH3OOῌ,CH3CH2OOῌ, and (CH3)3COOῌ.100) Facile decompositionof ethyl- and t-butyl peroxide anions to produce HOOῌ
was observed. Dehydration of peroxide anions was
also observed but it is a minor process occurring onlyat high collision energies.100) This is in contrast to the
CID of the [M῎H]ῌ anions of lipid hydroperoxides, inwhich loss of water is the key initial step in the decomposition.101) Combining the results of deuterium labeling,101) we propose that decomposition of lipid peroxideanions actually proceeds via an intramolecular ECO2
Incorporation of a powerful elecmechanism.100)
trospray ionization (ESI) source in the ﬂow tube7) is
expected to allow studies of large peroxides with the
SIFT setup. We are currently developing those ESI
sources.
6. Structure and Thermochemistry ofEnergetic and Unstable Anions
6.1 Peroxide anions
Hydroperoxyl (HOO) and alkyl peroxyl (ROO) radicals are key intermediates in combustion. HOO radicals also play a vital role in oxidation of volatile organic compounds in atmosphere. ROO radicals are usually
the ﬁrst oxidation products of hydrocarbons in troposphere, resulting in ozone formation through photochemical cycles. Heats of formation of the peroxyl
radicals are crucial in modeling these processes, andyet the experimental values have been controversial
and largely unestablished (these aspects reviewed anddiscussed in Blanksby et al.102) and Ramond et al.103)).
To accurately determine the peroxyl radical heats offormation, we used the acidity/electron a$nity thermochemical cycle104) which combines the gas phase
acidity measurements with SIFT and electron a$nity
measurements with negative ion photoelectron spec-

troscopy (NIPES).102), 103) The gas phase acidity of alkylhydroperoxide, DacidG298(ROOH), is determined at room
temperature by measuring the forward (kf) and reverse(kr) rate constants for the proton῎transfer equilibrium
between the peroxide anion and a reference acid (HA)
(Eq. (22)).
kf

ROOῌ῍HA ῌ ROOH῍Aῌ

(22)

kr

The equilibrium constant Kequil (῏ kf/kr) is connectedto DacidG298(ROOH) through a known acidity value for
HA (Eq. (23)). The enthalpy of deprotonation, DacidH298(ROOH), is then derived using computational entropies.
DrxnG298῏DacidG298(HA)῎DacidG298(ROO῎H)
῏῎RT ln Kequil(23)

The electron a$nity of the peroxyl radical, EA(ROO),
is measured by NIPES of the ROOῌ anion. The measured deprotonation enthalpy and electron a$nity,
along with the ionization energy of hydrogen IE(H),a#ord the bond dissociation enthalpy, DH298 (ROO῎H)(Eq. (24)). The thermal correction term arises from thefact that EA(ROO) and IE(H) are measured at 0 K. The
correction is usually very small (ῐ0.3 kcal molῌ1) andcan be neglected,104) or computed from the relevant
heat capacities. The 0 K bond dissociation energy, DH0(ROO῎H), is also calculated from the 298 K bond enthalpy following similar thermal corrections.
DH298(ROO῎H)῏DacidH298(ROO῎H)῎IE(H)῍EA(ROOῌ)῍(thermal correction)(24)

Finally, the heat of formation for ROO is obtained
from the bond enthalpy and known heats of formation
for hydrogen and ROOH (Eq. (25)), without the need for
relying on thermochemistry of other intermediate species.

῍ 194 ῍Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT Technique

Table 1.

a)Experimental Deprotonation Enthalpies, Electron A$nities, Bond Dissociation Energies, and Heats of Formation for
Alkyl Peroxyl Radicals, ROO, and Hydroperoxides, ROOHa)
R

DacidH298(ROOH)(kcal molῌ1)

EA(ROO)(eV)

DH0(ROO῎H)(kcal molῌ1)

DfH298(ROO)(kcal molῌ1)

H
CH3
CH3CH2(CH3)3C

376.5 ῏ 0.4
374.6 ῏ 1.0
371.0 ῏ 2.2
370.2 ῏ 2.1

1.078 ῏ 0.006
1.161 ῏ 0.005
1.186 ῏ 0.004
1.196 ῏ 0.011

86.6 ῏ 0.5
86.9 ῏ 1.0
83.9 ῏ 2.2
83.3 ῏ 2.1

3.2 ῏ 0.5
4.8 ῏ 1.2
ῌ 6.8 ῏ 2.3
ῌ 25.2 ῏ 2.5

Ramond et al.103) for R ῐ H and Blanksby et al.102) for R ῐ CH3, CH3CH2, and (CH3)3C.

DfH298(ROOῌ)ῐDH298(ROO῎H)῎DfH298(H)῍DfH298 (ROOH)(25)

Table 1 summarizes the thermochemical parametersfor HOO, CH3OO, CH3CH2OO, and (CH3)3COO radicals
determined to within an accuracy of a few kcal molῌ1or better. The uncertainties in bond dissociation energies and heats of formation arise primarily fromthose in the measured heats of deprotonation. It isnoted that the SIFT measurement of the relative acidity (DrxnG298) is very accurate to within a fraction of
kcal molῌ1, and the uncertainty in DacidH298(ROOH) isalmost entirely due to the error bars in the reported gas
phase acidities for the reference acids (Eq. (23)). The
heat of formation for HOO is nearly 3 kcal molῌ1higher103) than the previous recommended value 105) of
0.5῏2.0 kcal molῌ1, but is consistent with several
recent values obtained using di#erent techniques (reviewed in Ramond et al.103)). The measured heats offormation for the alkyl peroxyl radicals 102) also agreewith several previous reports, settling the controversy
in the thermochemistry of these reactive species.
6.2 Nitrogen-containing anions
Compounds containing many nitrogen atoms or nitrogen substituents (e.g., ῎NO2 and ῎N3 groups) areenergetic species. Anions and radicals of those species
are also considered important intermediates in combustion. The SIFT technique has been employed to
study, through an anionic approach, the structure andthermochemistry of diazocarbene (CNN) and the diazomethyl radical (HCNN),106) small nitroalkanes,107) and aseries of ﬁve-membered azole compounds.108)
HCNNῌ and CNNῌῌ anions can be generated, forexample, by HOῌ deprotonation and Oῌῌ dehydrogena-

Scheme 8.

tion of diazomethane, respectively (Scheme 7). Proton
a$nities of these anions were determined by protontransfer equilibrium (for HCNNῌ) and acidity bracketing (for CNNῌῌ) measurements, to be 372 ῏ 2 and 352
῏ 4 kcal molῌ1, respectively.106) It is noted that only
acidity bracketing is feasible with CNNῌῌ because thereverse reaction requires the HCNN radical reactant.
Photodetachment of HCNNῌ and CNNῌῌ gives accessto [HCNN] and [CNN], which are conjectured to be
combustion intermediates that provide low energy
paths to cleave N2 to produce N atoms (Scheme 7). The
nitrogen atoms are rapidly oxidized to nitric oxide, amajor source driving urban air pollution. Electron
a$nities of HCNN and CNN were measured by NIPES
of these anions. Given the heat of formation of diazomethane, heats of formation for the HCNN radical and
CNN carbene were accurately determined. It turns outthat the HCNN intermediate provides a deep well of῎31῏3 kcal molῌ1 relative to the CH῍N2 reactants in

Scheme 7.

Formation of HCNN and CNN intermediates
using gas phase ion schemes (solid arrows),
along with formation and subsequentdissociation in combustion (broken arrows).

Competitive proton transfer and E2 elimination reactions of nitroalkane, and the chemical reactivity of the
deprotonated nitroalkane anion.
῍ 195 ῍

S. Kato

the spin-forbidden CH(2P) ῍ N2 ῎ HCN῍N(4S) reaction, which is slightly endothermic, DrxnH0ῑ1.6῏0.7
kcal molῌ1. The detailed mechanism of how the doublet/quartet crossing occurs within the deep potential
well remains an intriguing question.
Nitro compounds are prototypical propellants andexplosives, as exempliﬁed by high-performance explosives trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5triazacyclohexane (RDX). Apart from acidity determinations, reactivity studies of nitroalkanes and theiranions are relatively few in the gas phase. We areespecially interested in the stability and reactivity of
deprotonated nitroalkanes (aci-anions), which havebeen considered a key intermediate in the condensed
phase, base-sensitized detonation of nitroalkanes.
109)ῐ111)
Small nitroalkanes (nitromethane, nitroethane,
2-nitropropane, 2-methyl-2-nitropropane, and nitrocyclopropane) were allowed to react with gas phase bases
HOῌ, CH3Oῌ, and HOOῌ. We ﬁnd the reactions toproceed very rapidly, k ῑ 2.5῎4.3ῐ10ῌ9 cm3 sῌ1, primarily producing [M῎H]ῌ anions via proton transfer(where an a-hydrogen is available) as well as NO2ῌ via
the E2 pathway (where a b-hydrogen is available).107)
SN2 formation of NO2ῌ is ruled out because NO2ῌ is notobserved from CH3NO2. The acyclic nitroalkanes are
similarly very acidic because of resonance stabilization
involving the aci form 14 for the deprotonated anions,in addition to the strong inductive e#ect by the nitro
group.112) The anomalously low acidity for nitrocyclopropane, which has the RῐRῑ bonding, is explained by
the destabilizing strain present in the aci form.
Reactivity studies revealed the importance of the aci
form in the [M῎H]ῌ anions of acyclic nitroalkanes.107)
In the SN2 reactions of [M῎H]ῌ anions with CH3I, thee$ciencies do not decrease but rather gradually increase with larger substituents R and Rῑ (Scheme 8).

Fig. 8.

This is an interesting result because the larger attacking anions have greater crowdedness67) around the deprotonated carbon atom and hence are expected todisplay a reduced SN2 reactivity. The observation isrationalized if the nucleophilic attack is actually madeby the oxo anion in the aci form (Scheme 8). The major
products CS2O῍ῌ and SO2῍ῌ observed from the reactions with CS2 and SO2 , respectively, are also consistent with the aci form being the important resonant
structure. The [M῎H]ῌ anion of nitrocyclopropane
displays di#erent SN2 and bimolecular reactivities thando acyclic anions, which is explained by the reduced
contribution of the aci form in the cyclic anion.107)
Polynitrogen compounds have been of both fundamental and practical interests.113) In theory those species can derive from isoelectronic substitution of N
atoms for CH units; however, polynitrogen compounds
are considerably less stable because of the nitrogenlone pair repulsion that is introduced. Only a handfulof homonuclear polynitrogen species has been discovered,113) e.g., azide (N3ῌ). This, in turn, indicates thatthese species are promising high energy density materials (HEDM). For example, ﬁve-membered rings pentazole (HN5) and the pentazolide anion (N5ῌ) have been
predicted to be metastable.114), 115) In fact, the N5ῌ anion(DHfῑ62῏4 kcal molῌ1)114) has recently been generated in the gas phase by CID116) and laser ablation117) ofappropriate precursor molecules containing N5 units.
N5ῌ has also been formed as a zinc salt in a ῎40ῒsolution.118)
Despite the observation of N5ῌ anions, experimental
characterization of N5ῌ and its derivatives has been
extremely di$cult. We have taken a di#erent approach in which we study deprotonated anions oflower azoles in the gas phase by measuring the proton
a$nity (PA) and electron binding energy (EBE), then

Pentazole (HN5), pentazolide anion (N5ῌ), pentazolyl radical (N5ῌ), and lower azolide anions (N1ῐN4).
῏ 196 ῏Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT Techniqueextrapolate the results to the pentazole family (Fig. 8).
CID threshold energies and fragmentation patterns canalso be examined for these lower azolide anions inorder to evaluate the pentazolide stability against
metastable decomposition: N5῍῍N3῍ῌN2. We havegenerated all azolide anions in Fig. 8. While this is arelatively new project with most of the work currently
in progress, our ﬁrst experiments on the pyrrolide
anion (C4H4N῍)108) have determined its proton a$nity
and electron binding energy. The anion is very stable(EBE῎2.145῍0.010 eV), and strong vibronic couplingis indicated between the ground and ﬁrst excited states
for the detached pyrrolyl radical.108) The SIFT instrument is also being used for CID studies of these azolide
anions. Preliminary experiments indicate an increasing trend for EBE and decreasing trends for DacidH298
and the CID threshold as the heterocyclic ring incorporates more nitrogen atoms.
6.3 Cyclooctatetraene and cyclopentanone
Conformational changes in 1,3,5,7-cyclooctatetraene(COT, C8H8) and its substituted derivatives have been asubject of considerable research interest for several
decades; tub-like COT (D2d) undergoes ring inversion
via a planar D8h structure.119), 120) Relatively little is
known, however, about the thermochemical propertiesof COT and, in particular, its radical and anionic derivatives. We generated anionic derivatives of COT using
electron impact, HO῍ deprotonation, and the Oῌ῍ reaction to abstract H2ῌ (Scheme 9). Determination of theadiabatic electron a$nity (EA) of COT has been challenging for many years because of the substantial conformational change (D2d ῍ D4h) upon electron attachment [COTῌe ῍ COTῌ῍(1)]. In fact, photodetachmentof COTῌ῍ has been shown to leave COT in a planartransition state for the ring inversion,121) with noFranck῏Condon overlap with the neutral vibrational
ground state. We measured both the electron῏transfer
equilibrium and forward/reverse kinetic rates for the
COTῌO2ῌ῍ ῌ COTῌ῍ῌO2 system. Under a truly thermal condition, the adiabatic EA of COT was determined unambiguously as 0.55 ῍ 0.02 eV,122) in excel-

Scheme 9.

lent agreement with later measurements using the kinetic method123) and thermal-energy electron attachment/detachment,124) and with high-level theoretical
calculations.125)
The radical anion of cyclooctatetraene [COTῌ῍(1)] is
one of the ﬁrst open-shell annulenes observed experimentally. The gas phase proton a$nity for COTῌ῍(1),along with those for C8H7῍(2a) and C8H6ῌ῍(3), was alsomeasured. Combining the values of proton a$nities
with electron binding energies measured using photoelectron spectroscopy, we determined the C῏H bonddissociation energies of COT and its derivatives, C8Hn(n῎7 ῏ 10).122), 126) Photoelectron spectroscopy and molecular orbital calculations revealed that the C8H7῍(2a)anion is a novel structure incorporating a p-electronicsystem and a bent allenic moiety within the eightmembered ring.122)
The large C8 ring systems undergo unexpected transformations upon collisional excitation. When SIFTinjected at high collision energies, COTῌ῍(1) and C8H6ῌ῍(3) radical anions appear to detach an electron for
signal loss. Remarkably, the C8H7῍(2a) anion undergoes a collision-induced isomerization to C8H7῍(2b) via
transannular bridging (Scheme 9),126) instead of usual
ring opening or fragmentation. Measured chemical
reactivity and molecular orbital calculations characterized C8H7῍(2b) as a fairly stable bicyclic species (DHf ῎
33 kcal mol῍1) with EBE and PA very similar to thosefor cyclopentadienide anion. Duplication of this transannular bond formation process in solution would provide entry into bicyclic ring systems that pose signiﬁcant synthetic challenge.
At even higher SIFT injection energies the C8H7῍(2b)product is replaced by a C8H6ῌ῍(g) species, apparently
following loss of a hydrogen atom. This species exhibits distinctly di#erent chemical reactivities from theisomeric C8H6ῌ῍(3). If it retains the skeletal structure of
C8H7῍(2b), this species may be the radical anion ofpentalene, a prototypical Hückel antiaromatic molecule which eluded chemists for several decades.127)
With appropriate tuning of the injection energy, theCyclooctatetraene, its anionic derivatives, and their collision-induced transformations.
῎ 197 ῎

S. Kato

Scheme 10.

Anions formed from the reaction of cyclopentanone with the oxygen radical anion.

C8H7῍(2a) anion is completely converted to C8H7῍(2b)with minimal fragmentation to C8H6῍῍(g). The C8H7῍(2b) anion can then be subjected to CID under a single
collision condition with the triple quadrupole (Fig. 1).
Interestingly, formation of C8H6῍῍(g) has been very
di$cult to observe under experimental conditions
where similar m/z ions are easily being detected. This
result may suggest that C8H6῍῍(g) can only be stabilized
in the high-pressure collisional environment in the ﬂow
tube. Pressure-dependent product branching has been
observed by a separate group.128)
As illustrated above, the atomic oxygen radical
anion (O῍῍) reacts with many organic compounds,
formally abstracting H2ῌ and producing a variety ofcarbene and distonic radical anions129), 130) as well as
anions with bent triple bonds.131) We have shown thatO῍῍ reacts with cyclopentanone to form a mixture ofthree C5H6O῍ isomers (Scheme 10).132) The major product cyclopentanone-2,5-diyl radical anion (16) is thermodynamically most stable. Photodetachment of thisspecies will determine the singlet-triplet splitting of the
2,5-diyl radical, an ethano-bridged analog of oxyallyl
which has been postulated as an intermediate in manyorganic reactions. NIPES experiments are under way
in the laboratory of Prof. W. C. Lineberger.
7.

New Directions

7.1 Reactions of ions with organic radicals
Thousands of ion reactions have been studied withstable neutrals and with some atomic radicals in thegas phase.40) In contrast, reactions of ions with polyatomic organic radicals have rarely been studied because of the inherent experimental di$culties. The
fundamental chemistry of ionῑpolyatomic radical reactions, where the charge and radical sites are split between the collision partners, is thus largely unexplored.
Ionῑorganic radical reactions are expected to be important in several environments, including in combustion
processes where both positive and negative ions havebeen identiﬁed133) at densities of 108῎1010 cm῍3. In theinterstellar medium more than a hundred organic species have been detected, among them about one-third
are radicals or carbenes and several are ions.134) Undervery low temperature conditions in space, barrierlessionῑradical association and reactions may play a signiﬁcant role for the formation and evolution of thoseinterstellar species.
Investigations of ionῑorganic radical reactions arechallenging primarily because of the di$culty in generating the radicals cleanly and in high densities. This
contrasts with generation of atomic radicals, which canbe achieved relatively easily by dissociation of theirprecursors in a microwave discharge or over a heatedmetal surface. These atomic sources have been combined with the SIFT135), 136) and Fourier transform ion

cyclotron resonance (FT-ICR)137) instruments to study
reactions of H, N, and O atoms.
For polyatomic radicals, chemical ionization mass
spectrometry (CIMS) has begun to use some ionῑradical
reactions to probe the atmospheric chemistry of alkyl
peroxyl (ROOῌ)138)ῑ144) and peroxyacetyl (CH3C(O)OOῌ)radicals.145) For example, when ROOῌ radicals areundergoing neutral radical chemistry with NO, theirconcentration changes are monitored by using chemical ionization with O2ῌ, O2῍, or H3Oῌ(H2O)n . The alkyl
peroxyl radicals are prepared by addition of O2 to thecorresponding alkyl radicals, which have been generated from suitable precursor molecules by pyrolysis,
low-power radio-frequency discharge, or atom reactions.138)ῑ144)
Besides those CIMS applications, a versatile method
for polyatomic radical generation is expected to open a
fertile research ﬁeld of ionῑradical chemistry. We have
employed a new approach that combines a supersonicpyrolysis nozzle146) with the SIFT instrument.147), 148)
Thermal decomposition of a variety of organic compounds in a pulsed heated nozzle (300ῑ1800 K) produces clean and intense beams of organic radicals or
diradicals.149) The high temperature and short residence time in the nozzle (῎30 ms) ensures clean pyrolysis of the precursor molecules while retaining highconversion e$ciencies (60ῑ90῏). The produced radicals are pulsed (῎1.3 ms in duration, 20ῑ40 Hz) into thehigh-pressure SIFT ﬂow tube, where both the ions andradicals are quickly thermally equilibrated before thereaction initiates (ῌ300 K). The ions and radicals arethen contained for a moderately long reaction time ofabout 10 ms. While the ionῑradical reaction products
can easily be detected and quantiﬁed, depletion of thereactant ion is too small to be measured directly underthe experimental conditions. However, a simple treatment of pulsed kinetics allows evaluation of the reaction rate constants based on the product yields.147), 148)
A simple radical allyl (C3H5) and a diradical orthobenzyne (o-C6H4) have been generated by pyrolysis ofallyl iodide (C3H5I῍D ῏ C3H5῍I) and benzoyl chloride(C6H5COCl῍D ῏ o-C6H4῍CO῍Cl), respectively, where
D represents heat. The strained triple bond in obenzyne carries a triplet diradical character, which
renders the bond highly reactive. These radicals encounter hydronium (H3Oῌ) or hydroxide ions (HO῍) inthe ﬂow tube.147), 148) The allyl radical and o-benzyne
react with H3Oῌ via exothermic proton transfer (Eqs.
(26) and (28)) at nearly 100῏ e$ciencies relative to theion-radical collision rates. No charged products wereobserved from allyl ῍ HO῍ within detection limits (Eq.
(27)). The ionῑradical encounter might be essentiallynonreactive; the heat of deprotonation for the allyl
radical is unknown. Alternatively, this ﬁnding could
result from associative/reactive detachment pathways

ῐ 198 ῐ

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using SIFT Technique(e.g., CH2῎CHCH2ῌῌHO῍ ῎ CH2῎CHCH2OHῌe),which are highly exothermic. The reaction is thenanalogous to associative detachment of anions withatomic radicals, which is commonly very rapid.
40), 136), 150)

Scheme 11.

῍
CH2῎CH῍CH2ῌH3Oῌ ῎ C3H6ῌῌH2O(26)

῍
CH2῎CH῍CH2ῌHO῍ ῎ no product ions(27)

ῌH3Oῌ ῎ C6H5ῌῌH2O(28)

ῌHO῍ ῎ C6H3῍ῌH2O(29)

The reaction of o-benzyne with HO῍ shows an intriguing result. Although abstraction of the most acidic a
proton is exothermic by more than 10 kcal mol῍1, theobserved product C6H3῍ (Eq. (29)) accounts for only
about 15῏ of the collision. This is an unexpected
result because exothermic proton abstraction is usually a barrierless process (Fig. 3a) that takes place atevery collision. The observation may be rationalized
by competing associative detachment; the HO῍ ion
could add to the triple bond to form o-hydroxy phenyl
anion (o-C6H4OH῍), which could subsequently lose theelectron because of the large reaction exothermicity far
exceeding the binding energy.147), 148) Thus, this is a
novel fundamental reaction channel for HO῍ ions thatis facilitated by the high energy content in o-benzyne.
Future experiments include trapping of detached electrons with electron scavengers such as SF6. Other
radicals methyl and phenyl as well as diradical cyclopropenylidene (c-C3H2) are among the reactants thatcan be generated and studied using the SIFTsupersonic pyrolysis technique.
7.2 Chemical ionization mass spectrometry
Compared to other ionization methods such as electron impact or hollow cathode discharge, chemical ionization is a soft ionization that bestows charge to neutral molecules with minimal fragmentation.151) Chemical ionization mass spectrometry, which conveniently
uses proton transfer/abstraction reactions of H3Oῌ or
HO῍ reagent ions in ﬂow-drift tube reactors,152), 153) has
become increasingly popular for highly sensitive, online detection of a variety of volatile organic compounds (VOCs). We utilized H3Oῌ proton transfer reactions to measure aldehydes from human cancer
cells154)ῐ156) and VOCs from fermenting bacteria.157) We
also demonstrated that negative ion chemistry withHO῍ is an alternative tool to monitor plant emission of
VOCs158)ῐ160) and to detect small, volatile nitroalkane
explosives.107) These analytical applications and bio/
medico-chemical implications, as well as ionῐmolecule
chemistry of interstellar relevance, will be discussed
elsewhere.161)
A new reaction scheme of CIMS interest is given
here. While reactions with H3Oῌ and HO῍ are facileand versatile in converting a majority of organic species to respective [M ῌ H]ῌ and [M῍H]῍ product ions,
identiﬁcation of isobaric products is usually a di$culttask. CID and H/D exchange reactions may be used todi#erentiate some product isomers, for example, deprotonated acetone versus propanal.157) Another approach

Gas phase reaction of hydrazinium ionwith carbonyl to form protonatedhydrazone.

to structure identiﬁcation is to devise a CI scheme withchemical speciﬁcity. In analogy to acid-catalyzed, solution phase chemistry of hydrazine and its derivatives,
we have shown that reactions of protonated hydrazine(N2H5ῌ) can be used to speciﬁcally detect carbonylcompounds in the gas phase.162), 163) N2H5ῌ reacts witha carbonyl compound and forms a characteristic product, protonated hydrazone (Scheme 11). This reaction,
while exothermic, is slow and observed only when
excess neutral N2H4 molecules are present.163) Further,formation of hydrazone is dramatically enhanced athigh ion kinetic energies in the drift ﬁeld.162) Additional experiments using deuterated N2D4 and density
functional calculations163) have identiﬁed the origin of
the slowness of the reaction: Transition states for thebimolecular reaction (Scheme 11) present kinetic andenergetic barriers that can be overcome by translational and/or chemical activation. The energies forthe strained transition states are likely to be lowered in
the presence of a second N2H4 molecule within theintermediate complex.163) The fundamental studies
have provided an important guideline for this CIMS
scheme to be widely used for future applications.
8.

Conclusions

Various aspects of fundamental gas phase ion chemistry have been discussed. The SIFT technique is aversatile tool with which to study the kinetics andmechanisms of ion reactions in a very clean, truly
thermal environment. This feature has been fully utilized, for example, to reveal nonstatistical behaviors in
SN2 kinetics and to prove otherwise controversial ECO2
reactions of biological interest, as well as address classical questions in organic chemistry and explore newschemes for CIMS applications. By incorporating LIF
and triple quadrupole detections and through a powerful coupling with NIPES experiments, the SIFT technique has continued to provide detailed information on
ion structure, energetics, and dynamics. These results
have stimulated theoretical interest as well. We haveshown that a hyperthermal radical source can be usedwith the SIFT instrument for studies of ion-polyatomic
radical reactions. ESI sources are also expected to be
useful for loading ions of nonvolatile species in the
ﬂow tube. These technical developments will further
extend the research horizon with the SIFT technique.
Acknowledgments
The author acknowledges Prof. Stephen R. Leone for
introduction to the ﬁeld of gas-phase ion chemical dynamics as studied by SIFT-LIF, and Profs. Venonica M.
Bierbaum and Charles H. DePuy during the subsequent
thermochemical and physical organic studies using
more classical SIFT designs. The author thanks Profs.
W. Carl Lineberger, G. Barney Ellison, Stephen J. Blank-

῏ 199 ῏

S. Kato

sby, Weston Thatcher Borden, and Dr. Carleton J.
Howard, among others, for the work in this article.
Invitation and encouragement from Prof. Kenzo Hiraoka to write this article is gratefully appreciated. The
work was supported by the National Science Foundation (CHE-0349937).

27)
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Keywords: Gas phase ion chemistry, Selected ion ﬂow tube,
Vibrationally state-selected chemistry, Organic reaction
mechanism, Thermochemistryῌ 202 ῌ