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Cite this: Phys. Chem. Chem. Phys.,2022, 24, 5774
Received 17th December 2021,
Accepted 7th February 2022
DOI: 10.1039/d1cp05762f

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Collision-assisted stripping for determinationof microsolvation-dependent protonation sites
in hydrated clusters by cryogenic ion trap infrared
spectroscopy: the case of benzocaineH+(H2O)n†‡
Keisuke Hirata, abc Fuad Haddad,d Otto Dopfer,and Masaaki Fujii *ace

*cd Shun-ichi Ishiuchi*abc

rsc.li/pccp

The protonation site of molecules can be varied by their surrounding
environment. Gas-phase studies, including the popular techniques ofinfrared spectroscopy and ion mobility spectrometry, are a powerfultool for the determination of protonation sites in solvated clusters but
often suﬀer from inherent limits for larger hydrated clusters. Here, we
present collision-assisted stripping infrared (CAS-IR) spectroscopy as a
new technique to overcome these problems and apply it in a proof-ofprinciple experiment to hydrated clusters of protonated benzocaine(H+BC), which shows protonation-site switching depending on the
degree of hydration. The most stable protomer of H+BC in the gas
phase (O-protonated) is interconverted into its most stable protomer
in aqueous solution (N-protonated) upon hydration with three watermolecules. CAS-IR spectroscopy enables us to unambiguously assign
protonation sites and quantitatively determine the relative abundanceof various protomers.

Introduction
Protonation is a ubiquitous phenomenon in chemistry andbiology. The reliable determination of protonation states ishighly significant because protonation aﬀects the optical,
redox, and catalytic properties of molecules.1–5 For example,

pH indicators change their color by protonation. Protonation is
also a fundamental process in organic synthesis, and stereoselective synthesis can be achieved by controlling the protonationsite of a reaction intermediate.3,4 In general, the protonation site ofan organic molecule highly depends on the surrounding
solvent.6–22 This solvent-dependent behavior is particularly prominent for molecules with multiple basic sites. For example, benzocaine (BC), which has both carbonyl and amino groups, is moststable in its NH2-protonated form (N-protomer, H+BC(N)) in polar
solvents, whereas the CO-protonated form (O-protomer, H+BC(O))
is most stable in the gas phase (Fig. 1).15 The stability of the Nprotomer in a polar solvent can readily be rationalized by thehigher basicity of the NH2 group. On the other hand, the isolated Oprotomer is stabilized by the resonance effect, in which the lonepair of the N-atom is donated to the protonated O-atom via thearomatic phenyl ring.14,15 This solvent-dependent protonation-site
switching is not only found in BC but prevails in other (bio-)organic
molecules.16–33 Much effort has been devoted to microsolvatedclusters, in which an ion is solvated by a few to several hundreds ofmolecules,14,34–36 because the degree of solvation can be tuned bycontrolling the type and numbers of solvent molecules.
Gas-phase mass spectrometry and laser spectroscopy have thepotential to determine the protonation site in microsolvatedclusters. In particular, infrared photodissociation spectroscopyaLaboratory for Chemistry and Life Science, Institute of Innovative Research,
Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama,226-8503, Japan. E-mail: mfujii@res.titech.ac.jp, ishiuchi.s.aa@m.titech.ac.jpbDepartment of Chemistry, School of Science, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japanc
Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovation
Research, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, 4259,
Yokohama, 226-8503, Japan. E-mail: dopfer@physik.tu-berlin.dedInstitut für Optik und Atomare Physik, Technische Universität Berlin,
Hardenbergstrasse 36, Berlin 10623, GermanyeSchool of Life Science and Technology, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa, 226-8503, Japan† Submitted for publication to PCCP on 17 December 2021.
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cp05762f

5774 | Phys. Chem. Chem. Phys., 2022, 24, 5774–5779

Fig. 1 Molecular structures of N- and O-protomers of H+BC, H+BC(N)and H+BC(O).

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Communication(IRPD) and ion mobility mass spectrometry (IMS) have frequently been applied to assign protomers of desolvatedions.8–10,12–21,23–27,30,32–35 For example, the protomers of isolated
H+BC could clearly be separated by both IRPD and IMS.15 In
principle, IMS can separate isomers based on the collision crosssection (CCS) with inert buffer gas, which strongly depends on
charge distribution and shape of the molecule. For H+BC, thedifference in the structures of the two protomers of BC is too
small to be detected by their CCS with He at the currently available
resolution. Instead, the protomers were separated by their different dipole moments because the more polar protomer travels
slower through a drift cell due to its stronger interaction with thepolarizable buffer gas.15 Although this method has successfully
been applied to other bare ions, such as protonated p-aminobenzoic acid (H+PABA), it has not been extended to solvatedclusters so far. In fact, the protomers of hydrated clusters of
H+PABA could not be distinguished by their dipole moment.34
This may be because the difference in dipole moment is‘‘masked’’ by surrounding water molecules. IRPD spectroscopy
is an alternative approach to probe the protonation site. IRPD
spectra of H+PABA(H2O)n clusters exhibit intense and broad NH
stretch bands in the 2950–3300 cm1 range at n = 6, whichsuggests the appearance of a new protomer.14 However, theauthors mention that the new protomer may also be alreadypopulated for n o 6 because of the minor contribution of
unassigned bands in the same spectral range. The ambiguity inthe band assignment is ascribed to overlapping vibrational bands
arising from the hydration network. In conclusion, both IMS and
IRPD are not generally applicable for the reliable determination of
the protonation site of organic molecules in a microsolvation
environment due to lack of sensitivity, resolution and/or spectralcongestion.
To overcome the deficiencies of IMS and IRPD for microsolvated clusters, we developed a new technique denoted
collision-assisted stripping (CAS). In this approach, we determine the protonation site of microsolvated clusters by collisionally removing all solvent molecules in a cryogenic ion trap
and probing the resulting protomer by IR spectroscopy. We
validate herein the new, simple, reliable, and generally applicable method for the prototypical example of H+BC(H2O)n. As
mentioned above, H+BC(O) is most stable in the gas phase(n = 0), whereas H+BC(N) is dominant in aqueous solution(Fig. 1),15 i.e., H+BC(O) must be converted to H+BC(N) at a
critical size of the hydration shell (n Z nc). We demonstratethat IRPD spectra measured for bare H+BC generated by CAS of
H+BC(H2O)n can reliably determine the protonation sites ofhydrated clusters as a function of n. CAS-IRPD spectroscopy isshown to be a new methodology to probe the protonationsite in a more quantitative way, with special focus on
microhydration-induced intracluster proton transfer.

Experimental
In a proof-of-principle experiment, CAS-IRPD and conventional
IRPD spectra of H+BC(H2O)n clusters were measured in aThis journal is © the Owner Societies 2022

PCCP
cryogenic ion trap setup (Fig. S1, ESI‡).37–43 Details of theexperimental procedures are described in ESI.‡ In brief, H+BC
ions generated in an electrospray ionization source were introduced into a linear ion trap (LIT)44,45 kept at 170 K, where they
were hydrated by introducing water vapor. The hydrated clusters were mass-selected by a quadrupole and guided into acryogenic quadrupole ion trap (QIT) held at 4 K. For CAS-IRPD
spectroscopy, the injection energy of the ions was set highenough to induce water dissociation (eqn (1)) by collisions with
the H2/He buffer gas.
CASþHþ BCðH2 OÞn !
 H BCþnH2 O
@QIT

e 10K

(1)

Subsequently, H2 was attached to the water-stripped ions, andthe H2-tagged H+BC ions were irradiated with an IR laser pulse.
The H2-detached photofragments generated by resonant IR
absorption were monitored as a function of IR wavenumber.
The weak interaction with the H2 tag has essentially no impact
on both the structure and energetics of the probed ions.46,47 For
conventional IRPD of mass-selected H+BC(H2O)n clusters, lossof a single H2O was monitored.

Results and discussion
Consistent with previous IRPD and IMS studies,15 all vibrational bands in the conventional IRPD spectrum of bare H+BC
shown in Fig. 2 are assigned to H+BC(O). Specifically, the bands
at 3441, 3541, and 3561 cm1 are ascribed to symmetric andantisymmetric NH stretches (nNHs/as(O)) and ionic OH stretch(nOH(O)), respectively, by comparison to computed IR spectra
for the most stable H2-tagged H+BC(O) and H+BC(N) conformers (Fig. S2 and S3, ESI‡). Conventional IRPD spectra of
H+BC(H2O)n measured for n = 1–6 are shown in Fig. 2 andFig. S4 (ESI‡). For the vibrational and isomer assignments,
density functional theory (DFT) calculations were performed at
the oB97X-D/6-311++G(d,p) level for all possible geometries inwhich water binds via a hydrogen bond (H-bond) to the acidic
NH and OH sites of H+BC. For n = 1 and 2, all observed
transitions are again assigned to the most stable clusters ofthe H+BC(O) protomer (Fig. S5–S8, ESI‡). For example, thebands at 3454 and 3555 cm1 for n = 1 are attributed to nNHs(O)and nNHas(O), while the bands at 3636 and 3721 cm1 arise fromfree symmetric (nOHs) and antisymmetric (nOHas) OH stretchesof water. The computed free energy gap (DG at 170 K) between
the most stable H+BC(O) and H+BC(N) isomers decreases as
34.8 4 27.3 4 11.6 kJ mol1 for n = 0–2, consistent with thelack of detection of the H+BC(N) isomers in this size range(Fig. 3). In the most stable BCH+(H2O)1 conformer, water is
H-bonded to the protonated OH site of H+BC(O) (Fig. 2 andFig. S5, S6, ESI‡). As a result, the broad band in the2900–3200 cm1 range can be ascribed to the strongly redshifted H-bonded protonated OH stretch (nOHHB(O)). The prominent ionic nNH(N) bands in the 3300–3400 cm1 range
predicted for the most stable monohydrate of H+BC(N) areclearly absent in the IRPD spectra (Fig. S5, ESI‡). The spectral

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Fig. 2 Conventional IRPD spectra of H+BC (H2O)n with n = 0, 1–3, and 6. The spectrum for n = 0 was recorded by H2-tagging. Spectra for n Z 1 wererecorded in the H2O loss channel. The red and blue-colored area correspond to vibrational bands derived from O- and N-protomers, respectively. The
question marks in the n = 6 spectrum indicate the problems of vibrational and isomer assignments arising from spectral congestion.

Fig. 3 Calculated Gibbs free energy for the most stable conformers of
H+BC(H2O)n at 170 K. Red and blue lines represent O- and N-protomers,
respectively (oB97X-D/6-311++G(d,p)).

pattern of H+BC(H2O)2 is similar to that of H+BC(H2O)1. The
intense bands observed at 3456 and 3558 cm1 are assigned
to nNHs(O) and nNHas(O) (Fig. S7, ESI‡), while the bands above
3600 cm1 and the broad band at B3300 cm1 arise from nOH
of water and ionic nOHHB(O).
In comparison to n r 2, new bands emerge in the lower
frequency range (2950–3140 cm1) of the IRPD spectrum of
H+BC(H2O)3, which can clearly be assigned to ionic nNHs(N)5776 | Phys. Chem. Chem. Phys., 2022, 24, 5774–5779

modes by comparison to calculations (Fig. S9, ESI‡). Together
with the fact that nNHs(O) (3445 cm1) and nNHas(O) (3552 cm1)
are observed, one can conclude both N- and O-protomers
coexist for H+BC(H2O)3. Indeed, the most stable trihydrated
H+BC(N) protomer is essentially isoenergetic with the moststable O-protomer (DG = +3.8 kJ mol1, Fig. 3 and Fig. S10,
ESI‡). The decrease in DG between H+BC(O) and H+BC(N) as afunction of n arises from better hydration of the NH3+ group of
H+BC(N) offering three proton donor sites to H2O compared to
the OH+ group of H+BC(O) offering only one proton donor site
for ionic H-bonding to H2O. As a result of progressive hydration, the IRPD spectra become substantially more congested
and the vibrational and isomer assignments become more
problematic starting from n = 3 (Fig. S4, ESI‡) and virtually
impossible for n = 6, concerning both the protonation site andthe structure of the hydration shell. This severe problem
illustrated here for H+BC(H2O)n is a general phenomenon forhydrated clusters of protonated (bio)organic molecules andprevents the reliable determination of the critical size nc forthe hydration-induced switch in the preferred protonation site,in particular for molecules with larger values of nc.
CAS-IRPD spectroscopy oﬀers a simple and general solution
to this problem, as shown in Fig. 4 for the case of H+BC(H2O)n.
The spectra are simple because the bands in the spectra are
only derived from desolvated H+BC (n = 0) and thus lack any
congestion. For example, the CAS-IRPD spectra of H+BC(H2O)1,2

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Fig. 4 (a) CAS-IRPD spectra of H+BC(H2O)n compared to (b) calculated IR stick spectra of the N- and O-protomers of H+BC (oB97X-D/6-311++G(d,p)).
The minor band marked by the asterisk is tentatively ascribed to an overtone/combination vibrational band of the O-protomer.

are similar to the IRPD spectrum of bare H+BC. The three bands
ascribed to H+BC(O) appear also in the CAS-IRPD spectra of
H+BC(H2O)1,2, indicating the predominant population of the
O-protomer in the mono- and dihydrate. On the contrary, thenew bands ascribed to the N-protomer are observed in the CASIRPD spectrum of H+BC(H2O)3 at 3232, 3280, and 3306 cm1and assigned to ionic NH stretches (nNH(N)). Thus, CAS-IRPD
clearly reveals the appearance of the N-protomer in
H+BC(H2O)3.
In addition to the reliable protomer assignment, CAS-IRPD
allows for the quantitative determination of their population
ratio. The latter was obtained by normalizing the band intensities using the calculated IR oscillator strengths of desolvated
H+BC (Fig. S11 and Table S1, ESI‡) assuming the same photofragmentation eﬃciency, and yields 60 : 40 in favor of the
N-protomer for n = 3 with an error of the order of 10%. The
relative abundance of N- and O-protomers was also deduced
from the conventional IRPD spectra (Fig. 2) using the calculated
IR oscillator strengths of O- and N-protomers in H+BC(H2O)3(Fig. S9, ESI‡). This results in a population ratio of B3 : 2 infavor of the N-protomer, which is consistent with CAS-IRPD andsuggests that intramolecular proton transfer hardly happens
during the CID process. It should be noted that the estimationof the populations from conventional IRPD spectra requires thefull assignments of the vibrational transitions in terms of thecoexisting isomers of clusters. In general, the number ofpossible isomers of solvated clusters increases dramaticallywith increasing number of solvent molecules, and furthermore,
H-bonding often causes broadening of the vibrational bands.
Thus, such analysis becomes very diﬃcult (or often impossible)
for the larger solvated clusters. This aspect highlights theThis journal is © the Owner Societies 2022

superior performance of CAS-IRPD for protomer identificationand quantification over conventional IRPD. Theoretical populations were calculated using the Boltzmann equation for thermal equilibrium (eqn (2)):

DG
P / exp RT(2)

where DG, R, T represent the Gibbs free energy of an isomer, thegas constant, and temperature (170 K), respectively (Table S1,
ESI‡). These population ratios of 0, 0, 0, and 13% for the
N-protomers predicted for n = 0 = 3 match well with theexperimental ones (0, 0, 0, 60%), supporting the absence of
the N-protomer for n r 2 and its substantial appearance forn = 3. The mismatch in the quantitative population ratio forn = 3 may arise from several factors and is here mainly
attributed to inaccurate computational free energies. For example, the relative population of N- and O-protomer can readily bevaried by the choice of DFT functional14 or computational
method (Table S2, ESI‡). Similar to the case of H+PABA(H2O)n,14
the oB97X-D functional is best to reproduce the experimental
results for H+BC(H2O)n. On the other hand, ab initio calculations at the MP2 level result in 99% dominance of the
N-protomer (Fig. S12 and Table S1, ESI‡). To this end, ourreliable experimental results provide a benchmark for highlevel conformational search on the Gibbs free energy surface,which is highly challenging for floppy systems such as hydrated
clusters.

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Conclusion
In summary, we demonstrate the superior performance of
CAS-IRPD over conventional IRPD and IMS for assigning protomers in hydrated clusters and quantifying their relative
abundance for the prototypical example of hydrated H+BC. As
for this molecule the switch in preferred protonation site
occurs at small degree of hydration (nc = 3), it was feasible tocompare IRPD with CAS-IRPD in a benchmark experiment.
CAS-IRPD will, however, develop its full potential in particularfor substantially larger hydrates, for which IRPD will notwork anymore. In fact, we also measured CAS-IRPD spectra
for n = 4–6 (Fig. S13, ESI‡), which clearly show the vibrationalsignatures of the N-protomer, in sharp contrast to the conventional IRPD spectra (Fig. 2). These spectra indicate that the
N-protomer becomes more abundant with increasing numberof water molecules. Moreover, CAS-IRPD can be applied not
only to fully desolvated species (n = 0) but also to partiallydissociated ones (n Z 1). In this way, one may efficiently
produce and probe higher-energy metastable species, whichappear only in larger hydrated clusters or in solution (e.g.,
N-protomer in this case) and are not readily produced directlyin the ion source. Although IRPD spectroscopy of fragment ionsgenerated by collision-induced dissociation has been applied tovarious systems,42,48,49 we extend herein the methodology tohydrated clusters to reliably determine the protonation site.
The cooling efficiency in the QIT is high enough to freeze theconformations of a molecule.45 Because intracluster protontransfer involves substantial reaction barriers, rapid coolingin the QIT leads to kinetic trapping of protomers. In principle,
CAS-IRPD spectroscopy is applicable to any solvated cluster freefrom the shortcomings of conventional IMS and IRPD techniques and provides an opportunity to address fundamental
questions related to the protonation site and its solvation effecteven in large solvated clusters.

Conflicts of interest
There are no conflicts of interest to declare.

Acknowledgements
This work was supported in part by KAKENHI (JP19K23624,
JP20K20446, JP20H00372, JP21H04674, and JP21K14585), theCore-to-Core program (JPJSCCA20210004) from Japan Society
for the Promotion of Science, research grant from World
Research Hub Initiative (WRHI) of Tokyo Institute of Technology, the Cooperative Research Program of the ‘‘Network Joint
Research Center for Materials and Devices’’ from the Ministryof Education, Culture, Sports, Science and Technology (MEXT),
Japan, and the RIKEN Pioneering Project, ‘‘Fundamental Principles Underlying the Hierarchy of Matter: a Comprehensive
Experimental Study’’. The computations were performed at the
Research Center for Computational Science, Okazaki, Japan.
M. F. is grateful for support from the Alexander von Humboldt
foundation. O. D. acknowledges support from Deutsche

5778 | Phys. Chem. Chem. Phys., 2022, 24, 5774–5779

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Forschungsgemeinschaft (DFG, DO 729/10) and the World
Research Hub Initiative (WRHI) of Tokyo Institute ofTechnology.

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