2A09
Proton-transfer from CH Bonds of IonizedTetrahydrofuran (THF) Clusters(Tohoku Univ.) ○Min Xie, Yoshiyuki Matsuda, Tomoya Endo, Asuka Fujii[Introduction]

Proton-transfer, which is one of the most fundamental reactions, has been

concerned intensively, because of its key role in a variety of biological and chemical processes. It
has been well-known that proton-donor abilities of cationic OH and NH bonds are higher thanthose of neutrals. Upon the ionization of clusters of protic molecules such as methanol, ammonia,and carboxylic acids, the barrier-less proton-transfer reactions have been identified from OH andNH bonds.[1] These results demonstrate strong acidities of cationic OH and NH.
Neutral CH bonds are ordinarily regarded as aprotic because of their low acidities. However,
likewise cationic OH and NH, high acidities of CH may be expected in the cationic state.
Tetrahydrofuran (THF) is composed only of the alkyl group and the oxygen atom which can acts
as a proton acceptor. Therefore, THF is a simple system to examine the proton donating abilityof CH and the possibility of proton-transfer from CH.
In this study, we have performed infrared (IR) spectroscopy of the THF monomer and dimercations, which are generated through the vacuum-ultraviolet (VUV) photoionization.
Theoretical calculations of geometry optimization, vibrational spectra, proton-transfer potential
energy curves, and natural bond orbital (NBO) analysis also have been performed for them. The
acidity of CH of cationic THF is discussed through comparison of these IR spectroscopic andtheoretical results.
[Experiments and Calculations] The IR spectra of the THF monomer and dimer cations wereobserved by IR predissociation spectroscopy of VUV-pumped ions (IRPDS-VUV-PI).[1] In thisspectroscopy, size-selective IR predissociation spectra of cations, which were generated with theVUV photoionization at 118 nm, are recorded by monitoring the size-selected ion signal intensityof the parent ion under investigation or the fragment ion by use of the time-of-flight mass
spectrometer.
The isomerization reaction of the ionized THF dimer was simulated by the global reaction
route mapping (GRRM) program [2] mounted on the Gaussian09 (G09) program. The structures,energies, and vibrational spectra were calculated with G09. The simulated IR spectra werecalculated at the UPBE1PBE/6-31+G(d) level with the scaling factor of 0.94.
[Results and Discussion]
Fig. 1 shows (a) the IR spectrum of the cationic THF monomer
obtained by monitoring the dehydrogenated THF cation channel, and (b) the calculated IR
spectrum of the THF cation. In the observed spectrum, the intense CH stretch band is seen at
2700 cm-1, which is out of the normal CH stretch frequency range. This band is assigned to thesymmetric stretch vibration of the CH bonds next to the oxygen atom. The extraordinarily low
frequency of the band is due to the weakening of the CH bonds by the delocalization of the σbonding electrons of CHs. This delocalization is caused by the hyperconjugation between the σorbital and the singly occupied
molecular orbital (SOMO) of the O
atom. The hyperconjugation increases
the acidities of the CH bonds.
Fig. 2 presents (a) the observed IR(a) [THF – H]+ monitor

spectrum of THF dimer cation and (b,c) the simulated spectra of the stablestructures of the THF dimer cations.
In the observed spectrum, the broadand intense feature appears from 2900
cm-1 to the lower frequency region.
This broad feature is assigned to the(b) calc. THF cation
Wavenumber /cm -1

Figure 1. (a) Observed and (b) calculated IR spectra of cationic THF andH-bonded OH stretch. Among isomers
its optimized structure.
of the THF dimer cation, two types ofthe most stable structures have H-bonded OH, as depicted in Fig. 2(b, c). One is the C···HO type,in which the C and O atoms intermolecularly share the proton, and the other is the O···HO type,in which the proton is located between the two O atoms. The spectral carrier of the observed
spectrum is assigned to them because
the broad and intense band is assigned
to the H-bonded OH of these structures.
Both of the structures are formed(a) IR spectrum of (THF)2 cation

through the intermolecular protontransfer from a CH bond of the ionized
THF. This result demonstrates that(b) C···HO type(2.54 kcal/mol)the proton-transfer from CH occurs in
the ionization of the THF dimer. Thus,
both the results of the THF monomerx 40(c) O···HO type(0 kcal/mol)x 80

and dimer cations indicate CH of THF
cation is highly acidic.

Wavenumber /cm -1

We have also performed theFigure 2. (a) Observed and (b, c) calculated IR spectra of the THF
intrinsic reaction coordinate (IRC)
dimer cation and its two optimized structures.
calculation for the proton-transfer
potential curve and the GRRM reaction path search for the isomerization of the ionized THF
dimer. We will discuss the acidity of THF cation and the isomerization of the dimer cation withthese experimental and theoretical results.
[Reference][1] Y. Matsuda, Phys. Chem. Chem. Phys., 2009, 11, 1279-1290.
[2] K. Ohno, S. Maeda, Chem. Phys. Lett., 2004, 384, 277–282.