1. Infrared spectroscopic investigation
International Symposium on Molecular Spectroscopy st Meeting - June 20-24, Champaign-Urbana, Illinois
Infrared spectroscopic investigation
on CH bond acidity in cationic alkanes
Yoshiyuki Matsuda, Min Xie, and Asuka Fujii
Department of Chemistry, Graduate School of Science,
Tohoku University, Japan
Acknowledgements: The Grant-in-Aid for Scientific Research from JSPS
The Grant-in-Aid for Scientific Research on Innovative
Area from MEXT 1

2. Introduction In radical cations
In neutral state, CH bonds are normally regarded as aprotic, because of their low acidity.
In radical cations
Acidity enhancement of CH bonds (barrierless proton transfer from CH bonds) have been found in cluster cation of amine, ether, and so on. (ex. PCCP 16, 9619 (2014))
Potential energy curve of ionized
diethylether dimer in IRC
In the dimer of diethylether, proton-transfer from CH occurs without the energy barrier from the vertically-ionized structure
Vertically ionized
Proton transferred
CH of cationic diethylether is
highly acidic
UPBE1PBE/6-31+G(d) 2 2

3. Introduction: IR spectrum of diethylether cation
proton
donor
site
JPCA, 119, 4885 (2015).
typical alkyl CH
stretch range a
A very intense CaH stretch band is seen at 2700 cm-1, which is out of the typical alkyl CH stretch frequency range.
nCH of
CaH
CaH is proton donor site in
ionized dimer.
ωB97X-D/ G(3df,3pd) level
　 　 　　
Wavenumber / cm-1
scaled by 0.94 e-
Hyperconjugation between the σ orbital of CaH and SOMO of the O atom
σ orbital
of CaH
Delocalization of σ electrons
Weakening CH bond strength
Increase of positive charge of the H atoms
The hyperconjugation enhances the acidities of the CH bonds.
SOMO
(nonbonding)

4. Introduction: IR spectrum of diethylether cation
proton
donor
site
JPCA, 119, 4885 (2015).
typical alkyl CH
stretch range a
A very intense CaH stretch band is seen at 2700 cm-1, which is out of the typical alkyl CH stretch frequency range.
nCH of
CaH
CaH is proton donor site in
ionized dimer.
ωB97X-D/ G(3df,3pd) level
IR spectroscopy is a good tool to examine an acidity of CH bond.
　 　 　　
Wavenumber / cm-1
scaled by 0.94 e-
Hyperconjugation between the σ orbital of CaH and SOMO of the O atom
σ orbital
of CaH
Delocalization of σ electrons
Weakening CH bond strength
Increase of positive charge of the H atoms
The hyperconjugation enhances the acidities of the CH bonds.
SOMO
(nonbonding)

5. Aims This study pentane hexane heptane
Alkanes are composed by only C and H atoms.
(No nonbonding orbital)
hexane
Aims
We want to explore:
The acidity of the CH bonds in cationic alkanes .
The correlation of the acidic CH and the length of the alkyl chain.
heptane
IR spectroscopy of n-alkane (pentane, hexane, and heptane) cations with different alkyl chain lengths

6. Experiments & calculations
Argon tagged IR predissociation spectroscopy of VUV-pumped ions
(IRPDS-VUV-PI)
Tandem Q-mass spectrometer V’
dissociation
ion signal Dt IR
VUV S0 IE
experimental setups
PCCP 11, 1279 (2009)
Calculations
All conformers were obtained from the GRRM* (Global Reaction Route Mapping) programs at the PBE1PBE/6-31+G(d) level. They are recalculated at the ωB97X-D/ G(3df,3pd) level with Gaussian09. Atomic spin density was analyzed by Multiwfn 3.3.8†.
*Ohno and Maeda, CPL 384, 277 (2004).
†Lu and Chen, J. Comp. Chem. 33, 580 (2012).

7. IR spectrum of cationic pentane
typical CH
stretch range tt
Observed IR spectral carriers are assigned to the two conformers, tt and tg. tg
Low frequencies and high intensities of the bands indicate the high acidity of the CH bonds. tt
Two in-plane CH stretch frequencies of tg conformer are lower than those of the acidic CH bonds of the tt conformer. tg
Wavenumber / cm-1
scaled by 0.945
ωB97X-D/6-311+G(3df,3pd)
The tg conformer is more acidic.
Acidities of CH bonds correlate
with the conformation.
tt (0.21 kcal/mol)
tg (0.0 kcal/mol)

8. Mechanism of acidiy enhancement of CH
typical alkyl CH
stretch range tt
Delocalization of spin density tg
Delocalization of positive charge
Delocalization of s electron
of in-plane CH bonds tt
Acidity enhancement tg
In the tt conformer, the spin density (positive charge) delocalizes more widely.
Wavenumber / cm-1
scaled by 0.945
ωB97X-D/6-311+G(3df,3pd)
Magnitude of delocalization
of s electron is small.
0.056
0.056
0.058
0.08
The CH bond in the tg conformer is more acidic.
tt (0.21 kcal/mol)
tg (0.0 kcal/mol)
Spin density (isoval = 0.007) of cationic pentane conformers

9. IR spectrum of cationic hexane
typical alkyl CH
stretch range
ttt
Similar to pentane, two conformers coexist.
The low frequency CH bands are assigned to the in-plane acidic CH bonds.
ttg
ttt
ttg
scaled by 0.945
Wavenumber / cm-1
ωB97X-D/6-311+G(3df,3pd)
The ttg conformer is more acidic than the ttt conformer.
ttt (0.0 kcal/mol)
ttg (0.80 kcal/mol)

10. IR spectrum of cationic heptane
tttt
Two conformers coexist.
(similar to pentane and hexane)
tttg
(a) obs. IR
The low frequency CH bands are assigned to the in-plane acidic CH bonds.
(b) calc. tttt
The tttg conformer is more acidic than the tttt conformer.
(c) calc. tttg
Wavenumber / cm-1
scaled by 0.945
ωB97X-D/6-311+G(3df,3pd) level
The same trends for all the alkane cations.
tttt (0.0 kcal/mol)
tttg (0.72 kcal/mol)

11. Blue shift of CH stretchtt
Pentane cation
Acidic CH bands in both the types of isomers show blue shifts with increasing alkyl chain length. tg
ttt
Hexane cation
ttg
tttt
The acidity of the CH bonds decreases with increasing the alkyl chain length.
Heptane cation
tttg
Wavenumber / cm-1

12. Mechanism of blue shifttt
0.056
The positive charge is mainly shared by
in-plane orbitals.
The positive charge is shared more widely in the longer alkyl chains.
0.056 tg tt
ttt
0.041
ttg
0.041
ttt
tttt
0.03
0.03
Stretch bands of the acidic CH bonds shift to higher frequency with increase of the alkyl chain length.
tttg
tttt
Spin density (isoval = 0.007)
of cationic alkanes
Wavenumber / cm-1

13. Summary IR spectroscopic investigations of the cationic n-alkanes.
IR spectra indicate enhancement of CH acidities in cationic 　
　　 alkanes.
2) Internal rotation changes the acidity of CH in the alkyl cations.
CH3CH2 rotation
acidity increase
Pentane cation, tt
Pentane cation, tg
3) The acidity of CH tends to decrease with increase of the alkyl
chain length.
longer alkyl chain
acidity decrease
Pentane cation
Hexane cation

14. TI01 : Pentane+-(H2O)n
TI01 : INFRARED SPECTROSCOPIC STUDY FOR THE HYDRATED
CLUSTERS OF PENTANE CATION
TOMOYA ENDO, YOSHIYUKI MATSUDA, ASUKA FUJII
Pentane+-(H2O)n, n=2,3 form the proton-transferred structures.
Pentane+-(H2O)2
Proton-trasnferred
These structures are formed
though the barrierless proton transfer.
Pentane+-(H2O)3
Proton-trasnferred
CH of pentane cation is also highly acidic, although it has no nonbonding orbital. 14 14