1. Infrared spectroscopic study
for the hydrated clusters of pentane cation
Tomoya Endo, Yoshiyuki Matsuda and Asuka Fujii
Department of Chemistry, Graduate
School of Science, Tohoku University, Japan

2. OH・・・X, NH・・・X CH・・・X Introduction (CH3OH)2+ (NH3)2+
Neutral OH and NH act as proton donor. In the cationic state,
they act as stronger hydrogen bond donors.
Structures of (NH3)2+ & (CH3OH)2+
(NH3)2+
(CH3OH)2+
H+-transferred
structure
H+-transferred
structure
JCP 125, (2006).
JCP 129, (2008).
Formed through barrierless H+-transfer from NH or OH
Proton donor abilities of cationic NH and OH are enhanced.
CH・・・X
　Neutral alkyl groups normally do not act as proton donor.
Does cationic CH increase proton donor ability? 2

3. How can we investigate the proton donor ability of cationic CH?
1. Study of alkane monomer cation. (TE11)
Spin density
IR spectroscopic study on pentane (C5H12)monomer cation indicates
the delocalization of sigma electron causes
the enhancement of the proton donor ability of CH.
2. Study of intermolecular interaction of alkane cation with
proton accepter molecule. (This study)

4. This study Investigate the proton donor ability of pentane cation Aim
Infrared spectroscopies and theoretical calculations of
pentane-(H2O)n (n=1~3) cluster cation
Theoretical calculations are carried out for optimization, vibrational
frequency, potential energy curve and reaction path search.
・Does pentane cation show high proton donor ability?
・How many water molecules are necessary for proton
transfer from pentane cation?　 4

5. Experiment
IR Predissociation Spectroscopy of VUV-Pumped Ion (IRPDS-VUV-PI)
PCCP 11, 1279 (2009)
dissociation V’ IR IE
ion signal Dt
VUV S0
・We monitored ion intensity of cation generated with VUV (118 nm).
・The cation under investigation is selected with 1st Q-mass.
・We monitored fragment ions generated by IR predissociation
through 2nd Q-mass.
IR predissociation spectroscopy of size-selected cations generated with VUV photoionization 5 5

6. IR spectrum of [pentane-(H2O)1]+
3520 cm-1
nCH
Structure (c) is eliminated because of
difference with the observed spectrum.
Relative energies of (a) and (b) are more stable than those of (c) and (d).
(a)
0 kcal/mol
Water molecule in the cluster has hydronium ion character due to the
proton from pentane cation.
(b)
0.10 kcal/mol
X 1/12
(c)
6.86 kcal/mol
Proton-shared type structures such as (a) and (b) are formed.
(d)
6.53 kcal/mol
2400
2600
2800
3000
3200
3400
3600
3800
Wavenumber/cm-1
wB97X-D/ G(3df,3pd)
scaled by 0.945

7. Energetics in 118 nm photoionization
10.48 eV
Min
10.38 eV
VIE TS
7.41
1.18 eV
6.34
6.07
5.94
5.77
0.99
VUV
(118 nm)
0 kcal/mol
(b)
(a) ~
neutral
GRRM is a program to explore potential energy surface.

8. Energetics in 118 nm photoionization
10.48 eV
Min
10.38 eV
VIE TS
7.41
1.18 eV
6.34
6.07
5.94
5.77
0.99
VUV
(118 nm)
0 kcal/mol
(b)
(a) ~
neutral
Energetically, (a) and (b) can be formed after 118 nm photoionization.

9. Potential energy curves along the proton transfer
Bond length of the neutral cluster
r(C-H)=1.09
donor
r(O-H) 6
accepter 5 4
r(C-H) 3
All parameters except the CH length are optimized.
DE(kcal/mol)
proton-shared 2
r(C-H)=1.48
r(O-H)=1.16 1
Single minimum is calculated in the proton-transfer coordinate.
1.2
1.4
1.6
1.8
r(C-H) (Å)
wB97X-D/ G(3df,3pd)
The proton is shared between pentyl radical and water molecule.

10. IR spectrum of [pentane-(H2O)2]+
Broad feature
nCH n3
Broad feature appears in the frequency region lower than 2800cm-1.
X 1/5
The observed broad feature can be
explained by the vibration of the proton.
(a)
0 kcal/mol
X 1/3
0.44 kcal/mol
No band is observed around 3450 cm-1.
(b)
X 1/3
Structure (a) and (b) are more stable
than structure (c).
5.34 kcal/mol
(c)
18.00 kcal/mol
[pentane-(H2O)2]+ forms
the proton transferred structure.
(d)
2400
2600
2800
3000
3200
3400
3600
3800
Wavenumber/cm-1
wB97X-D/ G(3df,3pd) scaled by 0.945

11. Potential energy curves along the proton transfer
Bond length of the neutral cluster
r(C-H)=1.09
accepter
r(O-H) 14
donor 12
r(C-H) 10
DE(kcal/mol) 8
All parameters except the CH length are optimized. 6
proton-transferred 4
r(C-H)=1.66
r(O-H)=1.06
Single minimum is calculated in the proton-transfer coordinate. 2
1.2
1.4
1.6
1.8
r(C-H) (Å)
wB97X-D/ G(3df,3pd)
Pentane cation has high proton donor ability.

12. IR spectrum of [pentane-(H2O)3]+
nCH n3
Intense broad band appears in low frequency region.
X 1/5
X 1/5
Structure (a),(b) and (c) show good
agreement.
(a)
0 kcal/mol
X 1/6
X 1/5
The relative energies of (a) and (b) are more stable than those of (c) and (d).
0.79 kcal/mol
(b)
X 1/6
X 1/6
5.12
kcal/mol
[pentane-(H2O)3]+ forms proton-transferred structure like (a) and (b).
(c)
26.49 kcal/mol
(d)
2400
2600
2800
3000
3200
3400
3600
3800
Wavenumber/cm-1
wB97X-D/ G(3df,3pd) scaled by 0.945

13. Potential energy curves along the proton transfer
Bond length of the neutral cluster
r(C-H)=1.09
accepter 20
donor
r(C-H) 15
All parameters except the CH length are optimized.
DE(kcal/mol) 10
No energy barrier exists in the proton-transfer coordinate. 5
proton-transferred
r(C-H)=1.80
1.2
1.4
1.6
1.8
r(C-H) (Å)
wB97X-D/ G(3df,3pd)
With increasing the cluster size, the proton is more strongly attracted to the water moiety.

14. Proton transfer in [pentane-(H2O)n]+
proton-shared structure
proton-transferred structure
Proton transfer occurs in the hydrated cluster of pentane cation .
Proton donor ability of pentane cation is highly enhanced.

15. Summary 1)
We observed the IR spectra of [pentane-(H2O)n]+ (n=1-3) with spectroscopy based on the VUV photoionization detection. The structures are analyzed by IR spectroscopic and theoretical results. 2)
The proton is transferred from the pentane cation to water moieties in n=2 and 3, while proton of CH is shared in the pentyl radical and water in n=1. 3)
No energy barrier exists in the proton transfer reactions from CH of pentane cation to the water moieties. These results indicate the high proton donor ability of CH of pentane cation.

16. Energetics in 118 nm photoionization
10.48 eV
Min
10.36 eV
VIE TS
1.79 eV
18.78
18.56
18.56
18.49
17.90
0.46
0 kcal/mol
VUV(118 nm)
AIE 8.57 eV
(b)
(a) ~
neutral
Energetically, proton transferred structures can be made after 118 nm photoionization.

17. Energetics in 118 nm photoionization
10.51 eV
Min
10.42 eV
VIE TS
25.97
25.85
1.94 eV
9.88
3.16
3.15
3.34
3.05
3.22
0 kcal/mol
VUV(118 nm)
AIE 8.57 eV ~
neutral
Energetically, the proton transferred structure can be made
after 118 nm photoionization.

18. Charge distribution of [pentane-(H2O)1]+
NBO calculation with wB97X-D/ G(3df,3pd)
Non-proton-shared structure
(a.u.)
H2O :
C5H12 :
Proton-shared structure
(a.u.)
H2O :
H :
C5H11 :

19. Charge distribution of [pentane-(H2O)2]+
NBO calculation with wB97X-D/ G(3df,3pd)
Non-proton-transferred structure
(a.u.)
H2O :
C5H12 :
Proton-transferred structure
(a.u.)
H5O2 :
C5H11 :

20. Charge distribution of [pentane-(H2O)3]+
NBO calculation with wB97X-D/ G(3df,3pd)
Non-proton-transferred structure
(a.u.)
(H2O)3 :
C5H12 :
Proton-transferred structure
(a.u.)
H7O3 :
C5H11 :

21. Potential energy curves along the proton transfer
accepter 18
r(C-H) 15
donor 12
DE(kcal/mol) 9
All parameters except the CH length are optimized. 6
proton-transferred
r(C-H)=1.84 3
No energy barrier exists in the proton-transfer coordinate.
1.2
1.4
1.6
1.8
r(C-H) (Å)
wB97X-D/ G(3df,3pd)

22. Simulated IR spectra of [pentane-(H2O)1]+
0 kcal/mol
6.53 kcal/mol
0.10 kcal/mol
7.68 kcal/mol
0.23 kcal/mol
7.03 kcal/mol
6.86 kcal/mol
0.16 kcal/mol
2600
3000
3400
3800
Wavenumber/cm-1
5.68 kcal/mol
wB97X-D/ G(3df,3pd) scaled by 0.945
2600
3000
3400
3800
Wavenumber/cm-1

23. Simulated IR spectra of [pentane-(H2O)2]+
kcal/mol
5.34
kcal/mol
0.32
kcal/mol
6.12
kcal/mol
0.20
kcal/mol
6.47
kcal/mol
18.00
kcal/mol
0.44
kcal/mol
2600
3000
3400
3800
Wavenumber/cm-1
0.18
kcal/mol
wB97X-D/ G(3df,3pd) scaled by 0.945
2600
3000
3400
3800
Wavenumber/cm-1

24. Simulated IR spectra of [pentane-(H2O)3]+
kcal/mol
2.16
kcal/mol
0.79
kcal/mol
2.94
kcal/mol
0.25
kcal/mol
5.12
kcal/mol
0.49
kcal/mol
26.59
kcal/mol
2600
3000
3400
3800
Wavenumber/cm-1
0.81
kcal/mol
wB97X-D/ G(3df,3pd) scaled by 0.945
2600
3000
3400
3800
Wavenumber/cm-1

25. Optimized structures of pentyl radical
0 kcal/mol
0.37 kcal/mol
3.67 kcal/mol
wB97X-D/ G(3df,3pd)