1. Yuichiro Nakayama, Yoshiyuki Matsuda, and Asuka Fujii
C…H…N hydrogen bond formation in trimethylamine dimer upon one-photon ionization
Yuichiro Nakayama, Yoshiyuki Matsuda, and Asuka Fujii
Department of Chemistry, Graduate School of Science, Tohoku University, Japan

2. Introduction Does a cationic alkyl group act
Proton-migration from a methyl group
Keto-enol tautomerization in the VUV photoionization of the acetone-water cluster
Most stable cation
Neutral
Proton-migration
VUV
photoionization
Matsuda et al. Angew. Chem. Int. Ed. 49, 4898 (2010)
A neutral alkyl group generally does not act as a proton donor
We have studied the ionization dynamics of clusters. Here, I show the result for the ionization process of the acetone-water cluster.
In the VUV photoionization process, the acetone moiety tautomerizes to the enol form through the long-distance proton migration. This migration is catalyzed by the water molecule.
Thus, the methyl proton is transferred in this reaction.
In general, an alkyl group in the neutral state does not act as a proton donor because the acidities of a neutral alkyl group is very low.
Then, how is in the cationic state. We have concerned the proton-donor ability or hydrogen-bond donor ability of the cationic alkyl group.
Does a cationic alkyl group act
as proton donor and/or H-bond donor?

3. This study Aim Sample Exp. & Calc.
Understand the ability of the cationic methyl group as H-bond donor
Sample
Trimethylamine (TMA) dimer cation
TMA has only the methyl groups and nitrogen.
Exp. & Calc.
Our aim of this study is to understand the hydrogen-bond donor ability of the methyl group in the cationic state.
To investigate this problem, we have studied trimethylamine dimer cation. Trimethylamine has only the methyl groups and nitrogen.
Therefore, we can simply study the interaction of the methyl group and the amine nitrogen.
To determine the cluster structure, we carried out IR spectroscopy based on VUV photoionization detection for neutral and cationic trimethylamine dimer.
We also performed the theoretical calculations of the structure optimization, the vibrational calculations, and the potential energy curves of the ionized state
to get the idea how the trimethylamine dimer favorably isomerize in the ionization process.
IR spectroscopy of (TMA)2 and (TMA) 2+ through the VUV photoionization detection
Theoretical calculations of structures, harmonic vibration, and potential energy curves of the cationic state

4. Experiment
IR spectroscopy for neutral clusters
IR spectroscopy for cationic clusters
VUV-ID-IRPDS
(VUV-Ionization-Detected
IR Predissociation Spectroscopy)
IRPDS-VUV-PI
(IR Predissociation Spectroscopy
of VUV-Pumped Ion)
IR spectroscopies of neutral and cationic clusters were performed by these schemes, respectively.
In both schemes, we monitored the ion intensity of the size-selected cluster cation, which was generated by the VUV photoionization at 118nm.
By introducing IR light prior to the VUV photoionization, we can perform the IR predissociation spectroscopy of neutral cluster, because the IR predissociation causes the decrease of the monitored ion intensity. By delaying the IR light injection, the IR predissociation spectroscopy of cationic cluster can be performed.
Thus, we can observe IR spectra of neutral and cationic clusters before and after the VUV photoionization.
Matsuda et al. Phys. Chem. Chem. Phys. 11, 1265 (2009)
We can observe IR spectra of both neutral and cationic clusters

5. IR spectrum of neutral (TMA)2
VUV-ID-IRPDS
First, I show the observed IR spectrum of neutral trimethylamine dimer, the most stable optimized structure, and the simulated spectrum at this calculational level.
These observed bands are assigned to the CH stretch bands.
Although the disagreements exist in the detailed comparison of the observed and simulated spectra, the overall observed spectra feature are reproduced by the harmonic vibrational calculation.
The most stable structure

6. IR spectrum of (TMA)2+ A broad H-bonded X-H stretch band is observed
Next, I show the IR spectrum of trimethylamine dimer cation. This is an observed IR spectrum of trimethylamine dimer cation. These are the optimized structures and the harmonic vibrational calculations for these structures.
In the observed IR spectrum, a broad absorption appears from 3000 cm-1 to the lower frequency region.
Generally, the stretching vibration of a cationic hydrogen-bonded X-H is observed with a broad bandwidth and high intensity in this region.
So this broad absorption can be assigned to the stretching vibration of the hydrogen-bonded X-H group.
These structure are the two most stable structures and these structures can have the hydrogen-bonded X-H group in the trimethylamine dimer cation.
This structure is the most stable structure where the methyl proton is transferred to nitrogen and this proton is shared between the carbon and nitrogen atoms.
In the other structure, the methyl group acts the hydrogen-bond donor and this proton is shared between carbon and nitrogen.
We cannot determine which structure is formed by comparison of this observed spectrum and these calculated spectra.
So we calculated the potential energy curve of the cluster cation to get the idea which the structure is favorably formed.
A broad H-bonded X-H stretch band is observed

7. Energy Diagram of the reaction of (TMA)2+
AIE
(C…H-N)
H+-transfer-type I
Most stable
Zero point energy
1.3 kcal/mol
Coexistence of two structures
(C-H…N) H-bond-type
Vertically ionized structure
Energy (kcal/mol)
This is the IRC potential curve which was calculated from the vertically ionized structure of trimethylamine dimer.
The horizontal axis is the intrinsic reaction coordinate, and the vertical axis shows the relative energy from the adiabatic ionization energy.
This potential curve indicates that the vertically ionized structure initially isomerizes to this structure without the energy barrier. In this structure, hydrogen-bond is formed between the methyl group and the nitrogen atom.
(click)
Next, I show the potential energy curve which we calculated by fixing the C-H distance and by optimizing all the other structural parameters.
This is the most stable structure where this proton is transferred to the nitrogen.
This potential curve corresponds to the proton-transfer potential. The energy barrier between these two structures is only 1.3 kcal/mol. This energy is smaller than the zero point energy of the stretch vibration of the N-H.
Therefore, these two structures would coexist. These structures are the most two stable structures.
These two structures are directly formed from the vertically-ionized state.
We have therefore concluded that trimethylamine dimer cation dominantly forms these types of structure.
AIE
Intrinsic Reaction Coordinate (Å1/2)

8. Isomerization reaction of (TMA)2 induced by photoionization
VUV
One-photon ionization
Neutral
Cation
A proton is shared between C and N
In the VUV one-photon ionization, trimethylamine dimer isomerizes this type of structure, where the methyl proton is shared between carbon and nitrogen atoms.
To our knowledge, this is the first result to find this type of structure.
And finally, we have concluded the methyl group can acts as a hydrogen-bond donor in the cationic state and its proton donor ability is high in the cationic state.
A methyl group acts as a hydrogen-bond donor in the cationic state

9. Summary
We observed the IR spectra of (TMA)2 and (TMA)2+ with spectroscopies based on the VUV photoionization detection.
We calculated the isomerization reaction potential energy curve following the one-photon ionization.
In the VUV photoionization process, (TMA)2 is isomerized to the structure in which a proton is shared between C and N. In this structure, the methyl group acts as the proton donor.
VUV
One-photon
ionization
The summary is as follows.
Neutral
Cation

11. IR spectrum of (TMA)2+ IRPDS-VUV-PI
Next, I show the IR spectrum of trimethylamine dimer cation. This is an observed IR spectrum of trimethylamine dimer cation. These are the optimized structures and the harmonic vibrational calculations for these structures.
In the observed IR spectrum, a broad absorption appears from 3200 cm-1 to the lower frequency region.
Generally, the stretching vibration of a cationic hydrogen-bonded X-H is observed with a broad bandwidth and high intensity in this region.
So this broad absorption can be assigned to the stretching vibration of the hydrogen-bonded X-H group.
These structure are the most stable structures and only these three structures can have the hydrogen-bonded X-H group in the trimethylamine dimer cation.
This structure is the most stable structure where the methyl proton is transferred to nitrogen and this proton is shared between the carbon and nitrogen atoms.
In the middle structure, the methyl group acts the hydrogen-bond donor and this proton is shared between carbon and nitrogen.
In the bottom structure, this transferred methyl proton is shared between nitrogen atoms.
We cannot determine which structure is formed by comparison of this observed spectrum and these calculated spectra.
So we calculated the potential energy curve of cluster cation to get the idea which the structure is favorably formed.