Theoretical Study on the Photoionization of Methanol Dimer

Studies on the properties, structures and reactivities of molecular cluster have been carried out in order to gain some understanding of the chemical dynamics in reactions systems lying between the gaseous and condensed phases. Although the characteristics of the detected ion cluster distribution have often been attributed to the properties and even structural features of the neutral precursors, much less is known about the detailed microscopic processes governed by pairwise molecular interactions. In spite of the apparent simplicity of the reactions in methanol clusters, the mechanisms and energetics of proton-transfer process remain a subject of controversy. In this respect, theoretical investigations on reactions intermediates will be helpful for understanding the proton transfer process in the gas phase hydrogen-bonded cluster ions. Since no detailed theoretical studies have been published so far, the purpose of this project is to investigate internal ion-molecule reactions in methanol clusters by using ab initio calculations at high level of theory on isotope-labelled clusters, leading subsequently to advanced molecular kinetics calculations.

Principal Investigator

Adriana Olleta
Chemistry
University of Queensland

Project

f57

Co-Investigator

Sean Smith
Chemistry
University of Queensland

RFCD Codes

250600

Significant Achievements, Anticipated Outcomes and Future Work

The energies and structures of neutral and ionic cluster as well as the dissociation products were calculated by using molecular orbital calculations. Experimentally methanol dimer (CD3OH)2 ion-molecule reaction is initialized by VUV (vacuum ultraviolet) laser photoionization. Most recently experimental results showed for the energetically allowed dissociation channels that proton transfer and deuteron transfer reactions are the dominant reactions.

Proton and Deuteron Transfer Reactions
CD3OH+ + CD3OH → CD3OH2+ + CD3O (1)
CD3OH+ + CD3OH → CD3OHD+ + CD2OH (2)

However later studies about the possible mechanism for the proton transfer reaction, the isotopic scrambling and energy randomization, showed some contradictory results. When methanol dimers are ionized by VUV photons with the energy between 10.91 and 10.49 eV, five different reactions channels are possible:

Un-reactive methanol dimer ion:
(CD3OH)2 + h ν (vuv) → (CD3OH)2+ + e- (3)
D atom or H atom loss reactions
(CD3OH)2 + h ν (vuv) → (CD2O.CD3OH)H+ + D + e- (4)
(CD3OH)2 + h ν (vuv) → (CD2O.CD3OH)D+ + H + e- (5)
Proton (H) transfer reaction and deuteron (D) transfer reaction
(CD3OH)2 + h ν (vuv) → CD2OH2+ + CD3O (or CD2OD) + e- (6)
(CD3OH)2 + h ν (vuv) → CD2OHD+ + CD2OH (or CD2HO) + e- (7)

Previous calculation of methanol dimer ion (CD3OH)2+ demonstrated that there two structures. One structure forms a complex, (CH3OH2+...OCH3), resulting from the proton transferred from the hydroxyl group, and the other (CH3OH2+...O(H)CH2) resulting from the transferred from the methyl group. This investigation demonstrated that the ionization of neutral dimer after vertical transition proceeds to form the first complex without passing an activation barrier

However, the same investigation observed that there is a barrier of 10.54 eV, from the ground state of the neutral dimer, for the formation of the second complex:
(CH3OH2+...OCH3) → (CH3OH2+...O(H)CH2) (8)
Thus, if the VUV photon energy is reduced from 11.00 to 10.49 eV, the proton transfer reaction from the methyl group must be reduced dramatically. The reported experimental data show that the deuteron transfer from the methyl group not only occurs below 10.54 eV, but also is a dominant channel at VUV photon energy lower than 10.54 eV. At this point, which process is dominant in the dissociation channel of methanol dimer ions cannot be determined. Complete potential energy surface and dissociation model simulations are necessary and are more reliable in order to reveal the dissociation processes.

 

Computational Techniques Used

Quantum chemical calculations are used to provide structural, vibrational and energetical information on the photo ionization of the methanol dimer system as well the minimum pathways for the different reaction channels. This system was studied employing the DFT(B3LYP) and MP2 method together with the 6-31+G(d) basis set and single point calculations at the B3LYP/ 6-311+G(3df,2p)//B3LYP/6-31+G(d) and MP2/ 6-311+G(3df,2p)//MP2/6-31+G(d) levels of theory. However, in order to obtain a better reference we have also performed calculation with the CBS-RAD(B3LYP,B3LYP) approach.

 

Publications, Awards and External Funding

External Funding and Awards

This work was supported in part by the Australian Research Council, the Australian Government, the US National Security Agency, the Advanced Research and Development Activity, and the US Army Research Office under contract number DAAD19-01-1-0653.
ARC Large Grant No: A29937112
Discovery Project Grant no: DP0211019

Publications

None.