Low-energy paths for the unimolecular decomposition of CH3OH:: A G2M/statistical theory study

The potential energy surface (PES) of the CH3OH system has been characterized by ab initio molecular orbital theory calculations at the G2M level of theory. The mechanisms for the decomposition of CH3OH and the related bimolecular reactions, CH3 + OH and have been elucidated. The rate constants for these processes have been calculated using variational RRKM theory and compared with available experimental data. The total decomposition rate constants of CH3OH at the high- and low-pressure limits can be represented by k(infinity) = 1.56 x 10(16) exp(-44310/T) s(-1) and k(Ar)(0) = 1.60 x 10(36) T-12.2 exp(-48 140/T) cm(3) molecule(-1) s(-1), respectively, covering the temperature range 1000-3000 K, in reasonable agreement with the experimental values. Our results indicate that the product branching ratios are strongly pressure dependent, with the production of CH3 + OH and (CH2)-C-1 + H2O dominant under high (P > 10(3) Torr) and low (P < 1 atm) pressures, respectively. For the bimolecular reaction of CH3 and OH, the total rate constant and the yields of (CH2)-C-1 + H2O and H-2 + HCOH at lower pressures (P < 5 Torr) could be reasonably accounted for by the theory. For the reaction of (CH2)-C-1 with H2O, both the yield of CH3 + OH and the total rate constant could also be satisfactorily predicted theoretically. The production of by the singlet to triplet surface crossing, predicted to occur at 4.3 kcal mol(-1) above the H2C...OH2 van der Waals complex (which lies 82.7 kcal mol(-1) above CH3OH), was neglected in our calculations.