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

Abstract

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 ¹CH₂ + H₂O, 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^∞ = 1.56 × 10¹⁶ exp(−44310/T) s⁻¹ and k_Ar⁰ = 1.60 × 10³⁶T^−12.2 exp(−48140/T) cm³ 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 ¹CH₂ + H₂O dominant under high (P>10³ Torr) and low (P<1 atm) pressures, respectively. For the bimolecular reaction of CH₃ and OH, the total rate constant and the yields of ¹CH₂ + H₂O and H₂ + HCOH at lower pressures (P<5 Torr) could be reasonably accounted for by the theory. For the reaction of ¹CH₂ with H₂O, both the yield of CH₃ + OH and the total rate constant could also be satisfactorily predicted theoretically. The production of ³CH₂ + H₂O 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.