Secondary kinetics of methanol decomposition:: Theoretical rate coefficients for 3CH2+OH, 3CH2+3CH2, and 3CH2+CH3

Direct variable reaction coordinate transition state theory (VRC-TST) rate coefficients are reported for the (CH2)-C-3 + OH, (CH2)-C-3 + (CH2)-C-3, and (CH2)-C-3 + CH3 barrierless association reactions. The predicted rate coefficient for the (CH2)-C-3 + OH reaction (similar to 1.2 x 10(-1) cml molecule(-1) s(-1) for 300-2500 K) is 4-5 times larger than previous estimates, indicating that this reaction may be an important sink for OH in many combustion systems. The predicted rate coefficients for the (CH2)-C-3 + CH3 and (CH2)-C-3 + (CH2)-C-3 reactions are found to be in good agreement with the range of available experimental measurements. Product branching in the self-reaction of methylene is discussed, and the C2H2 + 2H and C2H2 + H-2 products are predicted in a ratio of 4:1. The effect of the present set of rate coefficients on modeling the secondary kinetics of methanol decomposition is briefly considered. Finally, the present set of rate coefficients, along with previous VRC-TST determinations of the rate coefficients for the self-reactions of CH3 and OH and for the CH3 + OH reaction, are used to test the geometric mean rule for the CH3, (CH2)-C-3, and OH fragments. The geometric mean rule is found to predict the cross-combination rate coefficients for the (CH2)-C-3 + OH and (CH2)-C-3 + CH3 reactions to better than 20%, with a larger (up to 50%) error for the CH3 + OH reaction.