AN AB-INITIO STUDY OF THE 3-CHANNEL REACTION BETWEEN METHANOL AND HYDROGEN-ATOMS - BAC-MP4 AND GAUSSIAN-2 CALCULATIONS

Rate coefficients for the three-channel reaction of methanol with H at oms were calculated from ab initio saddle point properties using conve ntional transition state theory. The saddle point geometries and energ ies were determined using the BAC-MP4 and the Gaussian-2 methods. The( classical) ab initio barrier heights for the formation of CH2OH, CH3O, and CH3 are 11.3, 16.3, and 30.5 kcal mol(-1), respectively, as obtai ned using the BAC-MP4 method, and 10.6, 16.3, and 25.6 kcal mol(-1), r espectively, from Gaussian-2 calculations. The rate coefficients obtai ned from the G-2 calculations are 2.0 x 10(-16), 1.9 x 10(-20), and 1. 9 x 10(-28) cm(3) molecule(-1) s(-1) at 298 K. The BAC-MP4 rate coeffi cient at 298 K is similar for channel 2 and lower than the G2 rate coe fficient of the overall reaction agrees, within a factor of 2, with th e recommendation of Tsang in a broad temperature range. Its temperatur e dependence is represented by k = (1.57 +/- 0.56) x 10(-15)T(1.70+/-0 .05) exp(-2735 +/- 23 K/T) cm(3) molecule(-1) s(-1). Theory suggests t hat formation of CH2OH is the dominant channel, contributing to the ov erall reaction by over 95% below 1200 K and by about 90% at 2000 K. Th e formation of CH3 + H2O, which is the most exothermic channel, is uni mportant in the whole temperature range studied.