Theoretical Study on the Reaction of the Methylidyne Radical, CH(X_2Π), with Formaldehyde, CH_2O

摘要

A theoretical study of the mechanism and kinetics of the CH(X~2Π) + H_2C＝O reaction was carried out by ab initio molecular orbital theory based on the CCSD(T)/aug-cc-pVTZ//BHandHLYP/augcc-pVDZ method in conjunction with statistical theoretical kinetic VTST and RRKM Master Equation calculations. The potential energy surface for the cis/trans-HCOH + CH reactions was also examined. Calculated results show that the association reaction of CH and CH_2O occurs by addition of the CH radical onto the oxygen atom, cycloaddition onto the C＝O bond, and, for a small fraction, insertion of CH into a C-H bond, forming CH_2C-O-CH, cyclic H_2COCH, and CH_2CHO, respectively. These channels are all barrierless, leading to a rate coefficient near the collision limit with a slight negative temperature dependence, in excellent agreement with experimental data. The intermediates can undergo extensive isomerization across seven C_2H_3O isomers, many with multiple conformers, prior to fragmentation. Eight fragmentation product sets were characterized, where H_2CCO + H and CH_3 + CO were found to be the major products at lower temperatures, while ~3CH_2 + HCO started to contribute at higher temperatures. CCHO + H_2, C_2H + H_2O, HCCOH + H, C_2H_2 + OH, and HCCO + H_2 have negligible contributions for temperatures below 3000 K and pressures up to 100 atm. Collisional stabilization of the C_2H_3O isomers is negligible except at the highest of pressures and low temperatures.