REARRANGEMENT AND DISSOCIATIVE REACTIONS OF THE METHANOL RADICAL CATION (CH3OH.+) - A COMPARISON OF THEORY AND EXPERIMENT

Ab initio molecular orbital theory at the G1 and G2 levels has been used to examine in detail the rearrangement and dissociative reactions of ionized methanol. The G2 calculations perform slightly but consistently better than G1. Theoretical (G2) and experimental relative energies are found generally to agree to within 0.1 eV for stable structures and 0.15 eV for transition structures (a total of 31 comparisons). The G2 calculations provide new estimates of the energy differences (at 0 K) between CH3OH.+ and its distonic isomer CH2OH2.+(-32 kJ mol-1), between CH2O.+ and HCOH.+ (+25 kJ mol-1), and between HCO+ and COH+ (+156 kJ mol-1). The present calculations support a recently reported value of 968 kJ mol-1 for DELTA-H-(f) degrees 298 of HCOH.+ and suggest a value of 983 kJ mol-1 for DELTA-H(f) degrees 298 of COH+. Four separate pathways for fragmentation of CH3OH.+ to give HCO+ have been characterized, with calculated energy requirements in striking agreement with experimental appearance energies. An ion-neutral complex (CH2OH...H.+) is found to play an important role in the lowest energy pathway for production of CH2O.+ from CH2OH2.+.