Unimolecular dissociation of formyl halides HXCO → CO+HX (X = F, Cl):: An ab initio direct classical trajectory study

The dynamics of the unimolecular dissociation of formyl halides, HXCO (X = F, Cl), have been studied by classical trajectory calculations at the MP2/6-31G(d,p) level of theory and have been compared to similar calculations for H2CO. The calculated transition states are planar with elongated C-X and H-X bonds. Trajectories were started from the transition state with 12 kcal/mol of energy above the potential barrier, corresponding to the zero-point energy plus 2.25 and 2.87 kcal/mol of excess energy in the transition vector of formyl fluoride and formyl chloride, respectively. The CO fragments are produced rotationally hot with <J> = 40-50 but vibrationally cold with only 10-20% in v = 1. The HX fragment shows significant vibrational excitation with <v> = 1.0 for HF and <v> = 2.3 for HCl, compared to <v> = 1. 16 for H-2. The average rotational quantum number for HCl, <J> = 12.8, is considerably higher than for HF, <J> = 9.5, or H-2, <J> = 3.3. Product, translation receives about 42% of the available energy in formyl fluoride and about 31% in formyl chloride, compared to 70% for formaldehyde. The results show good qualitative agreement with the available experimental energy partitioning for HFCO; HF is observed to bevibrationally hot, but the distribution has not been measured. No experiments are available for HClCO dissociation.