Unimolecular Dissociation of Formyl Halides HXCO → CO + HX (X= F, Cl): An Ab Initio Direct Classical Trajectory Study

Abstract

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 H2. The average rotational quantum number for HCl, 〈J〉 = 12.8, is considerably higher than for HF, 〈J〉 = 9.5, or H2, 〈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 be vibrationally hot, but the distribution has not been measured. No experiments are available for HClCO dissociation.