Ab Initio Molecular Orbital and RRKM Calculations of the Thermal Unimolecular Dissociation of the CH2ClO Radical

Abstract

An ab initio molecular orbital study of the unimolecular elimination of HCl and Cl from the CH2ClO radical is reported. Geometry optimizations were carried out at the HF/6-31G(d), MP2(full)/6-31G(d), and MP2(full)/6-31G(d,p) levels, and total energies were calculated using G2 and G2(MP2) theories. The zero-point-energy-corrected energy barrier for HCl elimination is predicted to be 8 kcal/mol, and for Cl elimination it is predicted to be 10.5 kcal/mol. RRKM models for both unimolecular reactions were made from the ab initio vibrational frequencies (scaled), moments of inertia and barrier heights. The RRKM predicted thermal rate coefficients for HCl elimination are in good agreement with experimental data taken over the temperature range 265−306 K and the pressure range 5−35 Torr (Wu, F.; Carr, R. W. J. Phys. Chem. A 2001, 105, 1423.) when the barrier height is adjusted to 8.5 kcal/mol. Because of the low energy barrier and, to a lesser extent, the small size of the reactant, the falloff curve is very broad, and the high pressure limit is predicted to be closely approached only when pressures of 107 Torr and above are reached. RRKM calculations of the rate coefficient for Cl elimination predict that this reaction is negligible compared with HCl elimination at moderate temperatures, but should be taken into account in high-temperature reactions. G2 theory predicts the 298 K enthalpy of formation of CH2ClO to be −6.6 kcal mol-1.