Acetaldehyde photochemistry: The radical and molecular dissociations

Abstract

The dissociation of acetaldehyde to radical products, CH3+HCO and H+CH3CO, and to the molecular limit of CH4+CO has been investigated by ab initio SCF calculations. Effects of zero point energy corrections and of electron correlation on energy differences have also been considered. The computed T1 origin of acetaldehyde is in good agreement with recent experimental determinations. On the T1 surface, CH3CHO faces activation barriers of 12–15 kcal mol−1 for dissociation into radicals. The existence of and the quantitative determination of the barrier heights have been accurately computed for the first time. Vibrational excitation of T1 acetaldehyde formed with energy equal to that of its spectroscopic origin is anticipated to play an important role in acetaldehyde photochemistry as has been inferred experimentally. The barrier to the unimolecular dissociation to molecular products on the S0 surface is slightly lower than the T1 radical barriers. If hot ground state acetaldehyde can be formed with energies of ∼85 kcal mol−1 by some process then the molecular dissociation channel should also be accessible.