Photodissociation of acetaldehyde:: The CH4+CO channel

Ab initio quantum chemical calculations for the molecular dissociation channel of acetaldehyde are reported. The enthalpy change for the dissociation of acetaldehyde into methane and carbon monoxide was calculated to be exoergic by 1.7 kcal/mol. The transition state for this unimolecular dissociation, confirmed by normal mode analysis, was found to have an activation energy of 85.3 kcal/mol. Experimental measurements are reported for the vibrational and rotational state distribution of the CO product. No v=1 CO is found and the rotational temperature is 1300 +/- 90 K. The reaction coordinate at the transition state implies that the CO product is vibrationally cold and rotationally hot. This conclusion, which requires quantum dynamics calculations to confirm definitively, does agree with and aids in explaining the experimental results. (C) 2001 American Institute of Physics.