Theoretical study of the photochemistry of carbonyl compounds: Potential-energy surface for the photodissociation of propynal

The potential-energy surface of propynal (HC≡C-CHO) pertinent to its dissociation to molecular products acetylene and CO, to radical formation H + HC≡C-CO, and rearrangement to various carbenes have been studied by means of ab initio calculations. A basis set of double-zeta quality and a more flexible basis set including polarization functions were employed. The various stationary points of the energy hypersurface were obtained by a complete optimization of the geometry. Extensive CI calculations were used to obtain the energies of transition states and minima. The lowest-energy barrier to molecular products is 70 kcal mol^-1, and different in structure from the transition state of the similar reaction in formaldehyde. Characteristic features of the photodissociation of propynal are discussed.