The photodissociation of formaldehyde: Potential energy surface features

Features of the S0 potential energy surface of formaldehyde relevant to its dissociation to molecular products, H2+CO, to radical formation, H + HCO, and to rearrangement to hydroxycarbene, HCOH, have been studied by means of ab initio calculations. A gradient procedure was used to locate and to characterize both equilibrium and transition state geometries. Basis sets of at least double zeta (DZ) quality were employed throughout and many calculations involved more flexible basis sets including polarization functions. Force constants, normal modes and vibrational frequencies were calculated at the SCF level for stationary points on the surface. Extensive configuration interaction (CI) calculations were also carried out. For the molecular dissociation the energy barrier including the effects of polarization functions and electron correlation was 4.06 eV (93.6 kcal mole−1, 32 700cm−1). Correcting for changes in zero point vibrational energy gave an approximate activation energy of 3.76 eV (87 kcal mole−1, 30 300cm−1) with an estimated error of ±0.2 eV (±5 kcal mole−1, ±1700cm−1). The energy required for the rearrangement of formaldehyde to trans-hydroxycarbene was calculated to be 3.85 eV (89 kcal mole−1, 31 000cm−1) at the DZ + polarization + CI level with the inclusion of zero point corrections. The large imaginary frequencies associated with the reactive motion imply sharp and thin barriers through which tunneling is estimated to be of considerable importance. Based on the calculated features of the potential energy surface the mechanism of the photodissociation of formaldehyde is discussed.