Features of the H2CO potential energy hypersurface pertinent to formaldehyde photodissociation

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Abstract
In the light of intense current experimental and theoretical interest in the photodissociation of formaldehyde, several features of the H2CO potential surface have been explored using recently developed analytic configuration interaction (CI) gradient techniques. Employing a double zeta plus polarization basis set and CI including all valence shell single and double excitations, the transition state for the molecular dissociation H2CO→H2+CO has been precisely located and characterized by its vibrational frequencies. These results support previous, less sophisticated, theoretical predictions that the activation energy for this process is ∼87 kcal/mol. A similar theoretical treatment for the H2CO→HCOH isomerization suggests an activation energy of ∼84 kcal/mol for this process. Both transition states have also been examined at several simpler levels of theory and the comparisons are instructive. For the dissociative process, the reaction pathway has been mapped out in the vicinity of the saddle point. At the most complete level of theory, the structures and energetics of cis‐ and trans‐hydroxycarbene were determined along with some information concerning their (yet unobserved) electronic spectra. The lowest singlet–singlet vertical electronic transition is predicted to lie at ∼3.2 eV.