PNNL

Abstract

The unimolecular dissociation of formaldehyde to H2 + CO was studied using extended basis set calculations and a variety of medium-to-high accuracy correlation techniques, including second and fourth order perturbation theory, multireference configuration interaction wavefunctions, coupled cluster theory with perturbative triples and full iterative triples, and estimated full configuration interaction wavefunctions. The intrinsic error of the electronic structure methods was assessed by extrapolating total energies to the complete basis set limit. Our best estimate of the barrier height, including zero point vibrational effects, is 81.9 kcal/mol, almost 3 kcal/mol larger than the experimental value of 79.2 plus or minus 0.8 kcal/mol. This estimate includes corrections for the effects of finite basis set truncation (which is negligible at the quintuple zeta level), higher order correlation, core/valence correlation and scalar relativistic energy differences. Using the same theoretical approach, we estimate the exothermicity of the dissociation reaction to be -1.6 kcal/mol, compared to experimental values in the -0.4 to -2.2 kcal/mol range. New calculations of the unimolecular dissociation rate constants using a variety of techniques failed to reconcile theory and experiment.