Barrier for the H2CO→H2+CO reaction:: A discrepancy between high-level electronic structure calculations and experiment

The unimolecular dissociation of formaldehyde to H-2+CO was studied using extended basis set calculations and a variety of medium-to-high accuracy correlation recovery techniques. These included second and fourth order perturbation theory, multireference configuration interaction wave functions, coupled cluster theory with perturbative triples and full iterative triples, and estimated full configuration interaction wave functions. 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 +/- 0.3 kcal/mol, almost 3 kcal/mol larger than the experimental value of 79.2 +/- 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 recovery, core/valence correlation, and scalar relativistic effects. 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. (C) 2000 American Institute of Physics. [S0021-9606(00)30917-5].