Ab initio quantum chemical study of the formation, decomposition and isomerization of the formaldiminoxy radical (CH 2 NO): comparison of the Gaussian-2 and CASPT2 techniques in the calculation of potential energy surfaces

Abstract

The reaction between triplet methylene and nitric oxide, producing the formaldiminoxy (CH2NO) radical, and the subsequent decomposition and isomerization reactions of CH2NO have been studied using ab␣initio quantum chemical techniques that include the Gaussian-2 (G2), CASSCF and CASPT2 methods. Stationary points on the potential energy surfaces were located at MP2/6-31G(d) and CASSCF/cc-pVDZ levels of theory, while the electronic energies were determined using G2, G2(MP2), QCISD(T)/cc-pVTZ, RCCSD(T)/cc-pVTZ and CASPT2/cc-pVTZ approaches. G2 is believed to be reliable at equilibrium geometries, but the determination of certain transition state geometries and energies requires a MCSCF-based approach. The calculations suggest that CH2NO (2A′) forms in a barrierless reaction and could readily decompose to H+HCNO. A subsequent abstraction reaction then results in H2+CNO. No molecular elimination channel was found. An alternative pathway is the formation of CH2ON, which readily isomerizes to CH2NO.