Nitromethylene (H-C-NO2): a comparison of the lowest lying triplet and singlet states of a highly unconventional carbene

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The lowest lying triplet and single states of nitromethylene were studied with ab initio quantum mechanical methods employing large basis sets and high levels of theory. The lowest triplet state is predicted to have a planar C[sub s] symmetry structure. The lowest singlet state displays C[sub s] symmetry at the self-consistent field (SCF) and at the two-configuration self-consistent field (TCSCF) level of theory but shows C[sub 1] symmetry at correlated levels of theory. With a basis set of triple-[zeta] plus double polarization (TZ2P) quality and using the configuration interaction method including all single and double excitations (CISD), the zero point vibrational energy (ZPVE) corrected energy separation between the two electronic states is 3.5 kcal/mol, with the triplet state lying lower. However, Davidson corrected energies and single point energies at CISD geometries using the coupled cluster method including all single and double excitations (CCSD) suggest that the singlet state is about 7 kcal/mol lower in energy than the triplet state. The inclusion of f functions at the SCF level of theory showed little effect on the geometry of nitromethylene. 48 refs., 6 figs., 9 tabs.