DFT and ab initio study of the unimolecular decomposition of the lowest singlet and triplet states of nitromethane

The fully optimized potential, energy curves for the unimolecurar decomposi tion of the lowest singlet and triplet states of nitromethane through the C -NO2 bond dissociation pathway are calculated using various DFT and high-le vel ab initio electronic structure methods. We perform gradient corrected d ensity functional theory (DFT) and multiconfiguration self-consistent field (MCSCF) to conclusively demonstrate that the triplet state of nitromethane is bound. The adiabatic curve of this state exhibits a 33 kcal/mol energy barrier as determined at the MCSCF level. DFT methods locate this barrier a t a shorter C-N bond distance with 12-16 kcal/mol lower energy than does MC SCF. In addition to MCSCF and DFT, quadratic configuration interactions wit h single and double substitutions (QCISD) calculations are also performed f or the singlet curve. The potential energy profiles of this state predicted by DFT methods based on Becke's 1988 exchange functional differ by as much as 17 kcal/mol from the predictions of MCSCF and QCISD in the vicinity of the equilibrium structure. The computational methods predict bond dissociat ion energies 5-9 kcal/mol lower than the experimental value. DFT techniques based on Becke's 3-parameter exchange functional show the best overall agr eement with the higher level methods.