THE ISOMERIZATION OF NITROBENZENE TO PHENYLNITRITE

We report ab initio calculations on nitrobenzene (C6H5NO2) and phenylnitrite (C6H5ONO) performed at the MP2(FC) 6-31G* level. The results paint a detailed picture of the isomerization pathway between the two species on the ground electronic surface. They show that phenylnitrite is stable with respect to dissociation into phenoxy and nitric radicals by 80.6 kJ mol(-1) and lies only 20.2 kJ mol(-1) higher in energy than the nitrobenzene global minimum. We estimate the barrier between the two isomers to be less than 108.9 kJ mol(-1) (1.13 eV), but we are unable to locate a fully converged transition state. Calculation of the isomerization pathway shows that there is an early barrier close to the initial nitrobenzene geometry. It also provides a clear picture of the extensive rearrangement involving a C-N bond being broken, a new C-O bond being formed, and further changes in bond lengths, bond angles, and dihedral angles in the C-NO2 moiety of the nitrobenzene molecule. The harmonic frequencies of the two isomers and those for the phenoxy and nitric oxide radicals calculated at the same level of theory compare well with previous experimental and theoretical studies. These calculations also bear on our experimental study of the kinetic and internal energy disposal in the photodissociation of nitrobenzene (D.B. Galloway, et al., J. Chem. Phys., 98 (1993) 2107; J. Chem. Phys., 100 (1994) 1946).