The lower electronic states of nitrite and nitrate ion, nitromethane, nitramide, nitric acid, and nitrate esters

Abstract

CNDO /s –CI calculations are performed for the series of molecules: nitrite ion, nitromethane, nitramide, ethyl nitrate, methyl nitrate, nitric acid, and nitrate ion. The high intensity transitions, which occur near 6.5 eV in these molecules, are assigned to a transition to the 11B2(π0π*) state. The low intensity transitions, which occur below 6.5 eV, are assigned to transitions to 11B1(σ1π*) and 11A2(n 0π*) states. The 11A2(n 0π*) state remains at close to the same energy, 4.5 eV, throughout the series; whereas, the 11B1(σ1π*) state goes to increasingly high energy along the series. In nitrate ion, the 11A1(πD π*) and 21A2(σπ*) states, which lie above 6.5 eV in the other molecules, become degenerate with the 11B2(π0π*) and 11B1(σ1π*) states, respectively. Transitions to the 11A1(πD π*) state, which goes to lower energy across the series, may account for the high energy tail observed on transitions to the 11B2(π0π*) state. The first triplet state is assigned to the 13B1(σ1π*) and 13A2(n 0π*) state in nitrite and nitrate ion. respectively, the 13B1(σ1π*) state in nitromethane and the 13B2(π0π*) state in the nitric esters and nitric acid. The trends in the energies of these states along the series are related to the increasing electronegativity of the donor group attached to the nitro group. The 11,3A1(πD π*) states are characterized as intramolecular charge transfer states, which may be unstable to dissociation into donor and nitro group free radicals.