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

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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 1 1 B 2 (π 0 π*) state. The low intensity transitions, which occur below 6.5 eV, are assigned to transitions to 1 1 B 1 (σ 1 π*) and 1 1 A 2 (n 0 π*) states. The 1 1 A 2 (n 0 π*) state remains at close to the same energy, 4.5 eV, throughout the series; whereas, the 1 1 B 1 (σ 1 π*) state goes to increasingly high energy along the series. In nitrate ion, the 1 1 A 1 (π D π*) and 2 1 A 2 (σπ*) states, which lie above 6.5 eV in the other molecules, become degenerate with the 1 1 B 2 (π 0 π*) and 1 1 B 1 (σ 1 π*) states, respectively. Transitions to the 1 1 A 1 (π D π*) state, which goes to lower energy across the series, may account for the high energy tail observed on transitions to the 1 1 B 2 (π 0 π*) state. The first triplet state is assigned to the 1 3 B 1 (σ 1 π*) and 1 3 A 2 (n 0 π*) state in nitrite and nitrate ion. respectively, the 1 3 B 1 (σ 1 π*) state in nitromethane and the 1 3 B 2 (π 0 π*) 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 1 1,3 A 1 (π D π*) states are characterized as intramolecular charge transfer states, which may be unstable to dissociation into donor and nitro group free radicals.
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