NO+·O2 versus NO3+ – which is the more stable?

The geometries of the lowest stationary points on the triplet and singlet `NO3+ʹ potential energy surfaces are investigated. It is found that the skewed T-shaped NO+·O2(X̃ 3A″) state has the lowest energy, with a linear (Ã 3Σ−) minimum lying ∼1 kcal mol−1 higher in energy. About 1 eV higher in energy lie the cis and trans isomers of the closed-shell ã 1A′ state, with the cis state being lower in energy. Above the ã state lies the D3h X̃ 1A1′ state of NO3+ – the cation of the nitrate radical. It is found that the MPn series is rather oscillatory for NO3+, and so is probably unreliable. The binding energy, enthalpy and entropy of complexation of the X̃ 3A″ NO+·O2 complex are calculated and compared to available values. Ionospheric implications are noted.