Potential nonrigidity of the NO3 radical

Abstract

The equilibrium geometry and vertical excitation energies of the nitrate radical (NO3) have been determined at the coupled-cluster singles and doubles level. Unlike most previous theoretical methods used to study this problem, the present calculations circumvent the difficulties associated with symmetry-broken orbitals by using a reference function composed of orbitals obtained in a closed-shell self-consistent field (SCF) calculation for the NO3 anion. Although these orbitals do not satisfy the SCF equations for NO3 itself, they turn out to be more suitable for the correlation problem in the neutral molecule than the unrestricted Hartree–Fock solution for NO3. Nevertheless, our calculations agree with most previous studies in predicting that the high-symmetry D3h structure of NO3 is not a minimum on the potential energy surface. The potential near the D3h point is relatively flat and has seven stationary points: three (equivalent) C2v minima with one long and two short N–O bonds; three (equivalent) C2v transition states with two long and one short N–O bonds; and the D3h structure, which is unstable with respect to the in-plane degenerate mode (e′) and is consequently a saddle point of index two. Exchange of the oxygen positions via pseudorotation around the D3h stationary point is predicted to be an extremely facile process with a barrier height of ≊190 cm−1, suggesting that the molecule may be spectroscopically nonrigid, belonging to a molecule symmetry group which is isomorphic with D3h, as observed experimentally. Excitation energies are calculated for both the D3h structure and points on the pseudorotation pathway, in order to predict differences between values obtained from photodetachment spectroscopy of the NO3 anion and those determined by direct excitation of NO3.