The electronic spectrum and photodissociation of dinitrogen tetroxide (N(2)O(4)): Multireference configuration interaction studies.

Multireference configuration interaction (MRCI) calculations were performed for vertical excitation energies and potential curves of N(2)O(4) in D(2h) symmetry using the TZVPP basis set with diffuse functions on the nitrogens. The strong absorption of N(2)O(4) around 185 nm is assigned to the transition from the ground state to 1 (1)B(1u) (σ(O)→σ(∗) (N-N)) rather than 1 (1)B(2u) (π(O)→π(∗) (NO(2) ),n→σ(∗) (N-N)), as previously assumed. (N(2)O(4) is placed in the yz-plane, with N-N along z.) Transition to 1 (1)B(1u) is calculated to have an oscillator strength f=0.71 and is z-polarized, in agreement with the experimental observations. Another state, 2 (1)B(2u), lies close by, however, at a much lower f-value. The weak absorption around 340 nm is assigned to 1 (1)B(3u). Excitation to 1 (1)B(2u) is calculated at 227 nm. There is no clear assignment of a state for the observed shoulder around 260 nm. TD-DFT (time-dependent density functional theory) vertical excitation energies are close to MRCI values. MRCI singlet and triplet potential curves for the dissociation N(2)O(4)→2NO(2), combined with a table of NO(2) states correlating with those of N(2)O(4), indicate possible products of photodissociation at various wavelengths. The extensive literature on the photodissociation of N(2)O(4) is reviewed. DFT geometry optimizations have been performed on low-lying singlet and triplet states.