Ground and excited states of isodiazene – an ab initio study Original Research Article

The ground state properties of isodiazene have been determined by standard ab initio approaches. The vertical excitation energies of isodiazene for the low-lying valence and Rydberg excited states have been calculated by the CASSCF and related MR-CISD and MCQDPT methods. The assignment of the experimental value of 1.95 eV to the transition to the à 1A2 (2b2 n, 2b1 π*) state has been confirmed. The experimentally observed absorption occuring in the range 3.96–4.88 eV, possibly at 4.77 eV, is assigned to the transition to the lowest-lying singlet Rydberg state B̃ 1B2 (2b2 n, 6a1 3s). The equilibrium structures of isodiazene in the lowest-lying singlet and triplet excited states 1,3(n,π∗) are found to be twisted with the Cs symmetry. It can be expected that isodiazene, as well as 1,2-diazene, has no well-defined equilibrium structures on the 1,3A′′(n,π∗) potential energy surfaces due to the low inversion barriers. The relative stabilities of isodiazene and 1,2-diazene are found to be quite different in the 1,3(n,π∗) states than in the ground state. For the vertical spectra of the two isoelectronic molecules, isodiazene and formaldehyde, it was found that the orders of singlet electronic states are similar, but the relative stabilities of triplet states are different and the vertical excitation energies of isodiazene are generally lower. The 1(n,π*)–3(n,π∗) gap is much greater for isodiazene (1.30 eV) than for formaldehyde (0.38 eV) and hence intersystem crossing of these states of isodiazene is highly improbable.