Ground and excited states of isodiazene - an ab initio study

The ground state properties of isodiazene have been determined by standard ab initio approaches. The vertical excitation energies of isodiazene for th e low-lying valence and Rydberg excited states have been calculated by the CASSCF and related MR-CISD and MCQDPT methods. The assignment of the experi mental value of 1.95 eV to the transition to the (A) over tilde(1)A(2) (2b( 2) n, 2b(1) pi*) state has been confirmed. The experimentally observed abso rption 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 (B) over tilde (1)B(2) (2b(2) n, 6a(1) 3s). The equilibrium structures of isodiazene in th e lowest-lying singlet and triplet excited states (1,3)(n,pi*) are found to be twisted with the C-8 symmetry. It can be expected that isodiazene, as w ell as 1,2-diazene, has no well-defined equilibrium structures on the (1,3) A "(n,pi*) potential energy surfaces due to the low inversion barriers. The relative stabilities of isodiazene and 1,2-diazene are Found to be quite d ifferent in the (1,3)(n,pi*) states than in the ground state. For the verti cal spectra of the two isoelectronic molecules, isodiazene and formaldehyde , it was found that the orders of singlet electronic states are similar, bu t the relative stabilities of triplet states are different and the vertical excitation energies of isodiazene are generally lower. The (1)(n,pi*)-(3)( n,pi*) 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 hi ghly improbable. (C) 2000 Elsevier Science B.V. All rights reserved.