Theoretical studies of the low-lying electronic states of diazirine and 3,3 '-dimethyldiazirine

The low-lying electronic states of diazirine and 3,3' -dimethyldiazirine ha ve been studied by high level ab initio quantum chemical methods. The equil ibrium geometries of the ground state and the first excited singlet and tri plet states have been optimized using the Hartree-Fock (HF) and complete ac tive space SCF (CASSCF) methods, as well as using the Moller-Plesset second order perturbation (MP2) theory and the single configuration interaction ( CIS) theory. It was found that the first excited singlet state is of B-1(1) symmetry resulting from the n-pi* transition, while the first excited trip let state is of B-3(2) symmetry resulting from the pi-pi* transition. The h armonic vibrational frequencies have been calculated at the optimized geome try of each electronic state, and the scaled frequencies have been compared with the experimental frequencies available. The adiabatic and vertical tr ansition energies from the ground electronic state to the low-lying electro nic states have been estimated by means of multireference methods based on the CASSCF wavefunctions, i.e., the multiconfigurational quasidegenerate se cond order perturbation (MCQDPT2) theory and the CASSCF second-order config uration interaction (CASSCF-SOCI) theory. The vertical transition energies have also been calculated by the CIS method fur comparison. The computed tr ansition energies, particularly by MCQDPT2, agree well with the experimenta l observations, and the electronic structures of the molecules have been di scussed, particularly in light of the controversy over the existence of the so-called second electronic state.