An Ab Initio Study of the Ground and Excited State Chemistry of Phenyldiazirine and Phenyldiazomethane

Phenyldiazirine and phenyldiazomethane were studied at the B3LYP/6-31+G(d) and RI-CC2/TZVP levels of theory, and the three lowest singlet excited states of both compounds were optimized at the RI-CC2/TZVP level. The calculations predict that the SI state of phenyldiazirine is a sigma --> pi* state, with a quinoidal structure, and the C-N bonds of the diazirine group are slightly deformed from the C. symmetry of the ground state's geometry. Both the S-2 and S-3 states are predicted to be pi --> pi* states localized primarily on the phenyl group. The S, state was predicted to have an exceedingly large dipole moment with a strong and distinct aromatic C=C vibrational mode around similar to 1600 cm(-1), which is not present in any of the other electronic states examined in this study. The calculations are consistent with the assignment of the SI state of phenyldiazirine to the polar intermediate recently observed by ultrafast time-resolved UV-vis and IR spectroscopic studies of arylhalo- and arylalkyldiazirines. The excited states of phenyldiazomethane were also studied, and the implications of interconversion between phenyldiazirine and phenyldiazomethane are discussed. The calculations predict that the chemistry of the ground state and the SI excited state of phenyldiazirine are very different. Formation of phenylcarbene is favored on the ground state surface of phenyldiazirine, whereas the SI excited state favors isomerization to the first excited state of phenyldiazmethane, which rapidly extrudes nitrogen and forms carbene.