PHENYLNITRENE - ENERGETICS, VIBRATIONAL FREQUENCIES, AND MOLECULAR-STRUCTURES

Upon the generation of phenylnitrene (C6H5N) by photolysis of phenyl azide, it is possible for the conversion to the cyanocyclopentadienyl radical (C5H4CN) or dehydroazepine (seven-membered ring) to take place, depending on experimental conditions. Little is known about the mechanisms or electronic states involved. Characterization of these species by spectroscopic means is difficult because it is frequently unknown which of these intermediates is predominant. For example, a recent analysis of high-resolution electronic spectra leads to the conclusion that the electronic spectrum long attributed to triplet phenylnitrene is due to the cyanocyclopentadienyl radical. This conclusion is supported by the present research. The electronic spectra, vibrational frequencies, and optimized structures (and hence rotational constants) of phenylnitrene are predicted using ab initio quantum chemical techniques, involving double zeta plus polarization (DZP) basis sets and the single and double excitation configuration interaction (CISD) method. The 3A2 state is the ground state, in consensus with all previous work. The theoretical ground-state vibrational frequencies are used to assign the recent experimental IR spectrum for phenylnitrene. The S0 state is predicted to have the same orbital occupation as T0; i.e., it is the open-shell singlet 1A2 state. This 1A2 state is 6200 cm-1 above T0 at the highest level of theory but is structurally remarkably different from T0. The S1 1A1 electronic state is qualitatively a superposition of the nitrogen n(x)2 and n(y)2 configurations and is predicted to lie 11 300 cm-1 above the T0 3A2 ground state. The T1 state is predicted to be highly puckered with a T0-T1 transition energy of 18 600 cm-1 and a barrier to planarity of 8000 cm-1.