Spectroscopic Characterization of Matrix-Isolated Phenylcarbene and Cycloheptatetraene

The full UV/VIS and IR spectra are presented for phenylmethylene ( 1 ) and its rearrangement product, cycloheptatetraene ( 2 ). The spectra are assigned and discussed on the basis of CASSCF/CASPT2 excited-state and of scaled B3LYP force-field calculations, respectively. The optical spectrum of 1 is very similar in appearance to that of the iso-π-electronic benzyl radical, which demonstrates that the unpaired σ-electron does not interfere noticeably with the π-electronic structure. The IR spectrum of 1 is not very well reproduced by the unscaled B3LYP calculations because of pronounced anharmonicity of the Ph−C−H bending mode, and even a differentiated scaling of the harmonic force field cannot fully remedy this. In contrast, the IR spectrum of 2 is very well reproduced and the corresponding valence force field shows the expected properties for this cyclic allene.