Determination of the Symmetry of the First Excited Electronic State of Benzyl by Rotational Contour Analysis of Vibronic Bands of the Emission Spectra of C6H5CH2, C6H5CD2, and C6D5CD2

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Abstract
Theoretical calculations have differed in their predictions as to the symmetry of the first excited state of benzyl. Furthermore, conflicting interpretations have been given to previous experimental results. Rotational band contour analysis of high resolution gas phase emission spectra of C6H5CH2, C6D5CD2, and C6H5CD2 are here used to determine this electronic symmetry. A discussion is given of the treatment of new problems which arise in the band contour analysis of emission spectra and of free radicals as compared with previous contour work on absorption spectra of stable molecules, in particular, as regards the determination of molecular geometry, temperature effects and contour program modifications. Qualitative and quantitative criteria for matching experimental and calculated contours are made explicit. Nonmatching features are assigned to hot bands, using supporting assignments. The effect on calculated contours of the interpolation procedure used in Parkin's program is discussed in the Appendix. Good fit of calculated to experimental contours for type A and type B vibronic bands of the three isotopic benzyl radicals, including the vibronic origin band of the C6H5CH2 species, leads to the conclusion that the first doublet‐doublet electronic transition in benzyl is 1 2A2–1 2B2. This agrees with an earlier experimental assignment, based on the observation of a2 vibrations in the fluorescence spectrum of benzyl in crystalline cyclohexane solution but is in disagreement with the 2 2B2–1 2B2 assignment of Johnson and Albrecht based on three‐step photoselection studies in glassy solutions.