INFRARED-SPECTRUM OF ORTHO-BENZYNE - EXPERIMENT AND THEORY

The complete set of vibrational frequencies and absolute infrared intensities has been determined for omicron-benzyne and two of its isotopomers: C6D4 and 1,2-(C2C4H4)-C-13. In addition, for the majority of the transitions symmetries were assigned from infrared linear dichroism of the matrix-isolated samples, photooriented with polarized light during several photochemical transformations. Thermal relaxation of the high static pressure created by the initial photofragmentation causes dramatic changes of the fine site structure of each band of omicron-benzyne and results in a single-site infrared absorption spectra. A high-resolution, single-site vibrational spectrum was also obtained independently from laser hole-burning experiments. Band-shape analysis in different inert gas matrices (Ne, Ar, Xe, N2, and CO) greatly facilitates the correlation of isotopomer bands with those of unlabeled omicron-benzyne. The triple bond stretching vibration appears at 1846 cm-1 in a Ne matrix, with an experimental absolute intensity of 2.0 +/- 0.4 km/mol in the unlabeled omicron-benzyne and is polarized along the symmetry axis. It is red-shifted by 2 cm-1 in the perdeutero-omicron-benzyne and by 53 cm-1 in the doubly C-13-labeled compound, in very good agreement with our theoretical prediction (MP2/6-31G**) and previous gas-phase data for omicron-benzyne.