Infrared spectrum of o-benzyne: Experiment and theory

Article abstract of DOI:10.1021/ja00027a007

The complete set of vibrational frequencies and absolute infrared intensities has been determined for o-benzyne and two of its isotopomers: C₆D₄ and 1,2-¹³C₂C₄H₄. 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 o-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 o-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 o-benzyne and is polarized along the symmetry axis. It is red-shifted by 2 cm^-1 in the perdeutero-o-benzyne and by 53 cm^-1 in the doubly ¹³C-labeled compound, in very good agreement with our theoretical prediction (MP2/6-31G**) and previous gas-phase data for o-benzyne.