C-13 dipolar NMR spectrum of matrix-isolated o-benzyne-1,2-C-13(2)

The C-13 dipolar NMR spectrum of o-benzyne-1,2-C-13(2) in an argon matrix at a temperature of about 20 K is reported. The o-benzyne was generated from matrix-isolated phthalic anhydride-1,2-C-13(2) by UV irradiation. The average conversion to o-benzyne over the entire sample was about 53%; however, spectral simulations of the pure phthalic anhydride and of the o-benzyne-phthalic anhydride mixture were adequate to extract the NMR spectral parameters of both compounds. The simulation of the spectrum of o-benzyne gives a bond length of 1.24 +/- 0.02 Angstrom for the ''triple'' bond, in good agreement with the results of ab initio geometry optimizations. ?The experimental principal values of the chemical shift tensor of the labeled carbon in o-benzyne are 266, 240, and 73 ppm relative to TMS. The delta(33) component is not perpendicular to the molecular plane as is common in aromatic systems. Instead, the principal axis system is very similar to that of the alkyne carbon in natural abundance cycloctyne, with delta(33) in the molecular plane and about 7 degrees from the ''triple'' bond. Ab initio calculations of the chemical shift tensor of o-benzyne were completed at the SCF, MP2, and DFT levels using the GIAO method. The DFT calculations agree best with the experimental results.