Molecular structure of ethynylbenzene from electron diffraction and ab initio molecular orbital calculations

The molecular structure and benzene ring distortions of ethynylbenzene have been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G^* and 6-3G^** levels. Least-squares refinement of a model withC_2v, symmetry, with constraints from the MO calculations, yielded the following important bond distances and angles:r_g(C_i-C_o)=1.407±0.003 Å,r_g(C_o-C_m)=1.397±0.003 Å,r_g(C_m-C_p)=1.400±0.003 Å,r_g(Cr_i-CCH)=1.436 ±0.004 Å,r_g(C=C)=1.205±0.005 Å, C_o-C_i-C_o=119.8±0.4°. The deformation of the benzene ring of ethynylbenzene given by the MO calculations, including o-C_i-C_o=119.4°, is insensitive to the basis set used and agrees with that obtained by low-temperature X-ray crystallography for the phenylethynyl fragment, C₆H₅-CC-, in two different crystal environments. The partial substitution structure of ethynylbenzene from microwave spectroscopy is shown to be inaccurate in the ipso region of the benzene ring.