MOLECULAR-STRUCTURE AND LARGE-AMPLITUDE MOTION OF P-DIETHYNYLBENZENE FROM GAS-PHASE ELECTRON-DIFFRACTION AND THEORETICAL CALCULATIONS

The molecular structure of p-diethynylbenzene has been determined by g as-phase electron diffraction and ab initio MO calculations at the HF/ 6-31G and MP2/6-31G*(fc) levels. The two ethynyl groups undergo large -amplitude bending motions, making the equilibrium D-2h model inadequa te to describe the average structure from electron diffraction. Based on spectroscopic information on low-frequency modes, the electron diff raction data were approximated by a model consisting of a mixture of r igid conformers, differing only in the extent of the symmetric out-of- plane bending of the substituents. This gave the following geometrical parameters: angle C-ortho-C-ipso-C-ortho = 119.2 +/- 0.2 degrees, [r( g)(C-C)] = 1.402 +/- 0.003 Angstrom, r(g)(C-ring-C-sp) = 1.431 +/- 0.0 03 Angstrom, and r(g)(C = C) = 1.211 +/- 0.003 Angstrom. The computed r(e) values (MP2) are 119.2 degrees, 1.401 Angstrom, 1.430 Angstrom, a nd 1.223 Angstrom, respectively, with the C-ipso-C-ortho bond 0.016 An gstrom longer than the central C-C bond. The HF/6-31G geometries of e thynylbenzene and p-diethynylbenzene indicate that the interaction of the ethynyl group with the ring is not affected by the presence of ano ther ethynyl group in the para position. Comparison with solid-state r esults shows no appreciable effect of crystal environment on the ring deformation in these molecules.