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 gas-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 D2h model inadequate to describe the average structure from electron diffraction. Based on spectroscopic information on low-frequency modes, the electron diffraction data were approximated by a model consisting of a mixture of rigid conformers, differing only in the extent of the symmetric out-of-plane bending of the substituents. This gave the following geometrical parameters:  Cortho−Cipso−Cortho = 119.2 ± 0.2°, rg(C−C) = 1.402 ± 0.003 Å, rg(Cring−Csp) = 1.431 ± 0.003 Å, and rg(CC) = 1.211 ± 0.003 Å. The computed re values (MP2) are 119.2°, 1.401 Å, 1.430 Å, and 1.223 Å, respectively, with the Cipso−Cortho bond 0.016 Å longer than the central C−C bond. The HF/6-31G* geometries of ethynylbenzene and p-diethynylbenzene indicate that the interaction of the ethynyl group with the ring is not affected by the presence of another ethynyl group in the para position. Comparison with solid-state results shows no appreciable effect of crystal environment on the ring deformation in these molecules.