Molecular structure of p-diisocyanobenzene from gas-phase electron diffraction and theoretical calculations and effects of intermolecular interactions in the crystal on the benzene ring geometry

In this study, the molecular structure of p-diisocyanobenzene has been determined by gas-phase electron diffraction and quantum chemical calculations. The electron diffraction intensities from a previous study by Colapietro et al. (J Mol Struct 125:19-32, 1984) have been reanalyzed using geometrical constraints and initial values of vibrational amplitudes from computations. The equilibrium structure of the molecule has D (2h) symmetry, whereas the average geometry in the gaseous phase is best described by a non-planar model of C (2v) symmetry. The lowering of symmetry is due to large-amplitude motion of the substituents out of the plane of the benzene ring. The non-planar model has an internal ring angle at the ipso position, a (a)C2-C1-C6 = 120.6 +/- A 0.2A degrees, about 1A degrees smaller than that from the previous study, but consistent with the quantum chemical calculations. The mean length of the ring C-C bonds and the length of the triple bond are accurately determined as aOE (c) r (g)(C-C)> A = 1.398 +/- A 0.003 and r (g)(Na parts per thousand C) = 1.177 +/- A 0.002 , respectively. Comparison with the gaseous isoelectronic molecules p-diethynylbenzene and p-dicyanobenzene shows that the differences in the mean lengths of the ring C-C bonds and in the lengths of the triple bonds determined by electron diffraction are equal or closely similar to the corresponding differences from quantum chemical calculations. The present experimental value of the ipso angle in free p-diisocyanobenzene is slightly, but significantly smaller than that obtained by X-ray crystallography. The difference is confirmed by computational modeling of the crystal structure and appears to be due to -Na parts per thousand C center dot center dot center dot H-C intermolecular interactions in the crystal.