Rotational spectrum and structural analysis of CH⋯F interactions in the vinyl fluoride⋯1,1-difluoroethylene dimer

Abstract

The structure of the weakly bound dimer of 1,1-difluoroethylene with vinyl fluoride was studied using Fourier-transform microwave spectroscopy in the 6-19 GHz range. In the observed dimer structure each monomer acts as both a weak bond donor and acceptor, giving a cyclic arrangement of C-H…F contacts. The difluoroethylene C=C bond is roughly perpendicular to the vinyl fluoride C=C bond, forming the cross of a T-shaped carbon atom framework, resembling the lowest energy structure predicted by MP2/6-311++G(2d,2p) calculations. Observed C-H…F distances are similar to those of vinyl fluoride and 1,1difluoroethylene complexes with difluoromethane and HCCH. The dipole moment of the dimer was measured using the Stark effect, giving values of μ a = 0.9003(19) D, μ b = 0.030(8) D and μ total = 0.9008(22) D. A second ab initio structure, with the C=C bonds in a slipped parallel arrangement, was predicted to be about 38 cm-1 higher in energy than the T-shaped conformation. This higher energy arrangement has not been observed experimentally.