Evidence for a C-H...n type weak interaction:1:1 complex of styrene with acetylene studied by mass selective high-resolution UV spectroscopy and ab initio calculations

摘要

The 1:1 complex of styrene with acetylene has been studied by mass selective low-and high-resolution UV resonance-enhanced two-photon ionisation(R2PI)spectroscopy combined with genetic-algorithm-based computer-aided fit of the spectra with partial rotational resolution,and high level ab initio quantum chemistry calculations.Two stable conformeric geometries of the 1:1 complex of styrene and acetylene have been theoretically found:one with acetylene binding to styrene as a proton donor,and one with acetylene acting as a proton acceptor.From the analysis of the vibronic structure of the Si <-S_0 spectrum and the fit of the highly resolved spectrum of the 0_0~0 origin band of the complex it is shown that the favoured conformation is the one in which acetylene binds to the benzene ring of styrene through formation of a non-conventional hydrogen bond of C-H...pi type with no marked change of the transition moment orientation of styrene.The styrene moiety remains planar and the acetylene molecule is tilted by a small angle of 4° relative to the C6 symmetry axis of the benzene ring,most likely due to the reduced symmetry of the benzene ring pi electrons rather than to a direct interaction with the vinyl group.