Benzene-ethene interactions as studied by ab initio calculations

For a search for pi-pi* and/or CH-x interactions between benzene and ethene, ab initio calculations were carried out at HF/6-31G**, MP2/6-31G**, and B3LYP/6-31G** levels of theory. Thirteen configurations of benzene and ethene complexes were optimized by the energy gradient techniques, of which only two were real energy-minimum forms and the others were saddle points or conical points. The global minimum configuration is one in which one of the tilted ethene C-H groups interacts with the C-C Jc-bond of benzene (configuration F-1). A configuration, in which benzene and ethene planes are parallel, was not obtained as a minimum except for MP2 calculations. The intermolecular distances obtained by HF and B3LYP theories were longer than the sum of van der Waals radii, whereas those obtained by the MP2 method were close to the sum. Dipole moments were also obtained that suggest the existence of charge transfer interactions. It is concluded that the molecular interaction energy between benzene and ethene mainly consists of dispersion forces with minor contributions of CT interactions, while the configuration of the global minimum is determined by CH-pi interactions. From the frequency analysis for the energy-minimum forms, the normal modes associated with the intermolecular CH-pi interactions were obtained at low frequency regions.