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 B 3LYP/6-31G** levels of theory. Thirteen configurations of benzene and ethen e complexes were optimized by the energy gradient techniques, of which only two were real energy-minimum forms and the others were saddle points or co nical points. The global minimum configuration is one in which one of the t ilted ethene C-H groups interacts with the C-C Jc-bond of benzene (configur ation F-1). A configuration, in which benzene and ethene planes are paralle l, was not obtained as a minimum except for MP2 calculations. The intermole cular 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 o f charge transfer interactions. It is concluded that the molecular interact ion 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 ana lysis for the energy-minimum forms, the normal modes associated with the in termolecular CH-pi interactions were obtained at low frequency regions.