Exploring the size dependence of cyclic and acyclic pi-systems on cation-pi binding

MP2(FULL)/6-311++G** calculations are performed on the cation-pi complexes of Li+ and Mg2+ with the pi-face of linear (ethylene, butadiene, hexatriene, and octatetraene) and cyclic (benzene, naphthalene, anthracene, phenanthrene and naphthacene) unsaturated hydrocarbons. The interaction energy is found to increase systematically as the size of the pi-system increases. The higher interaction energy is in good correlation with the extent of charge transfer. The increase in the interaction energy is more dramatic in the case of acyclic systems. The computations reveal that larger pi-systems tend to have higher complexation energy with the metal ions, which will have important implications in our understanding of the structural and functional aspects of metal binding.