Theoretical studies on cation-π interactions III.: Quantum chemistry investigation on the configurations of and interactions in alkali metal cation-benzene complexes

ab initio Hartree - Fock SCF, MP2 and density - functional theory B3LYP methods with different basis sets have been used to optimize the possible geometries of the alkali metal cation - benzene (M+... C6H6) complexes. The calculation results show that the most stable geometries of alkali metal cation - benzene complexes are the configuration with C-6v, symmetry axis. where the alkali metal cations are located above the benzene ring. The calculated IR spectrum show that these structures are reasonable. The bond lengths, total atomic charge, the coefficients of the molecular orbitals, frontier orbital energies, and Mullicken bonding population (MBP) of the complexes indicate that the interaction between alkali metal cations and benzene involve p - pi interaction. On the other hand, electron is transfered from the benzene to the alkali metal cations, and thus gives charge transfer complexes. The calculated results also imply that the interaction characteristic of the alkali metal cation - benzene complexes are similar to hydrogen bonding. However, the theoretical thermodynamic parameters demonstrate that the interaction strength beteween the alkali metal cation and benzene is larger than that of the typical hydrogen bond, especially for the lithium cation - bezene, of which the formation enthalpy is as large as that for a chemical bond. The normal mode analysis of the predicted vibrational frequency shows that the characteristic vibration made of the complex is located at about 200 cm(-1), which corresponds to the back and forth vibration of the alkali metal cations above the bezene ring plane.