π-systems as lithium hydrogen bond acceptors:: Some theoretical observations

Ab initio calculations at the Hartree-Fock and correlated levels and density functional theory calculations have been performed with 6-31 + + G(d,p) and 6-311 + + G(d,p) basis sets on LiF and HF complexes of benzene, ethylene, and acetylene. Complex binding energies have been corrected for basis set superposition error, and zero point energy corrections have been done on Hartree-Fock binding energies. Computed results indicate that the complexes exist in different conformations and among them those with pi-lithium and pi-hydrogen bonds are the most stable. pi-lithium bonds are stronger than pi-hydrogen bonds. The computed binding energies and geometry of KF complexes correlate well with the available experimental results. LiF complexes with these pi systems are found to be weaker than Li+ complexes but they are stronger than Li atom complexes. Natural bond orbital analysis traces the origin of the weak interactions that stabilize the complex. Li, as Found in earlier cases, prefers the most symmetric site for interaction whereas proton prefers a nonsymmetric site in benzene complexes. Surprisingly, such a change of interaction geometry in LiF and HF complexes is found to change the donating pi-orbitals in the benzene complexes. (C) 1998 American Institute of Physics. [S0021-9606(98)30646-7].