Basis Set and Electron Correlation Effects on Lithium Carbenoid Dimerization Energies

A systematic investigation was performed to determine which levels of theory are required to obtain accurate geometries and dimerization energies for lithium carbenoids. Dimerization free energies of six lithium carbenoids were calculated in the gas phase at the B3LYP and MP2 levels with several different basis sets and at the CCSD(T) level with the aug-cc-pvdz basis set. It was found that the calculated thermal corrections to the dimerization free energy, obtained from frequency calculation, were relatively independent of the basis set for basis sets larger than 6-31+G(d) and that smaller basis sets often generated qualitatively correct results. The dimerization free energies were usually overestimated by small basis sets compared to those using polarization and diffuse functions, whereas only modest gains were obtained by using basis sets larger than 6-31+G(d). The B3LYP DFT method generated geometries consistent with the MP2 and CCSD methods for halomethyllithium carbenoids, but not always for 1-halovinyllithium carbenoids. The B3LYP calculations also severely underestimated the dimerization free energies for the halovinyllithium carbenoids, compared to the MP2, and coupled cluster methods.