Vibrational spectra of methyllithium and its aggregates: a new interpretation from ab initio anharmonic calculations

The complete quartic force field of methyllithium (CH₃Li) is computed at the B3LYP/cc-pVTZ level of theory. The vibrational energy levels calculated from a perturbational and a variational procedure are in agreement with the observed spectra except for the C–Li stretching and the symmetric methyl deformation modes for which a disagreement with the experimental assignment given by Andrews is apparent. This discrepancy between experiment and theory is so large that questions are raised either about a correct characterization of, or correct calculations for the monomeric species CH₃Li. Our theoretical study of methyllithium aggregates (CH₃Li)_n, with n = 2, 3, 4 and 6, gives a new interpretation of the experimental data.