Theoretical investigation of the structure and vibrational spectrum of the Li₂CO₂ molecule

Abstract

The special features of the potential-energy surface (PES) and the paths for the intramolecular rearrangements of the Li₂CO₂ molecule have been investigated by the SCF method in the double-zeta Huzinaga—Dunning basis supplemented by d-type polarization functions in the carbon and oxygen atoms (DZ + P). It has been shown that there are two minima, which correspond to C_s and C_2ν structures, on the PES of the molcule. All the remaining structures of the molecule considered either correspond to saddle points or do not represent special points on the PES at all. The force constants, vibrational frequencies, and intensities of the vibrational transitions in the IR spectra have been calculated for both isomers in the DZ basis. The theoretical values of the vibrational frequencies and the isotope shifts are in good agreement with the experimental data obtained with matrix isolation: the mean deviation of the scaled theoretical frequencies for the C_s isomer amounts to 13 cm⁻¹ or 1.6%. An analysis of the Mulliken populations, as well as a comparison of the geometric parameters and force constants of the isomers of Li₂CO₂ with the corresponding parameters of the Li₂O and CO molecules and of the CO₂ ²⁻ ion, have shown that the chemical bonds in both isomers may be represented by the scheme (Li⁺)₂CO₂ ²⁻. The similarity between the PES's of the Li₂CO₂ and Li₂CO₃ molecules has been demonstrated and explained in the framework of the Gillespie—Nyholm model.