Isomerism and vibration spectra of M2XO4 molecules (M = Li, Na, K; X = S, Se, Te, Cr, Mo, W)

Abstract

The equilibrium geometries, isomerization energies, force fields, vibration frequencies, and band intensities in the IR spectra of M2XO4 molecules (M = Li, Na, K; X = S, Se, Te, Cr, Mo, W) were calculated ab initio by the Hartree-Fock method in extended basis sets using relativistic effective core potentials. The relative energies of alternative structures were refined by the configuration interaction method taking into account single- and double-excited configurations, with the Davidson correction for quartic excitations. The results show that the chemical bonds between the metal atom and the acid residue XO4 are highly polar. The majority of M2XO4 molecules have two isomers. In both isomers the XO42- anion coordinates the metal cations M+ in the bisbidentate (bb) fashion. The equilibrium configurations ofthe nuclei in the ground (bb) and excited (bb') isomers have the D2d and Cs symmetry, respectively. In the bb isomer, the cations coordinate at the opposite, and in the bb' isomer, at the adjacent edges of the XO42- anion, having the shape of a distorted tetrahedron. The relative energy of the bb' isomer is 9-28 kJ mol-1. The energy barriers to intramolecular rearrangements bb'(C4s) → bb(D2d) are also low: 15-35 kJ mol-1. These results show that the M2XO4 molecules are structurally nonrigid, with a “polytopic” character of the M-XO4 chemical bonds. The calculation results were compared to the published experimental data on the structure and vibration spectra of the M2XO4 molecules.