The equilibrium geometries, isomerization energies, force fields, vibration frequencies, and band intensities in the infrared spectra of M₂XO₄ molecules (M = Li, Na, K; X = S, Se, Te, Cr, Mo, W) were calculated by the Hartree-Fock method in extended basis sets using relativistic effective core potentials. The relative energies of alternative structures were refined by the CI 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 XO₄^2- anion coordinates the metal cations M+ in the bisbidentate (bb) fashion. The equilibrium configurations of the nuclei in the ground (bb) and excited (bb') isomers have the D_2d and C_s symmetry, respectively. In the bb isomer, the cations coordinate at the opposite, and in the bb' isomer, at the adjacent edges of the XO₄^2- anion, having the shape of a distorted tetrahedron. The relative energy of the bb' isomer is 9-28 kJ/mol. The energy barriers to intramolecular rearrangements bb'(C_s) ==> bb(D_2d) are also low: 15-35 kJ/mol. These results show that the M₂XO₄ molecules are structurally nonrigid, with a "polytopic" character of the M-XO₄ chemical bonds. The calculated results were compared to the published experimental data on the structure and vibration spectra of the M₂XO₄ molecules.

See more...