Isomerism and Vibrational Spectra of Alkali Metal Perrhenates: Ab Initio CISD+Q Calculations

Abstract

The equilibrium geometric parameters, isomerization energies, force fields, vibrational frequencies and intensities in the IR spectra of MReO4 molecules (M = Li, Na, K) are calculated by the configuration interaction method including all singly and doubly excited configurations with Davidson's correction for quartic excitations in expanded basis sets using effective relativistic core potentials. The calculations indicate that the chemical bonds between the metal atoms and the ReO4 anion group are highly polar. It is found that the MReO4 molecule has two isomers. The basic isomer with C2v symmetry has bidentate b coordination of the M+ cation to the ReO 4 - anion. The excited isomer is of C3v symmetry and corresponds to monodentate m coordination of M+. The low relative energies of the m isomers (17-29 kJ/mole) and the low energy barriers to the intramolecular m→b rearrangements (5-11 kJ/mole) indicate that MReO4 molecules are structurally nonrigid systems where the M–XO4 chemical bonds are “polytopic.” The results of calculations are compared with the available literature data on the structure and vibrational spectra of MReO4 molecules.