The equilibrium geometric parameters, isomerization energies, force fields, vibrational frequencies and intensities in the infrared spectra of MReO₄ molecules (M = Li, Na, K) are calculated by the CI 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. The MReO₄ molecules have two isomers. The main isomer with C_2v symmetry has bidentate b coordination of the M⁺ cation to the ReO₄^- anion. The excited isomer is of C_3v symmetry and corresponds to monodentate m coordination of M⁺.  The low relative energies of the m isomers (17-29 kJ/mol) and the low energy barriers to the intramolecular m ==> b rearrangements (5-11 kJ/mol) indicate that the 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 the MReO₄ molecules.

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