AB-INITIO STUDY OF THE MOLECULAR-STRUCTURES, FORCE-FIELDS, AND VIBRATIONAL-SPECTRA OF ALKALINE METAL FLUORIDE DIMERS MM'F-2 (M, M' = LI, NA, K)

Ab initio calculations of the equilibrium geometrical parameters, forc e constants, and IR vibration frequencies and Intensities of Li2F2, Na 2F2, K2F2, LiNaF2, LiKF2, and NaKF2 are reported. The calculations use the Hartree-Fock-Roothaan method and second-order Moller-Plesset pert urbation theory along with configuration interaction theory including singly and doubly excited configurations and corrections for quartic e xcitations with basis sets of grouped Gaussian functions: Li - (9s3p1d /4s3p1d), Na - (12s8p1d/6s4p1d), K - (14s11p3d/9s8p3d), F - (9s5p1d/4s 2p1d). According to the results of calculations, the equilibrium struc tures of the molecules are planar cyclic structures of D-2h (for M2F2) and C-2v (for MM'F-2) symmetries. The linear configurations M-F-M'-F (of C-infinity v symmetry) are 70-190 kJ/mole less stable than the cyc lic ones; for all molecules except M-F-K-F, these configurations corre spond to local minima on the potential energy surface. The role of cor relation effects in ab initio calculations of the geometry, force fiel ds, and LR characteristics of molecules with highly polar chemical bon ds is discussed. The theoretical force fields of the molecules are rep resented in canonical form in a system of redundant natural vibrationa l coordinates. The force fields of MM'F-2 molecules are studied. The r esults of the ab initio calculations are compared with the experimenta l structural data and vibrational spectra available in the literature.