Ab initio study of the molecular structures, force fields, and vibrational spectra of alkaline metal fluoride dimers MM′F₂ (M, M′=Li, Na, K)

Abstract

Ab initio calculations of the equilibrium geometrical parameters, force constants, and IR vibration frequencies and intensities of Li₂F₂, Na₂F₂, K₂F₂, LiNaF₂, LiKF₂, and NaKF₂ are reported. The calculations use the Hartree-Fock-Roothaan method and second-order Möller-Plesset perturbation theory along with configuration interaction theory including singly and doubly excited configurations and corrections for quartic excitations with basis sets of grouped Gaussian functions: Li — (9s3p1d/4s3p1d), Na — (12s8p1d/6s4p1d), K — (14s11p3d/9s8p3d), F — (9s5p1d/4s2p1d). According to the results of calculations, the equilibrium structures of the molecules are planar cyclic structures of D_2h (for M₂F₂) and C_2v (for MM′F₂) symmetries. The linear configurations M-F-M′-F (of C_∞v symmetry) are 70–190 kJ/mole less stable than the cyclic ones; for all molecules except M-F-K-F, these configurations correspond to local minima on the potential energy surface. The role of correlation effects in ab initio calculations of the geometry, force fields, and IR characteristics of molecules with highly polar chemical bonds is discussed. The theoretical force fields of the molecules are represented in canonical form in a system of redundant natural vibrational coordinates. The force fields of MM′F₂ molecules are studied. The results of the ab initio calculations are compared with the experimental structural data and vibrational spectra available in the literature.