Molecular Structures and Vibrational Spectra of ScF₃, YF₃, and LaF₃ as Calculated by the CISD+Q Method

Abstract

Molecular structures and vibrational spectra of ScF₃, YF₃, and LaF₃ were studied by the Hartree–Fock– Roothaan method in terms of second-order Möller–Plesset perturbation theory and by the configuration interaction method including all singly and doubly excited configurations and Davidson's correction for quartic excitations (CISD+Q). The atomic core orbitals are defined in terms of the effective relativistic potentials suggested by Stevens et al. The equilibrium nuclear configuration is shown to be planar (D _3h symmetry) for ScF₃ and YF₃ and pyramidal (C _3v ) for LaF₃ with a bond angle $\alpha _e (F{\kern 1pt} - {\kern 1pt} La{\kern 1pt} - {\kern 1pt} F) = 117.5^ \circ$. The inversion barrier of LaF₃ is very low: h = E(D _3h )- E(C _3v ) = 38 cm ^–1 (CISD+Q). The vibrational spectra were calculated by the variational method using the vibrational Hamiltonian describing the “nonrigidity” of a molecule with respect to out-of-plane deformation. The calculation results are compared with the previously published experimental data. The IR band assignments for matrix-isolated molecular conformations in a vapor over yttrium trifluoride were corrected.