Structure, vibrational spectra, and energetic stability of LnX4− ions (Ln=La, Lu; X=F, Cl, Br, I).

Abstract Molecular structure and properties of La and Lu tetrahalide ions LnX4-) are studied by the configuration interaction singles-and-doubles method augmented with quadruple excitation correction (CISD+Q) and by the fourth-order Möller-Plesset perturbation theory with account for single, double, triple, and quadruple excitations (SDTQ-MP4). The atomic inner shells are described by Stevens relativistic effective core potentials. Valence basis sets are augmented with diffuse s-, p-, and polarization d-, f-, and g-functions. The equilibrium configuration of nuclei in LnX4- ions was found to be tetrahedral. The equilibrium internuclear distances, quadratic force constants, vibrational frequencies, and IR intensities of LnX4- ions are compared with the corresponding parameters of La and Lu trihalide molecules (LnX3), calculated within the same approximations. Regularities in the behavior of molecular parameters on going along the LnF4-?LnCl4-?LnBr4-?LnI4- series and from La compounds to Lu compounds are revealed. Heights of the energy barriers to the LnX4- inversion through the square planar structures (Td? D4h?Td) are evaluated: 100–110 and 130–150 kJ/mol for LaX4- and LuX4-, respectively. Enthalpies of dissociation reactions LnX4-?LnX3+X- are calculated and the results obtained are compared with the available experimental data. [ABSTRACT FROM AUTHOR]