Cornelius Peter Groen, Ad Oskam, and Attila
Kovács

**Theoretical study of mixed LiLnX _{4}
(Ln=La, Dy; X=F, Cl, Br, I) rare earth/alkali halide complexes.**

*Inorganic Chemistry*, 39 (2000) 6001-6008

The structure, bonding and vibrational properties
of the mixed LiLnX_{4} (Ln = La, Dy; X = F, Cl, Br, I) rare earth/alkali halide complexes were
studied using various quantum chemical methods (HF, MP2 and the
Becke3-Lee-Yang-Parr exchange-correlation density functional) in conjunction
with polarized triple-zeta valence basis sets and quasi-relativistic effective
core potentials for the heavy atoms. Our comparative study indicated the
superiority of MP2 theory while the HF and B3-LYP methods as well as less
sophisticated basis sets failed for the correct energetic relations. In
particular, *f* polarization functions
on Li and X proved to be important for the Li^{…}X interaction in
the complexes.

From the three characteristic structures
of such complexes, possessing one- (*C*_{3v}), two- (*C*_{2v}) or three-fold coordination (*C*_{3v}) between the alkali metal and the
bridging halide atoms, the bi- and tridentate forms are located considerably
lower on the potential energy surface then the monodentate isomer. Therefore
only the bi- and tridentate isomers have chemical relevance. The monodentate
isomer is only a high-lying local minimum in the case of X = F. For X = Cl, Br
and I this structure is found to be a second-order saddle-point. The bidentate
structure was found to be the global minimum for the systems with X = F, Cl and
Br. However, the relative stability with respect to the tridentate structure is
very small (1-5 kJ/mol) for the heavier halide derivatives and the relative
order is reversed in the case of the iodides. The energy difference between the
three structures as well as the dissociation energy decrease in the row F to I.
The ionic bonding in the complexes was characterized by natural charges and a
topological analysis of the electron density distribution according to
Bader’s theorem. Variation of the geometrical and bonding characteristics
between the lanthanum and dysprosium complexes reflect the effect of
’lanthanide contraction’. The calculated vibrational data indicate
that infrared spectroscopy may be an effective tool for experimental
investigation and characterization of LiLnX_{4} molecules.