Geometry and Infrared Spectra of Matrix-Isolated Rare-Earth Halides. I. LaF3, CeF3, PrF3, NdF3, SmF3, and EuF3

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Abstract
The infrared absorption spectra (40–800 cm−1) of the trifluorides of lanthanum,cerium,praseodymium,neodymium,samarium, and europium isolated in solid argon, krypton, and nitrogen matrices have been obtained. For LaF3, CeF3, SmF3, and EuF3 only the asymmetric stretching mode and the bending modes were observed; these molecules are assigned a planar configuration. For PrF3 the symmetric and asymmetric stretching modes were observed together with the bending modes; PrF3 is accordingly assigned a pyramidal configuration. At present the spectrum of NdF3 cannot be satisfactorily interpreted, but a number of possibilities are explored. Using a valence force field approximation, force constants have been derived from the measured fundamental frequencies and the infrared‐inactive symmetric stretching frequency of the planar molecules has been estimated. Statistical entropies have also been calculated from the vibrational data and molecular geometries and are compared with third‐law values.