The infrared spectra and symmetry of fluorocompounds adsorbed on lanthanide halides: The fluorine bond

Abstract

When covalent fluorine-containing compounds are brought in contact with rare earth metal halides they adhere strongly to the surface. We have studied the i.r. spectra of the following compounds adsorbed on rare earth metal halides: CF 4CF 3Cl,CF 2Cl 2, HCF 2Cl,C 2F 4Cl 2, and C 2F 2H 2. In all cases the spectra of the adsorbed species resemble quite closely those of their gaseous analogs. The fundamental stretching modes remain essentially unaltered, but the bending modes are usually not observed; neither are the combination and overtone bands. We have seen no previous reports of this kind of bonding in the literature. We shall refer to it as fluorine bonding. Fluorine containing molecules would be expected to sit on the surface of rare earth metal halides in a manner which permits the maximum number of fluorine atoms to contact the surface consistent with the most probable structure. This requirement stabilizes the adsorbed molecule in a definite conformation, thus permitting the i.r. spectrum of that conformation to be measured. This prediction led us to conclude that our experimental observations could lead to a method of conformational analysis. We verified this conclusion by measuring the spectrum of C 2F 4Cl 2 on the surface of ErCl 3 and ErF 3. The molecule would be expected to sit on the surface in either the cis or gauche form. By comparison with published data we ascertained that the gauche form of the molecule is stabilized on adsorption. In the discussion below we have drawn many conclusions about the adsorbed molecules listed above. In the future we shall test other molecules and attempt to verify the suitability of the method for conformational analysis. Of particular interest was the failure of C 2F 2H 2 to be adsorbed. We hope to examine similar compounds and see if this is a peculiarity of unsaturated compounds.