’’Effective’’ site symmetry in molecular crystals. Orientational energies of dilute guest probes in isotopically mixed, partially deuterated ethylene crystals

Abstract

The midinfrared and Raman spectra of dilute mixed crystals of the partially deuterated ethylenes (C2H3D; cis-, trans- and 1,1-C2H2D2; C2HD3) in C2H4 and in one another have been obtained. The guest modes often exhibit multiplet structure arising from different relative orientations of the neighboring molecules. The number of energetically inequivalent orientations is diagnostic of the ’’effective’’ site symmetry of the host lattice (i.e., symmetry in the spectroscopic rather than the mathematical sense), which was found to be Ci for each of the partially deuterated ethylenes. Consistent with the predictions of the virtual crystal approximation, the number and energies of the orientational components of the guest molecules are independent of the isotopic nature of the host crystal, which indicates that the intermolecular force field is insensitive to the isotopic composition of the interacting ethylene molecules. The orientational effect can be used to distinguish among several possible effective site symmetries for any crystalline lattice, provided that the number of predicted orientational components differ.