Raman study of crystalline ethylenes and the structure determination through model calculations of the lattice spectra

Abstract

Complete Raman spectra of the intra‐ and intermolecular regions of pure ethylene, ethylene‐d₄, and their dilute mixed crystals have been obtained. The observations agree with group theoretical predictions based on a $C_{\text{2h}}^5$ space group. Model calculations of the Raman lattice spectra for the two possible molecular orientations consistent with this space group were undertaken to evaluate which of these two structures more satisfactorily reproduces the experimental results. Various sets of carbon‐carbon, carbon‐hydrogen, and hydrogen‐hydrogen interaction parameters were utilized in the calculation of crystal energies, librational frequencies, and relative Raman scattering intensities. It is noteworthy that the predicted relative intensities are only very slightly dependent on the nature of the intermolecular potential chosen. This investigation provides strong evidence that the crystalline ethylene structure with ${\text{P\hspace{0.17em}12}}_1{\text{/n\hspace{0.17em}1\hspace{0.17em}(C}}_{\text{2h}}^5)$ space symmetry, the so‐called ``b structure,'' is correct.