Knowledge Bank

Crystals of the partially-deuterated ethylenes have random molecular packing and lack translational symmetry; rigorously, the site symmetry is $C1$. However, our vibrational spectra of the neat, partially-deuterated ethylene solids suggested that a higher effective site symmetry” may be $operable.^{1}$ We have obtained infrared and Raman spectra of dilute mixed crystals of the isotopic ethylenes, where the guest molecule acts as a probe of the effective site symmetry” of the host. The number of energetically-inequivalent guest orientations depends on whether the guest distinguishes hydrogen from deuterium on neighboring molecules. Our spectra in each case can be interpreted on the basis of a centrosymmetric site symmetry, indicating myopic intermolecular interactions. In a $Ci$ site, cis- and 1,1-$C2H2D2$ should have only one spectroscopic component; whereas, the three other partially-deuterated ethylenes should have two energetically-inequivalent orientations. Thus, in their solid phases, the $-d1$, trans $-d2$ and $-d3$ isotopes can be considered as 50:50 mixed crystals; i.e., in the neat crystals 50% of the molecules assume one orientation and 50% the other. The ``nearsightedness” of the ethylenes suggests that the magnitude of the orientational splitting is independent of the host. We have obtained the splittings by observing the infrared and Raman spectra of 1% mixed crystals in a $C2H4$ host. The coherent potential approximation can be used to calculate the spectra of the 50:50 mixed crystals. Predictions based on this approximation will be compared to the observed vibrational spectra.