INFRARED AND RAMAN SPECTRA OF PURE AND MIXED ISOTOPIC ETHYLENE CRYSTALS: MYOPIC INTERMOLECULAR INTERACTIONS
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Date
1977
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Ohio State University
Abstract
Crystals of the partially-deuterated ethylenes have random molecular packing and lack translational symmetry; rigorously, the site symmetry is $C_{1}$. 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 $C_{i}$ site, cis- and 1,1-$C_{2}H_{2}D_{2}$ 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 $-d_{1}$, trans $-d_{2}$ and $-d_{3}$ 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 $C_{2}H_{4}$ 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.
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$^{1}$R. G. Whitfield and G. E. Leroi, Paper RR'1, 31st Symposium on
Author Institution: Department of Chemistry, Michigan State University
Author Institution: Department of Chemistry, Michigan State University