Knowledge Bank

The vibrational spectra of the polycrystalline ethylenes: $-d1$, cis-$d2$ trans-$d2$. asym-$d2$, and $-d3$, are being investigated by infrared and Raman techniques. The absence of a center of symmetry in all but the trans-$d2$ molecule leads co obvious differences in the spectra, when compared to the $-doand-d4$ molecules previously reported. $\mathrm{<mrow\; data-mjx-texclass="ORD">1,2</mrow>}$ In addition, the loss of the $D2h$ symmetry in these compounds gives rise to the possibility that the molecules will assume non-translationally equivalent orientations in the crystal, and that therefore it may not be possible to interpret the spectra on the basis of perfect crystal theories. The more subtle consequences require a careful analysis of the number, spectroscopic activity, and frequency of the vibrational Davydov components, as well as comparison of the relative band widths. The only previously reported work on a complete series of isotopic molecular crystals concerned the deuterated $benzenes,3,4$ where Fermi resonance and other difficulties precluded a generalizable interpretation. Vibrational spectra of the partially deuterated polycrystalline ethylenes obtained up to the date of the symposium will be presented and analysed.