Knowledge Bank

“Infrared and Raman spectra of cyclooctane have been obtained under several conditions of temperature and state of aggregation. The infrared spectra of gaseous, liquid, and solid cyclooctane contain essentially the same bands. No unequal dependence of band intensities on temperature is observed in either the Raman or infrared spectrum of the liquid. This behavior is taken to imply that only one structural isomer of cyclooctane is present under ordinary conditions. The number of lines observed in the vibrational spectrum and the presence of several coincidences serve in a general way to eliminate from consideration those structures for cyclooctane which possess either very high or very low symmetry or a center of symmetry. Actual selection of the most probable structure for the molecule is made by dividing the vibrational spectrum into regions characteristic of the group frequencies present in the system, and by comparing in each region the observed spectrum with that predicted for the various possible models. This approach reduces the number of reasonable structures to two, the “tub” form (point group $D2d$) and the “distorted crown” form (point group $D4$). Further consideration of coincidences to be expected from these two structures gives some evidence that the “tub” form is the most probable structure for cyclooctane. The assignment of frequencies to the fundamental vibrations and overtones of the molecule on the basis of the $D2d$ “tub” model is found to proceed without difficulty.”