Normal hydrocarbon spectra vibrations

The infrared active v (CH2), v (CH2), 8 (CH2), and yr (CH2) fundamentals can be readily assigned as a result of the extensive spectroscopic studies on hydrocarbons which have been undertaken [Sheppard and Simpson (795)]. In addition, because of the polarized radiation studies on single crystals of normal paraffins [Krimm (95)], it is possible to assign uniquely the components of the doublets found in the spectrum for these bands to symmetry species. Similarly, the Raman active va(CH. ), vs(CH2), (CHg), v+ (0), and v+ (n) fundamentals can be unambiguously assigned, the latter two on the basis of normal vibration calculations... 

We shall not be concerned here with the exact treatment of spectra by normal coordinate analysis 24, 29). Instead an attempt will be made to give some semiempirical rules, which will enable us to predict the correct number and types of vibrations in the infrared or Raman spectrum for a given symmetry. If the observed spectrum coincides with theoretical expectations, we can be reasonably sure that we predicted the correct symmetry. However, we cannot be absolutely certain, since one can never be sure that the number of frequencies found is not too low (or more seldom too high) for an assumed symmetry. It is not so much the correct application of the rules, but rather the interpretation of the experimental findings that is decisive. It is, therefore, in most cases involving hydrocarbon complexes, advisable and sometimes necessary to obtain additional evidence from measurements on deuterated compounds, from comparisons of the spectra of similar compounds, or from considerations of chemical facts, in order to make sure that the determination of a symmetry group is well-founded. 

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