HYDROCARBONS, C6 AND UP, NON-AROMATIC
The alkanes that have six carbon atoms or more all have similar spectra. The main feature of the spectra is the C-H stretch band near 3000 cm-1, which is usually the band used for measurement. When a complex mixture of these higher molecular weight alkanes is examined, as in measurements of gasoline vapor, the infrared spectrum does not readily distinguish one molecule from another. The spectrum does, however, give an estimate of the total amount of alkane, which is usually expressed in hexane equivelants. If the mixture contains only a small number of alkanes--perhaps three or four--the individual molecules may be distinguished from each other and measured by means of the skeletal vibration bands. These fall in the region 1200 to 500 cm-1; they are much weaker than the main C-H bands. The intensity ratio of the two bands between 1500 and 1400 cm-1 may be used as an indicator of degree of chain branching, which is related to the octane number. Compare the spectra of n-Octane and 2,2,4-trimethylpentane.
In the alkenes, the occurrence of the double bond in the carbon skeleton affects the vibrational frequencies and absorption intensities in significant ways. This creates distinguishing characteristics in the spectra. The differences between the spectra of 2-methyl-1-pentene and 2-methyl-2-pentene offer a good example.
The different positions of the double bond in the molecules of the alpha and beta pinenes cause distinct differences in the spectral region 900 to 700 cm-1 that allow one isomer to be distinguished from the other. See SPECTRA, chapter C.