Some Characteristic Frequencies in the Raman Spectra of Saturated Aliphatic Hydrocarbons

Abstract

An analysis of the data presented in a recent compilation of Raman spectra of saturated hydrocarbons has been carried through with a view to the determination and assignment of vibration frequencies that are characteristic of particular molecular groups. Suggested approximate assignments have been made for the rocking and wagging modes of CH₂ groups (720–760 cm⁻¹ and 1300–1340 cm⁻¹, respectively) and for the deformation modes of CH groups (1330–1360 cm⁻¹) in such hydrocarbon molecules. Assignments previously made for the analysis of the Raman spectra of isobutane (trimethyl methane) and neopentane (tetramethyl methane) have been reviewed, and the results applied to the interpretation of observed characteristic frequencies of singly and double branched paraffins. Single branching of the hydrocarbons gives rise to characteristic frequencies in the regions 1180–1140 cm⁻¹ and 830–790 cm⁻¹, whereas double branching gives similar frequencies in the regions 1260–1190 cm⁻¹ and 760–710 cm⁻¹. These can probably be assigned to C–C stretching modes of vibration of the molecules. Other characteristic frequencies in the regions 970–950 cm⁻¹ and 930–910 cm⁻¹ occur in the spectra of single and doubly branched hydrocarbons, respectively, when these have at least two methyl groups attached to the branching carbon atom. They are probably caused by methyl wagging modes of these groups.