Raman Spectra of Benzene and Isotope Effect

A faint line accompanying the strong line in the region of 990 cm—1 is observed in the Raman spectra of benzene, chlorobenzene, bromobenzene, toluene and cyclohexane. The relative intensities of the weak 984 cm—1 line and the strong 992 cm—1 line of benzene are measured to be in the ratio 6 : 100. This together with an approximate theoretical estimate support the suggestion that the weak line is due to molecules in which one of the carbon atoms is an isotope of mass 13. The exceedingly low intensity of the corresponding faint line in cyclohexane seems to point to a nonplanar structure. Similar study on the 944 cm—1 line in C6D6 is suggested.