Note on the Spectra of the Disubstituted Acetylenes and of the Mustard Oils

It has been observed that in the Raman spectra of many disubstituted acetylenes the line corresponding to the so-called C≡C frequency is split into two components of not greatly different intensity (1), though this is not the case in the monosubstituted compounds. In the mustard oils the C≡N Raman line is double (2), and in allyl mustard oil (3), the one substance which appears to have been studied with sufficient dispersion, the corresponding infrared band shows definite indications of splitting into two components. This doubling is not observed in the organic thiocyanates. These facts have caused some speculation so far appear never to have received a satisfactory explanation. The presence of two fundamentals in the 2200 cm^-1 region appears quite impossible for most of the molecules showing the doubling, and on the other hand the coexistence of two forms of molecule with slightly different frequencies appears unlikely, especially in the case of the acetylenes. Consequently the author proposes an explanation involving a Fermi resonance interaction between the C≡C or C≡N fundamental, as the case may be, and the overtone of a certain other vibration of approximately one-half or one-third the frequency.