CHAPTER 6 - TRIPLE BONDS AND CUMULATED DOUBLE BONDS

Abstract

Triple bonds (X≡Y—Z) and cumulated double bonds (X=Y=Z) absorb in the same region, 2300–1900 cm −1 . The vibrations involved in them are the stretching of the triple bond and the out-of-phase stretching of the cumulated double bonds. This chapter illustrates these vibrations and presents the bands expected for various triple bond stretching vibrations and cumulated double bond stretching vibrations. Many of the compounds of these types are actually resonance hybrids of triple–single and double–double bond forms. Usually, compounds that are predominantly X=Y=Z absorb in the infrared considerably more strongly than the compounds that are predominantly X≡Y—Z. The reverse intensity relationship tends to hold for Raman spectra. As this region of the spectrum is relatively free of bands, even weak bands in that region are distinctive and reliable. Sometimes multiple bands appear in that region where a single band is expected. When it occurs, it is the result of Fermi resonance type interaction. The chapter presents a list of the spectral regions for such types of groups.