Raman frequencies of cumulated double bonds

Table 7.32 Raman Frequencies of Cumulated Double Bonds...

Allenes like other cumulative double bond systems do not show a normal double bond absorption. They have instead, a high frequency antisymmetric absorption which is usually of medium intensity in the infra-red, and a symmetric band at lower frequency which is usually very weak or forbidden in the infra-red and is best seen in the Raman spectra. The antisymmetric band occurs at 1980—1945 cm" in mono-substituted allenes, and at 1930—1955 cm" when di-substituted so that there is no terminal =CH2. Asymmetric di-substitution of the latter type gives this band at 1930— 1915 cm". The symmetric mode appears strongly in the Raman spectra near 1080 cm". ... 

Chapter 4 examines the group frequencies of triple bonds and cumulated double bonds. The alkyne hydrocarbons (acetylenes, —C=C—) are treated in I and the important nitrile group, —C=N, in detail in II. In the case of the alkynes the amount and type of substitution can often be identified from vibrational data (I, D, E). Systems containing cumulative double bonds (so-called back-to-back double bonds, >C=C=C<) such as the allenes in the hydrocarbon series are considered in III. A number of them contain hetero-atoms as for example, the ketenes (>C=C=0, III, B). Section F presents a problem that allows the reader to determine which of two possible structures is the correct one based on infrared and Raman data. 

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See also in sourсe #XX -- [ , ]

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