Gas-Phase Infrared Spectroscopy for Determination of Double Bond Configuration of Monounsaturated Compounds

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Abstract
Gas-phase Fourier-transform infrared spectra allow unam- biguous determination of the configuration of the double bonds of long-chain unsaturated compounds bearing RCH=CHR' type bonds. Although the infrared absorption at 970-967 cm-l has been used previously for the identification of trans bonds, the absorption at 3028-301 1 cm-l is conventionally considered to be incapable of distinguishing cis and trans isomers. In this paper, we present a large number of gas-phase spectra of monounsaturated long-chain acetates which demonstrate that an absorption, highly characteristic for the cis configuration, occurs at 3013-3011 cm-l, while trans compounds fail to show any bands in this region. However, if a double bond is present at the C-2 or C-3 carbon atoms, this cis =CH stretch absorption shows a hypsochromic shift to 3029-3028 and 3018-3017 cm-l, respectively. Similarly, if a cis double bond is present at the penultimatecarbon atom, this band appearsat 3022-3021 cm-l. All the spectra of trans alkenyl acetates showed the expected C-H wag absorption at 968-964 cm-l. In addition, the spectra of (E)-Z-alkenyl acetates show a unique three-peak "finger- print" pattern which allows the identification of the position and configuration of this bond. Furthermore, by synthesizing and obtaining spectra of appropriate deuteriated compounds, we have proved that the 3013-3011 cm-l band is representative of the C-H stretching vibration of cis compounds of RCH=CHR' type. Sex pheromones of moths (Lepidoptera) are usually mixtures of long chain aldehydes, acetates, and alcohols bearing one or more double bonds. Synthetic pheromone mixtures are now used for monitoring insect populations. An essential prerequisite for a successful synthetic attractant is the correct formulation. Very often the presence of minute amounts of a stereoisomer with the opposite geometric configuration inhibits drastically the attractivity of a synthetic pheromone. Therefore, the exact determination of the composition of natural and synthetic pheromone mixtures is vital for the successful application of pheromone technology to integrated pest management. However, this is not a trivial task, particularly for natural pheromones, since these com- pounds are often available only in nanogram quantities. Although excellent techniques are now available for deter- mining the position of the double bonds, thenumber of methods usable for elucidating double-bond configurations are rather

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