Gas-phase infrared spectroscopy for determination of double bond configuration of monounsaturated compounds.
Gas-phase Fourier-transform infrared spectra allow unambiguous 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-1 has been used previously for the identification of trans bonds, the absorption at 3028-3011 cm-1 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-1, 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-1, respectively. Similarly, if a cis double bond is present at the penultimate carbon atom, this band appears at 3022-3021 cm-1. All the spectra of trans alkenyl acetates showed the expected C-H wag absorption at 968-964 cm-1. In addition, the spectra of (E)-2-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-1 band is representative of the C-H stretching vibration of cis compounds of RCH=CHR' type.
Attygalle
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Baker Laboratory, Department of Chemistry, Cornell University, Ithaca, New York 14853.
Svatos
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Wilcox
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Voerman
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D000475:Alkenes / Q000737:chemistry*; D005740:Gases / Q000032:analysis; D008968:Molecular Conformation; D010675:Pheromones / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods
D000475:Alkenes; D005740:Gases; D010675:Pheromones