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.