GAS-PHASE INFRARED-SPECTROSCOPY FOR DETERMINATION OF DOUBLE-BOND CONFIGURATION OF MONOUNSATURATED COMPOUNDS

Gas-phase Fourier-transform infrared spectra allow unambiguous determi nation of the configuration of the double bonds of long-chain unsatura ted compounds bearing RCH=CHR' type bonds. Although the infrared absor ption at 970-967 cm(-1) has been used previously for the identificatio n of trans bonds, the absorption at 3028-3011 cm(-1) is conventionally considered to be incapable of distinguishing cis and trans isomers. I n this paper, we present a large number of gas-phase spectra of monoun saturated long-chain acetates which demonstrate that an absorption, hi ghly characteristic for the cis configuration, occurs at 3013-3011 cm( -1), while trans compounds fail to show any bands in this region. Howe ver, 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 pre sent 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 whi ch allows the identification of the position and configuration of this bond. Furthermore, by synthesizing and obtaining spectra of appropria te deuteriated compounds, we have proved that the 3013-3011 cm(-1) ban d is representative of the C-H stretching vibration of cis compounds o f RCH=CHR' type.