Gas-Phase Infrared Spectroscopy for Determination of Double-Bond Configuration of Some Polyunsaturated Pheromones and Related Compounds

Gas-phase Fourier transform infared spectroscopy allows the determination of the geometric configuration of disubstituted carbon-carbon double bonds present in linear polyunsaturated compounds on the basis of the characteristic absorptions observed in the 3036-3011 and 982-947 cm -1 regions. Although the 3036-3011 cm -1 band, attributable to a =CH stretch vibration, is conventional considered to be unreliable for cis/trans determinations, data obtained from a large number of mono- and polyunsaturated acetates show that many useful deductions can be based on this absorption. Double bonds of the RCH=CHR' type separated by at least two methylene groups, present in polyunsaturated compounds, give rise to an absorption at 3013-3011 cm -1 if the double bonds are not proximal to a functional group or a terminal methyl group. Cis dienes those double bonds are separated by only a single methylene group show the cis =CH stretch band at 3017 cm -1 . For methylene-interrupted polyenes, this band shows a gradual hypsochromic shift as the unsaturation increases. Furthermore, a band at 1391 cm -1 was recognized to represent the deformation vibration of the CH 2 groups located between the double bonds. The intensity of this band is proportional to the number of skipped methylene groups. None of the isolated or methylene-interrupted dienes bearing only trans double bonds showed any significant absorptions above 3000 cm -1 . However, spectra of trans-trans conjugated compounds show a =CH stretch band at 3016-3012 cm -1 as a well-defined shoulder on the high-frequency side of the CH 2 asymmetric stretch band, in addition to the wag absorption observed at 984982 cm -1 , which is highly characteristic for trans-trans conjugated compounds. The spectra of cis-trans or trans-cis conjugated compounds are virtually identical and show a characteristic «fingerprint» consisting of two bands of similar intensity at 978-976 and 949-946 cm -1 , in addition to the =C-H stretch band observed at 3021-3017 cm -1 . Interestingly, not only are the spectra of conjugated cis-cis compounds devoid of any significant absorption in the 982-947 cm -1 region, but also the =CH stretch band is broad and appear as a poorly defined shoulder on the high-frequency edge of the CH 2 asymmetric stretch band