Characterization of vinyl-substituted, carbon-carbon double bonds by GC/FT-IR analysis

Vapor-phase infrared spectra allow the determination of the stereochemistry of carbon-carbon double bonds conjugated with a vinyl group, Cis and trans isomers of unsubstituted 1,3-alkadienes can be differentiated on the basis of the differences observed in the 900-1000 cm(-1) region (spectra of cis isomers show two bands at 993 and 906 cm(-1), while those of trans compounds show three absorptions at 998, 949, and 902 cm(-1)) and the 1590-1650 cm(-1) region (the C=C stretch bands are observed at 1595 and 1642 cm(-1) for cis compounds and at 1604 and 1650 cm(-1) for trans compounds), Compounds bearing CH2=CHC(CH3)=CHCH2- and CH2=CHC(=CH2)-CH2- structural moieties, referred to as alpha- and beta-type compounds, are frequently encountered as natural products, For compounds bearing alpha-type groups, the cis/trans configuration of the trisubstituted double bond can be determined unambiguously. An absorption at 3095-3091 cm(-1), for the =CH2 stretch vibration, is common to both of these groups; however, due to the presence of two =CH2 groups; the relative intensity of the band is much higher for beta-type compounds, For alpha-type compounds, a cis configuration at the C-3 carbon atom is characterized by a =CH2 wag absorption at 907-906 cm(-1), For beta-type compounds and 3E-alpha-type compounds, this band appears at 899-897 cm(-1). In addition, a wavy ''fingerprint'' pattern with two minima at 1632 (low intensity) and 1595-1594 cm(-1) (high intensity) is characteristic for beta-type compounds, Our generalizations are based on spectra of cis and trans ocimene, myrcene, and dehydration products of many 3-methyl-1-alken-3-ols. Six isomers of farnesene can be characterized by GC/FT-IR Furthermore, gas-phase IR allows the determination of the configuration of the trisubstituted double bond at C-3 in alpha-type farnesene congeners, For example, the homo- and bishomofarnesene isomers from Myrmica ants were shown to include a 3Z bond.