ABSTRACT
H stretch next to a double bond occurs at a higher wavenumber (lower wavelength) than saturated C
H stretch absorptions. This peak is strong and distinct in some structures, particularly the methylene group of terminal double. In most cases, however, it is weak and difficult to locate especially in the presence of methyl groups. The band position is near 6100-6200 cm
(1640-1612 nm). The C-H stretch first overtone of terminal methylene groups of vinyl and vinylidene structures
is isolated enough that it can be used in traditional quantitative analysis. Figure 3.1 provides one example, and Table 3.1 provides some typical peak locations. Goddu
provides tables of absorptivities for the first overtone absorption of the terminal methylene group in a variety of compounds and solvents. Molar absorptivities are about 0.2-0.5 l/mol-cm. Put another way, a 100-ppm amount of methylene gives an absorbance of 0.01 in a 10-cm cell. Analyses using this peak to measure the vinyl content of acrylate monomers,
butadienes,
and edible oils
have been reported. In studies of wavelength displacement of deuterium substitution, hexadeuteropropylene
(CD
CD
fi
CD
) gives an asymmetric stretching of the CD
at 4610 cm
(2170 nm).
