Methylene groups first overtone

The principal saturated hydrocarbon functional groups of concern are methyl, methylene and methyne (—CH3, —CH2—, = CH—). The spectra of typical hydrocarbon mixtures (for example as in gas oil and gasoline) are dominated by two pairs of strong bands in the first overtone and combination regions (5900-5500 cm-1 and 4350-4250 cm-1). These are predominantly methylene (—CH2—). The methyl end groups typically show up as a weaker higher-frequency shoulder to these methylene doublets. 

A further development in the characterization resulted from a study of the near infrared region, by Washburn and Mahoney who showed that the first overtone of the cyclopropyl C-H stretching frequency absorbs at 6097 cm with a combination band also present at ca. 4465 cm . The overtone band is sharp though weak and separated from the first overtone of the saturated aliphatics. Any possible ambiguity with terminal methylene groups can be removed in infrared spectroscopic terms by analysis of the fundamental of the methylene. 

The general absorptions of methylene groups are illustrated in Figure 2.7, a spectrum of n-decane. 23.1 First Overtone Region — Linear Molecules... 

Methylene groups in linear, aliphatic molecules have two primary peaks at about 5800 cm" (1723 nm) and 5680 cm" (1762 nm) in the first overtone region. The 5800-cm" peak is generally thought to be a combination band, " reported as The 5680-cm" peak is considered to be a pure... 

The weaker methylene peak in the first overtone region was said to be at 5671 + - 3 cm for linear hydrocarbons. This peak was thought to have contributions from both methyl and methylene groups (Tosi and Pinto), although others have assigned a 5680-cm band to be both the first overtone of the methylene symmetric stretch and of the asymmetric stretch shifted by Fermi resonance (Ricard-Lespade et al.). More recently, Parker et al. have discussed the origins of this peak in terms of local mode theory. 

The first overtone stretching vibrations of methylene groups of strained-ring cyclic compounds such as cyclopropane occur near 6135 cm (1630 nm). The effects of various substituents on the ring have been studied by several authors. Gassman and Zalar list the band positions of 37 cyclopropane derivatives. Gassman also published a table of first overtone CH band positions of aliphatic nortricyclene derivatives. These overtones were at sUghtly lower wavenumber maxima than the cyclopropanes — about 6024 cm (1660 nm). 

The first overtone of the C-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 61(X)-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 absorp-tivities 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 1/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. 

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