Overtone methylene group

An example of Fermi resonance in an organic structure is the doublet appearance of the C=0 stretch of cyclopentanone under sufficient resolution conditions. Figure 3.2 shows the appearance of the spectrum of cyclopentanone under the usual conditions. With adequate resolution (Fig. 3.3), Fermi resonance with an overtone or combination band of an a-methylene group shows two absorptions in the carbonyl stretch region. 

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. 

Skatteb l cited a number of examples including spiropentane (67) for which the absorption of the C-H overtone was 6082 cm (e = 1.20) the larger value of e is regarded as an indication of the greater number of methylene groups ... 

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. 

Almost simultaneously, Aldridge et al. [110] used NIRS to monitor monomer conversion in situ and in real time in bulk methyl methacrylate (MMA) polymerizations performed inside a mold. The authors used a dispersive instrument to observe the spectral changes at 868 and 890 nm, related to the disappearance of the third overtone of the CH stretch of the vinyl group and the concomitant appearance of the CH of the methylene group. Spectral changes were monitored with the help of the second-derivative spectra of the samples and posterior subtraction of the initial monomer spectrum. Simple linear calibrations were performed and shown to be in good agreement with conversion data obtained with independent procedures. 

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. 

Figure 5.8 Overtone spectrum of allyl isocyanide in the region of the sixth overtone. The three peaks from low to high energies correspond to C—H overtones at the following positions the methylenic CH2, nonterminal olefinic CH, and the terminal olefinic CH groups, respectively. Adapted with permission from Segall and Zare (1988).

The second overtone region (1150-1210 nm or 8264-8696 cm ) has also been used for quantitative measurements, in particular to measure methyl, methylene, methine, and aromatic contributions. The methyl groups of long-chain paraffinic hydrocarbons appear between 8365 and 8375 cm (1194-1195nm). In pentane and hexane, the methyl group absorbs at 8396 cm (1191 nm), in heptane it absorbs at 8388 cm (1192 nm), and in decane it is at 8378 cm (1194 nm). See Figure 2.4. 

In the second overtones, which are also shown in Figure 2.6, only one methyl and one methylene peak are normally observed at 8389 cm (1192 nm) and 8264 cm (1210 nm), although weaker peaks can be seen at 8673 cm (1153 nm) and 8503 cm (1176 nm) with higher resolution. In higher alkanes above dodecane, the methyl group becomes a shoulder in the methylene peak. ... 

The third overtone of terminal methylenes C-H stretch is fonnd at 11,390 cm (878 nm) in 1-alkenes. Its absorptivity is 0.002 1/mol-cm. The alkyl acrylate donblet occnrs at 11,905 and 12,500 cm (800 and 840 nm). The peak is at 10,776-11,360 cm (880-928 nm) in alkyl vinyl ethers, and 10,788-10,929 cm" (915-927 nm) in vinyl ethers and vinyl esters. The fourth through seventh overtones of terminal methylene C-H stretch peaks have been studied using photoacoustic spectroscopy." These were given as approximately 11,450, 14,090, 16,630, and 18,800 cm", respectively (873, 710, 600, and 532 nm, respectively). In the spectrum of propylene, this peak is split into a doublet of two peaks that are cis and trans to the methyl group. 

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