Methylene groups combination bands

Appreciable interaction between chromophores does not occur unless they are linked directly to each other, or forced into close proximity as a result of molecular stereochemical configuration. Interposition of a single methylene group, or meta orientation about an aromatic ring, is sufficient to insulate chromophores almost completely from each other. Certain combinations of functional groups afford chromophoric systems which give rise to characteristic absorption bands. 

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. 

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... 

First Combination Bands of Methylene Groups in CHjClj... 

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 4.6 Quantitative analysis by FTIR of propylene/ethylene (P/E) copolymers I IR absorbance spectrum of P/E with mainly isolated E groups II IR absorbance spectrum of P/E copolymer with mainly adjacent E units III IR absorbance spectra of blend of polypropylene and linear polyethylene a represents spectra at room temperature b represents spectra of melt , i.e. at 145°C c represents resolution enhanced melt spectra IV band area per unit thickness measurements of 720 cm and 733 cm absorption bands versus wt% of -CH2- as-(CH2)/i- ( 5) and -(CH2)3- determined from C-NMR data. V quantitative determination of -(CH2)3- groups as a proportion of the total combined ethylene content of P/E copolymers—correlation of absorbance ratio 733cm V(720cm + 733cm ) with C-NMR data. Note in IV, IR absorbance per methylene appears essentially independent of the length of the E sequence. Reproduced from refs. 96 and 97 by permission of Springer-Verlag, New York, and The Royal Society of Chemistry, London,...

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