Methylene bending

Methyl asymmetric bend Methyl symmetric bend Methylene symmetric bend Lactone C-O-C asymmetric bend Ester C-O-C asymmetric bend Lactone C-O-C symmetric stretch Ester C-O-C symmetric stretch Alcohol C-OH stretch Trisubstituted olefinic C-H wag... 

Morterra and Low109,110 proposed that thermal crosslinking may occur between 300°C and 500°C where phenolic hydroxyl groups react with methylene linkages to eliminate water (Fig. 7.43). Evidence for this mechanism is provided by IR spectra which show decreased OH stretches and bending absorptions as well as increased complexity of the aliphatic CH stretch patterns in this temperature range. 

Table 1 Optimized energy values for the and B states of the methylene as function of the HCH bending angle.

Fig. 12. Internal energy spectrum of the CH2 fragment from photolysis of the CH3 radical at 216.3 nm. The combs above the figure indicate the expected TOFs of H atoms formed, in association with triplet methylene CH2(X3Bi) or singlet CH2( i1Ai) respectively as a function of V2, the vibrational quantum number for their respective bending mode. (From Wilson et al,113)...

Due to the superimposing C-H bending vibrations of the various methyl and methylene groups. 

The methylene triplet adds to ethylene symmetrically through the triplet biradical directly (B). The central methylene group (formed from ethylene) is bent downwards by this process (Fig. 10). At this stage rotation or direct ring closure can occur, with loss of stereochemistry following bond formation to yield cyclopropane. The cyclo-addition of the triplet requires only a small activation energy of about 5 kcal/mole 52). 

The vibrational modes of alkanes are common to many organic molecules. Although the positions of C—H stretching and bending frequencies of methyl and methylene groups remain nearly constant in hydro-... 

The absorption band near 1375 cm-1, arising from the symmetrical bending of the methyl C—H bonds, is very stable in position when the methyl group is attached to another carbon atom. The intensity of this band is greater for each methyl group in the compound than that for the asymmetrical methyl bending vibration or the methylene scissoring vibration. 

Methylene Groups The bending vibrations of the C—H bonds in the methylene group have been shown schematically in Figure 1. The four bending vibrations... 

C—H Bending Vibrations Cyclization decreases the frequency of the CH2 scissoring vibration. Cyclohexane absorbs at 1452 cm-1, whereas n-hexane absorbs at 1468 cm-1. Cyclopentane absorbs at 1455 cm-1, cyclopropane absorbs at 1442 cm-1. This shift frequently makes it possible to observe distinct bands for methylene and methyl absorption in this region. Spectra of other saturated hydrocarbons appear in Appendix B hexane (No. 1), Nujol (No. 2), and cyclohexane (No. 3). 

Assignments have been made for a few of the more prominent and reliable in-plane bending vibrations. The vinyl group absorbs near 1416 cm-1 because of a scissoring vibration of the terminal methylene. The C—H rocking vibration of a cis-disubstituted alkene occurs in the same general region. 

The prospensity for the formation of bends in the flexible spacer in polyesters containing naphthyl units is examined by the study of the fluorescence of a series of diesters. The dependence of the degree of intramolecular excimer formation on the length of the aliphatic spacer, under circumstances where the dynamics of rotational isomerism in the flexible spacer is suppressed, is evaluated by extrapolation of the measurements to infinite viscosity n. The extrapolated results exhibit an odd-even effect, with the more intense excimer emission being observed when the number of methylene groups is odd. The odd-even effect is rationalized by an RIS analysis of the diesters. 

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