C—H Bending Vibrations

Figure C3.5.11. IR-Raman measurements of vibrational energy flow tlirough acetonitrile in a neat liquid at 300 K, adapted from [41], An ultrashort mid-IR pulse pumps the C-H stretch, which decays in 3 ps. Only 1% of the energy is transferred to the C N stretch, which has an 80 ps lifetime. Most of the energy is transferred to the C-H bend plus about four quanta of C-C=N bend. The daughter C-H bend vibration relaxes by exciting the C-C stretch. The build-up of energy in the C-C=N bend mirrors the build-up of energy in the bath, which continues for about 250 ps after C-H stretch pumping.

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

Table 6.4 Experimental0 and calculated frequencies0 of in-plane C-H bending vibrations of CgHg...

The spectra obtained from the chemisorption of methanol onto catalyst above 100°C indicated the progressive oxidation of methoxy species to formate via dioxymethylene/HCHO and finally to CO, CO2, and H2. Phenol adsorbed on the surface Lewis acid-base pair site and dissociated to phenolate anion and proton. The formation of phenolate anion and proton were discerned from the strong intense C-0 stretching vibration and the disappearence of phenolic 0-H stretching vibration, respectively. Importantly, there were series of definite low intensity bands between 2050 and 1780 cm" that were identified as the out-of-plane aromatic C-H bending vibrations [79, 84-85]. These bending vibrations are possible only if the phenyl ring of phenol is perpendicular to the catalyst surface. 

The IR spectra of methyllithium exhibited two C—H bending vibrational modes at 1480 and 1427 cm . Their assignment was again substantiated by significant isotopic shifts to 1100 and 1043 cm in the deuterium compound (Table 1). Only one weak band was observed for ethyllithium in the C—H deformation region at 1450 cm . Moreover, a new sharp peak was detected at 1385 cm and ascribed to the C—H symmetrical bending mode of CH3. Its absence in the IR spectrum of methyllithium is a further indication that free methyl groups exist only in ethyllithium. 

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). 

Alkene C—H Bending Vibrations Alkene C—H bonds can undergo bending either in the same plane as the C=C bond or perpendicular to it the bending vibrations can be either in phase or out of phase with respect to each other. 

The most characteristic vibrational modes of alkenes are the out-of-plane C—H bending vibrations between 1000 and 650 cm-1. These bands are usually the strongest in the spectra of alkenes. The most reliable bands are those of the vinyl group, the vinylidene group, and the trans-disubstituted alkene. Alkene absorption is summarized in Appendix Tables D-l and D-2. 

C—H Bending Vibrations The C—H bending vibration of alkynes or monosubstituted alkynes leads to strong, broad absorption in the 700-610 cm-1 region. The first overtone of the C—H bending vibration appears as a weak, broad band in the 1370-1220 cm-1 region. 

Box 3.4 Out-of-plane C-H Bending Vibration Frequencies of Mono- and Disubstituted Aromatic Rings... 

Further work is needed to obtain more information on quantum yields of two primary processes as a function of pressure and wavelength. It is also of interest to look into the vibrational excitation of H2. Since the process of H, formation involves the simultaneous excitation of the C H stretching and the H —C—H bending vibrations, the product H2 must be highly vibra-lionally excited. The radical production process (VI1-46) is expected to be faster than the molecular process (VII-47) if H + HCO is predissociated by way of a repulsive state as shown in Fig. II 11, p. 78. 

The infrared spectra of the alkanes show clearly absorptions corresponding to the C—H stretching frequencies at 2850 cm 1 to 3000 cm-1. The C—C stretching absorptions have variable frequencies and are usually weak. Methyl (CH3—) and methylene (—CH2—) groups normally have characteristic C—H bending vibrations at 1400 cm-1 to 1470 cm-1. Methyl groups also show a... 

C-H bending vibrations Asym 1450 20 cm-1 1445 Aromatics overtones and Two bands... 

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