Alkene C—H Bending Vibrations

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. 

Alkene (olefinic) structures introduce several new modes of vibration into a hydrocarbon molecule a C=C stretching vibration, C—H stretching vibrations in which the carbon atom is present in the alkene linkage, and in-plane and out-of-plane bending of the alkene C—H bond. The spectrum of Figure 3.10 is that of a typical terminal alkene. 

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 C—H bonds in alkenes can vibrate by bending both in-plane and out-of-plane when they absorb infrared radiation. The scissoring in-plane vibration for terminal alkenes occurs at about 1415 cm This band appears at this value as a medium to weak absorption for both monosubstituted and 1,1-disubstituted alkenes. 

Figure 2.22 shows the C—H out-of-plane bending vibrations for substituted alkenes, together with the frequency ranges. 

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