Rock bending vibration

Bending vibrations in one plane (symbol 8), changing bond angles but leaving bond lengths unaltered (in larger molecules further divided into rock, twist and wag vibrations),... 

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 vibrations of a molecule containing three or more atoms are more complex (Figure 13.12). Such molecules can experience symmetric and asymmetric stretches and bends, and their bending vibrations can be either in-plane or out-of-plane. Bending vibrations are often referred to by the descriptive terms rock, scissor, wag, and twist. 

The bending vibrational modes of methylene group The top two dii rams frrocking modes. The bottom two dii rams show wagging and twisting modes. 

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