CC Stretching Vibrations

The v, i(b2) vibration featuring the asymmetric CC stretching vibration, which is strongly coupled with the asymmetric CH in-plane bending vibration (at 1061 cm- experimentally) is shifted by 20-25 cm towards lower frequencies, each time deuterium is incorporated in the molecule. The intensity ratio of V3 between the vs (ba) and the V4(a,) dominant feature is almost unchanged with deuteration. 

A- B vibration is distinct from the stretching and bending vibrations of the AH and BHy groups. This is not the case with the molecule H2C=CH2. The CC stretching vibration at 1623 cm lies close to the CH2 deformation mode with the same symmetry (1342 cm ). Therefore, erroneous results are obtained for the force constant/(CC) if the mass of the CH2 group is used in the formula for the diatomic oscillator (Becher and Adrian, 1970). In contrast, the situation is simpler if the A and B atoms are heavier. In bonds in which elements of the third period are involved, the element-element vibrations are hardly influenced by AH vibrations of the same symmetry ... 

In organic molecules, the CC stretch frequency is usually between 1000 and 1800 cm" (Figure 4.5). The lowest CC stretch vibrations are at 1057 cm" for ethane and at 1488 cm" for ethylene. In both these molecules, the hydrogen atoms follow the carbon atoms in their motion. There is another CC stretch at 1564 cm" for ethane, and at 1851 cm" for ethylene, where the hydrogen atoms do not move. 

Fig. 4.21. The CC stretching vibrations of ethylene, allene, and butatriene. The vibrations (S for stretch and C for contract) are compared with the standing waves of a helical spring with mass.

Fig. 4.26. Raman-spectra measured for a sample of PE (M > 10 ) in the frequency ranges of the CC-stretching vibrations top left) and the CH2-twisting vibrations top right). Spectra were registered at 126°C, after completion of isothermal crystallization, and then successively at 115 °C, 100 °C and 25 °C. Fractions of methylene units in the orthorhombic-crystalline, amorphous and intermediate phase, as derived from a decomposition of the spectra bottom) [34]...

The nuclear frequency factor for ET in these organic systems is the frequency of the CC stretching vibration of 1600 cm-, or = 4.8X10 sec. The separation distance between the planes defined by the aromatic rings is typically the van der Waals contact distance... 

That surfaces with a linear HCC equilibrium geometry give rise to intrinsic non-RRKM lifetimes is consistent with the fact that the coupling between the HCC bend and the HC and CC stretch vibrational momenta is zero for 0 = 180° and a maximum for 0 = 90°. This can be seen by writing the vibrational kinetic energy in internal coordinates using... 

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