Vibrational energy ethylenic compounds

The C=C harmonic vibrational frequency is calculated at 1671 cm-1 in free ethylene and is infrared (IR) forbidden. Its IR intensity is therefore expected to remain low in the vinyl series of compounds. The C=C stretch energy is calculated to be 1687 cm-1 in propene and then decline to 1629 4 cm-1 for X = Si - Pb. As in the equilibrium bond distance, there is also a very small counter-trend change in the vibrational frequency going from X = Sn to X = Pb that indicates a slight strengthening of the C=C bond. 

The shape resonances described in the previous sections have all been associated with tt orbitals. Resonances formed by attachment into the O orbitals of unsubstituted hydrocarbons typically occur at high energy (E > 5 eV) and are usually broader than ir resonances. In compounds in which they are hard to discern in the total cross section, they may appear more readily in the cross sections for vibrational excitation, since direct excitation of vibration is weak, at least for levels which are not allowed optically, and hence the interfering background is small. Such broad 0 resonances, for example, have been observed in methane (42) and ethylene (22) using such measurements. 

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