Spectroscopic investigations of olefinic compounds: Part I. Rehybridization as a concept for the counterbalance of sterical hindrance and ring strain

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Abstract
Rehybridization as a possible means to alleviate the effects of sterical hindrance and ringstrain has been investigated. This concept, although general in nature, has been analysed in detail for ethylenic structures. It is shown that two out of the six fundamental distortions of a magnified image “molecule” can be counter‐balanced by rehybridization. One of these, the in‐plane rehybridization of the sp2 hybrid orbitals, seems to be the most useful mechanism. For small distortions the change in p character (as given by its mixing coefficient λk) for each of these sp2 orbits depends only on the change in bond angle (ΘY) opposite to the (k) hybrid orbital. It is shown that Δλk = √6ΔΘY.The theory allows the prediction of infrared frequency shifts for the  C‐H stretching, C  C stretching, C  C‐H in‐plane deformation, and C  C‐H out‐of‐plane deformation vibrations as a function of sterical hindrance. A comparison of the infrared spectra of a large variety of ethylenic hydrocarbons reveals a very satisfactory agreement between observed and predicted shifts.