Charge mobility in σ-bonded molecules: The infrared spectrum of polymethylene chains in the solid and liquid phases

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Abstract
The experimental observation we wish to explain and which is common in most of the polymethylene systems with no polar substituents is that the absorption intensity in the infrared of the CH2 wagging motions is extremely weak for solid all‐t r a n s chains and becomes much stronger in the liquid phase in which molecules are conformationally distorted. Using current theories for the interpretation of the infrared intensities, it is shown that the observed phenomenon arises from the fact that for all transplanar chains, a sizeable ‘‘σ conjugation’’ along the CC chain takes place. It is shown that ‘‘charge fluxes’’ from second nearest neighbors play the main role in determining such intensity behavior.