How Far Does Substituents' Impact Reach? The Transmission of Electronic Substituent Effects along the Polyphenylene Chain and the Evaluation of Through-Bond and Through-Space Contributions.

Abstract

Interactions between various parts of molecules determine their reactivities and are important from the point of view of molecular recognition. Concerning substituted aromatics, the correlation between the substituents' electrical properties with those of the rest of a molecule has been widely studied. However, the distances between the interacting sites generally did not exceed a dozen Å. This study deals with the long polyphenylene substituted hydrocarbons and aims to answer two questions. The first question involves how rapidly interactions between a substituent in the phenyl ring and a site on the other end of the molecule decline with increasing distance between the two sites. The second is regarding how this interaction would be changed after chopping out the benzene rings, spacing units, from inside the molecules, leaving only the first, substituted benzene ring, and the last. The interactions were gauged by the electrostatic potential at the carbon atom the most distant from the substituent, and at two points in space. It was found that the interaction between two sites on a long molecule, about 120 Å apart, is almost as precisely controlled by the substituent properties as in the substituted benzene molecules. The contributions of the through-bond and through-space interactions were that the latter increased as the molecules lengthened.