HINDERED ROTATION IN DIPHENYLMETHANE DERIVATIVES - ELECTROSTATIC VS CHARGE-TRANSFER AND HOMOCONJUGATIVE ARYL-ARYL INTERACTIONS

A series of p,p'-disubstituted 7-phenyl-7-(2-fluorophenyl)norbornanes 5xy has been prepared, and the barrier (Delta G(#)) to 160 degrees lib ration around the 2-fluoroaryl-norbornane bond has been measured by DN MR. There is spectroscopic evidence of strong homoconjugative and char ge-transfer (CT) interactions between both aryl groups of 5xy. However , the relationship between Delta G(#) and the nature of the substituen ts X and Y is accounted for only by electrostatic interactions between both aryl groups in the ground state as well as in the transition sta te of the libration. Therefore, the notion of CT and aromatic homoconj ugation as strong attractive forces between aryl groups should be defi nitively rejected.