KINETICS OF RAPID INTERNAL SUBGROUP ROTATION OF A GROUP OF PHENOXY RADICALS AS STUDIED BY ENDOR SPECTROSCOPY

Electron nuclear double resonance spectroscopy (ENDOR) has been used to study the rate of rotation of substituents at the para position of a group of phenoxy radicals. The para substituent of the molecules contain oxime groups with aliphatic substituents on the oxime carbon atom. The oxime nitrogen atom interacts with one of the phenoxy ring protons via a through space interaction. This interaction changes the electron-nuclear hyperfine coupling of this proton. Rotation of the para substituent with respect to the phenoxy ring interchanges the magnetically nonequivalent phenoxy ring protons and results in characteristic line shape changes of the ENDOR spectra from these protons. Analysis of the ENDOR Line shape allows one to determine the rate of rotation. Temperature dependence studies of rates of rotation allow one to determine activation parameters for rotation. These experiments show that the rate of rotation increases as the steric bulk of para substituent increases until the substituent becomes large enough to favor a nonplanar conformation. The ct-electron delocalization energy drives the molecules toward planar conformations which maximize delocalization of the spin into the oxime group while steric interactions between the aliphatic chains and the aromatic ring drive the molecule toward the conformation in which the oxime group is perpendicular to the ring. The activation energy for rotations is found to depend on the relative magnitude of these two types of interactions. The activation energy decreases with steric bulk until the aliphatic group is tert-butyl in which case the steric interaction is large compared to the pi-electron energy and the molecule assumes a perpendicular conformation. The activation entropy is found to be very dependant on the bulk of the substituent. These entropy changes are explained by reorganization of solvent molecules during rotation.