Proton Magnetic Resonance Studies of Rotational Isomerism in Halotoluene Derivatives. VI. Rotational Barrier and Conformational Energy Differences in α,α,2,3,6-Pentachlorotoluene

Abstract

Rotation by π radians about the sp2–sp3 carbon–carbon bond of the dichloromethyl group in α,α,2,3,6-pentachlorotoluene interconverts the two conformations which are characterized by coplanarity of the aromatic ring with the C—H bond of the sidechain. The ring protons undergo effective nonmutual exchange during this rotation. The rate of rotation as a function of temperature is extracted by line-shape fitting of the ring p.m.r. spectra. The rotation is characterized by a negative entropy of activation. The free energy difference between the two conformations is 50 ± 20 cal/mol at 240 °K.