Anomalous Chemical Shifts in the Proton Magnetic Resonance Spectra of the Dimethylcyclohexanes and Related Hydrocarbons

Abstract

High-resolution proton magnetic resonance spectra have been determined for the dimethylcyclohexanes and several related hydrocarbons between —130° and 130°C. All of the compounds which should undergo rapid ring inversion at room temperature produce spectra which change upon cooling because of ``freezing out'' of this motion. The assumption that appearance of the ring-hydrogen resonances as a relatively narrow band is invariably a symptom of rapid ring inversion is shown to be unfounded. Several of the ring spectra differ drastically from what is predicted using a bond-anisotropy model. A previously unrecognized effect must then make a significant contribution to the observed chemical shifts.