Internal rotation of methyl groups in terpenes. Variable-temperature carbon-13 spin-lattice relaxation time measurements and force-field calculations

Abstract

Carbon-13 spin-lattice relaxation times and nuclear Overhauser enhancement factors are reported for borneol, camphor, isopinocampheol, and α-pinene in deuterochloroform solution at several temperatures. The random jump rates characterizing the internal motion of methyl groups are evaluated and the corresponding Arrhenius activation energies are determined. The error sources involved in such a procedure are discussed and compared to the conventional treatment, using single-temperature data. The activation energies for the nonassociated systems are found to agree reasonably well with the potential barriers obtained from the empirical force-field calculations.