In order to study the internal rotation of methyl-type groups in molecules, the carbon-13 nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) and nuclear Overhauser enhancement factors (n) are measured for some terpenes (borneol, camphor, isopinocampheol and α-pinene), for octafluorotoluene and for 1-methylnaphtalene. In addition, deuterium NMR T1 data are presented for 1-methylnaphtalene-d10. All experiments are performed at several different temperatures. Parameters characterizing the internal motion in these model systems are extracted and fitted to Arrhenius type expressions to yield activation energies and preexponential factors. The results are discussed in terms of a) barrier heights from force-field calculations and results from single temperature NMR measurements (the terpenes), b) different models for overall motion and internal motion (octafluorotoluene) and c) absolute rate theory (1-methylnaphtalene-h10 and d10). It was found that our approach using temperature dependent T1 measurements provides reasonable estimates for internal barrier heights, provided the hindrance to methyl rotation is primarily of internal origin.
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