Carbon-13 Relaxation and Internal Rotation in Mesitylene and o -Xylene

A dynamic method for determining carbon-13 spin—lattice relaxation times and nuclear Overhauser enhancements from spectra of samples with natural isotopic abundance is presented. The method, along with adiabatic rapid passage and relative intensity measurements, is used to determine the relaxation times and important relaxation mechanisms of all carbons in neat samples of o-xylene and mesitylene. The results are interpreted in terms of over-all molecular rotational diffusion and internal methyl rotation. The relaxation times in mesitylene indicate that internal methyl rotation is very rapid with respect to over-all molecular diffusion. There is also evidence that the relatively free methyl rotation leads to relaxation of these carbons through spin—internal-rotation interaction. In the case of o-xylene the internal rotation is not well separated from the over-all rotation, but approximate calculations show that the results are consistent with a rate of hindered rotation which does not greatly exceed the over-all diffusion rate, with a barrier to rotation of 1 or 2 kcal/mole. Relatively large Overhauser enhancements and dipolar relaxation rates for the substituted ring carbons indicate that intermolecular dipole—dipole interactions are significant in the relaxation of these carbons. These intermolecular interactions are shown to be relatively unimportant, however, for carbons with directly bonded protons.