Molecular motion in liquid toluene from a study of 13C and 2D relaxation times

Abstract

The 13C nuclear spin-lattice relaxation times for ring and methyl carbons in liquid toluene were studied from −95°C to +60°C at frequencies of 14 and 61 MHz. Data were taken for protonated as well as deuterated toluene. The results were analyzed in terms of three relaxation mechanisms: intramolecular dipole-dipole coupling, spin-rotation interaction, and anisotropic chemical shift. The last mechanism gives a significant contribution only to the relaxation rate of the ring carbons of the deuterated species at 61 MHz and low temperatures. A tentative value of Δσ = 295 ppm is obtained in this case. In order to separate the contributions of the dipole-dipole and spin-rotation interactions the 13C data are compared with deuteron relaxation times. Comparison of the 13C data in the protonated and deuterated form of toluene shows that the correlation times for the ring differ by 20% and an even larger effect of isotopic substitution is found for the methyl group. It is demonstrated that the fast internal motion of the methyl group cannot be studied quantitatively using deuteron or 13C intramolecular dipole-dipole relaxation rates alone because of the sensitivity of the results to the angle, varpi, the Z-axis of the electric field gradient, or the internuclear vector, respectively, forms with the C3 axis. Analysis of the relaxation rates due to spin-rotation interaction yields τj (internal), the correlation time of angular momentum of the internal motion directly. The correlation time of reorientation τc (internal) is calculated from τj (internal) using Gordon's extended diffusion model which is applied to a symmetric rotor with a fixed axis. It is found that both τj (internal) and τc (internal) are of the same magnitude as the correlation time of the free rotor. The ratio of correlation times of the overall and internal reorientation ranges from approximately 200 at the melting point to approximately 13 at +60°.