Intramolecular and overall motion of proline: The influence of viscosity on carbon‐13 spin‐lattice relaxation times

Nuclear magnetic resonance of 13 C is used to probe the overall and internal motions of proline. Spin‐lattice relaxation times ( T 1 ) are reported for proline monomer dissolved in water/glycerol mixtures. Rates of overall molecular motion and internal motion depend on solvent composition but to different degrees. The effective correlation times (τ eff ) of the various proton‐bearing carbon atoms in proline vary linearly as a function of solvent composition (%v/v) rather than of solution viscosity. The effective correlation time for molecular motion (τ eff ) is separated into contributions from overall molecular motion (τ mol ) and internal motion (τ int ). The γ‐carbon of proline shows the smallest dependence of τ int on solvent composition. The data indicate a high degree of intramolecular motion for the γ‐carbon of proline. Inclusion of anisotropic molecular reorientation in the data analysis was found not to affect the above conclusions. The observed values of τ eff indicate that the rotational diffusion model of molecular reorientations should apply to proline. The values of τ eff calculated for proline using the Stokes‐Einstein relation are larger than those observed; the discrepancy is discussed in terms of solvent‐solute interactions.