Electron‐nuclear relaxation time T 1e for intramolecular distance evaluation

Carbohydrate molecules labelled by TEMPOL nitroxide radicals in dilute deuteriochloroform solutions are considered. The distances between 13 C nuclei and the nitroxide free electron are related to the measured contributions to the 13 C longitudinal relaxation rates which stem from the dipolar coupling with the electron spin. It is shown that a correct interpretation of these electron‐nuclear relaxation times rests on an accurate knowledge of the effective viscosity experienced by the labelled solute. This microviscosity can be roughly estimated from a simple hydrodynamic model which takes the difference in sizes of the solute and solvent molecules into account. A more accurate experimental determination is obtained by measuring the electron‐nuclear relaxation times of the cyanide 13 C spin on a rigid cyano‐TEMPOL radical used as a microviscosity probe. Various models of rotational diffusion are considered. The anisotropy of the rotational motion of the labelled sugar and a possible contraction‐expansion of the electron‐ 13 C distances due to dynamic conformational changes are examined, and their effects on the relaxation times are calculated. The delocalization of the paramagnetic free electron is taken into account, and a theoretical model for calculating its influence on the relaxation is presented. To our knowledge, this is the first study showing the importance of the electronic spin delocalization which reduces the relaxation times by a factor of 2 for the cyano‐TEMPOL. Using a theory which incorporates the correct microviscosity, rotational anisotropy and free electron delocalization, the electron‐ 13 C distances of the labelled sugar, which give theoretical relaxation times in agreement with the experimental data, are found to be nearly equal to the values determined by X‐ray studies. The solute appears to have a stretched rigid shape in chloroform over a large temperature range. Electron‐nuclear relaxation studies allow the evaluation of intramolecular interspin distances up to 10–15 Å in length with an accuracy of about 5%.