Abstract
In situ NMR spectroscopy can be applied to investigate chemical reactions during which free radicals occur as intermediates. In chemical systems of low molecular weight, nuclear‐spin polarization results from the spin selectivity of free‐radical reactions, because a pair of radicals has to obey the exclusion principle according to Pauli; therefore, this system reacts in a spin‐selective manner when forming a chemical single bond. As a consequence, strong transient absorption and emission lines occur in the NMR spectra acquired during free‐radical reactions. This phenomenon is known as ‘chemically induced dynamic nuclear polarization’ (CIDNP). However, long correlation times associated with short proton spin–lattice relaxation times T~1~ render it difficult to observe CIDNP in macromolecules. Therefore, in these and in many other cases, it can be attractive to utilize the simultaneously occurring heteronuclear polarization or to selectively transfer the ^1^H polarization to heteronuclei, since their T~1~ times can be substantially shorter than those of protons.
In this paper, we present examples of how CIDNP can be observed both in low‐molecular‐weight systems as well as in systems exhibiting a rather macromolecular character. Also, CIDNP can assist in obtaining useful information about macromolecular systems that normally is very difficult to obtain otherwise. Since CIDNP is primarily a qualitative method by which free‐radical intermediates may be identified, we have developed a procedure allowing the quantitative determination of the magnetic properties of the intermediate free radicals. This process is especially useful for very short‐lived radicals, which are frequently elusive to ESR spectroscopy conducted in solution. In particular, g values of a variety of radical ions have been determined in this fashion by CIDNP‐NMR spectroscopy. The data thus obtained provide an alternative to ESR and, hence, complement this traditional method (manuscript in preparation).