Exploiting Nuclear Spin Polarization to Investigate Free Radical Reactions via in situ NMR

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, like any other given set of particles, has to obey the exclusion principle. Therefore, this system reacts selectively in terms of the participating nuclear spins when forming a chemical single bond. As a consequence, strong transient absorption and emission lines occur in NMR spectra acquired during a reaction of free radicals. This extraordinary phenomenon has become known as chemically induced dynamic nuclear polarization (CIDNP). Ever since its experimental discovery and theoretical verification, CIDNP has been employed to study the mechanisms of free radical reactions in solution. As such it has proven to be a very valuable tool for the elucidation of the mechanism of these reactions and, more importantly, to discriminate reaction pathways that include the formation of transient radical species from those that exclusively follow a “diamagnetic” route, i.e., a pathway where no paramagnetic intermediates are formed whatsoever. More recently, the photo-CIDNP technique has also been employed extensively to probe the surface-accessibility of aromatic amino acid side-chains bound within a protein. As such, it can be used to study the dynamic features of a protein during folding, refolding, and also in the equilibrium or “steady state”, yielding both qualitative and quantitative information.

This review outlines the historical development of the CIDNP technique as well as its theoretical background. This is followed by a series of examples showing how CIDNP can be used to elucidate reaction pathways of chemical transformations comprising diamagnetic intermediates. Additionally, we present examples of how “biological” CIDNP experiments are usually performed and we show what kind of information can be extracted from these studies.