Chemically Induced Spin Hyperpolarization: Coherence Formation in Reaction Products

Chemically induced dynamic nuclear polarization (CIDNP) has emerged as a highly informative method to study spin-dependent radical reactions by analyzing enhanced NMR (nuclear magnetic resonance) signals of their diamagnetic reaction products. In this way, one can probe the structure of elusive radical intermediates and determine their magnetic parameters. A careful examination of experimental CIDNP data at variable magnetic fields shows that formation of hyperpolarized molecules in a coherent state is a ubiquitous though rarely discussed phenomenon. The presence of nuclear spin coherences commonly leads to subsequent polarization transfer among coupled spins in the diamagnetic products of radical recombination reaction that must be taken into account when analyzing the results of CIDNP experiments at low magnetic field. Moreover, such coherent polarization transfer can be efficiently exploited to polarize spins, which do not acquire CIDNP directly. Here we explain under what conditions such coherences can be generated, focusing on the key role of level anti-crossings in coherent polarization transfer, and provide experimental approaches to probing nuclear spin coherences and their time evolution. We illustrate the theoretical consideration of the outlined coherent spin phenomena in CIDNP by examples, obtained for the dipeptide tryptophan–tryptophan.