TR-CIDNP as tool for quantitative analysis of hyperfine couplings in elusive radicals

For many years Chemically Induced DynamicNuclear Polarization (CIDNP) has been successfullyused for investigating the kinetics of fast reactions ofradicals [1]. The origin of CIDNP is the nuclear spinselective recombination of radical pairs. This case isgiven when the electronic singlettriplet conversion isthe bottleneck for the radical pair recombination. As aconsequence, radical pairs have different reactivity indifferent nuclear spin states, with the result that thediamagnetic reaction products have nonthermal populations of their nuclear spin energy levels. CIDNPmanifests itself in anomalous phase and intensity ofthe NMR spectra of the diamagnetic molecules. Thetimeresolved (TR) version of the CIDNP techniquehas a particularly high content of information abouttransient radicals [2] as it can differentiate primary andsecondary reaction steps. First, CIDNP spectrarecorded right after recombination of the geminateradical pairs (geminate CIDNP) are a fingerprint ofthe transient radicals. Second, CIDNP effects can bedetected in liquids at ambient conditions, whereasconventional EPR spectroscopy often allows one toinvestigate the elusive radical species only in frozenmatrix at cryogenic temperatures. Third, the CIDNPtime dependence (kinetics) gives information aboutthe rates of radical termination, intramolecular electron migration and nuclear spin relaxation in transientradicals.In the present work, we will investigate theoretically and experimentally the possibility of extractinghyperfine coupling constants (HFCCs) in radicalsfrom the geminate CIDNP spectrum recorded at high