Field cycling NMR experiments with hyperpolarized multi-spin-systems

FT-NMR experiments with scalar coupled multi-spin systems employing variation of the external magnetic field B are described, allowing one to discriminate interaction parameters because of their different dependence on B. The mayor part of the work deals with spin ensembles far away from their thermal equilibrium. In particular, hyper-polarization (HP) generated by means of chemically induced dynamic nuclear polarization (CIDNP), and T1-relaxation in liquids (fast motional regime) are studied. The techniques described are applied to the analysis of the photolysis of the aromatic amino acids tyrosine (NATyrOH) and tryptophan (NATrpH) at λ=308 nm in aqueous media. From the field dependence of CIDNP and its time dependence at high magnetic field (TR-CIDNP), the photo-physics and -chemistry is analyzed. The arising amino acid radicals are further investigated in the presence of different triplet sensitizers, e.g. 2,2'-dipyridyl (DP) and anthraquinone-2-sulfonic acid (AQ2S), yielding the same amino acid radicals in spin correlated triplet radical pairs with the radicals of the dyes. Various combinations of the amino acid and dye radicals allow a comparison of the Δg-values from several CIDNP field dependences. Because of their inter-dependence, each g-value is extracted from more than a single field-dependence, and values of higher accuracy are obtained for the radicals under study. The mechanisms for efficient CIDNP formation upon restriction of molecular mobility are investigated on the photolysis of cyclic ketones. The prospects of further applications of field dependent magnetic resonance techniques are discussed. (orig.)