Non-classical disproportionation revealed by photo-chemically induced dynamic nuclear polarization NMR

Abstract

Abstract. Photo-chemically induced dynamic nuclear polarization (photo-CIDNP) was used to observe the light-induced disproportionation reaction of 6,7,8-trimethyllumazine starting out from its triplet state to generate a pair of radicals comprising a one-electron reduced and a one-electron oxidized species. Our evidence is based on the measurement of two marker proton hyperfine couplings, Aiso(H(6α)) and Aiso(H(8α)), which we correlated to predictions from density functional theory. The ratio of these two hyperfine couplings is reversed in the oxidized and the reduced radical species. Observation of the dismutation reaction is facilitated by the exceptional C–H acidity of the methyl group at position 7 of 6,7,8-trimethyllumazine and the slow proton exchange associated with it, which leads to NMR-distinguishable anionic (TML−) and neutral (TMLH) protonation forms.

Unravelling structures of radicals of kynurenic acid formed in the photoinduced reactions with tryptophan and N-acetyl tyrosine

“…A similar mismatch of slopes for proportionality dependences was previously found for CIDNP of two products detected during the light-induced disproportionation reaction of 6,7,8-trimethyl lumazine. 26…”

19 F MRI signal-enhancement by photo-chemically induced dynamic nuclear polarization enables spatially-resolved detection of sub-nmol amount of the antiviral drug favipiravir

“…Interestingly, the structural similarity of the hetero-aromatic ring in favipiravir to the middle part of ribo avin is immediately apparent [33]. Wörner et al published photo-CIDNP measurements for non-classical disproportionation [41]. The effect of charge redistribution within the molecule is re ected in the hyperpolarizability of these systems.…”

Expanding Reaction Horizons: Evidence of the 5‐Deazaflavin Radical Through Photochemically Induced Dynamic Nuclear Polarization

“…[39][40][41] This method often reveals elusive paramagnetic species that even escape detection by EPR. [39,[42][43][44][45] Photo-CIDNP in solution relies on the fact that a radical pair state generated by photo-induced electron transfer is formed with a multiplicity that is equal to that of its precursor state due to the conservation of spin angular momentum. This could be an (excited) singlet state (spin quantum number S = 0) or, if intersystem crossing precedes electron transfer, a triplet state (S = 1).…”

“…Driven by the lack of knowledge of the unsubstituted 5‐deazaflavin radical and motivated by a recent report on the catalytic capabilities of C5‐derivatized 5‐deazaflavins [21] we applied photo‐CIDNP, an NMR technique frequently used to study cyclic chemical photoreactions involving short‐lived radicals [39–41] . This method often reveals elusive paramagnetic species that even escape detection by EPR [39,42–45] . Photo‐CIDNP in solution relies on the fact that a radical pair state generated by photo‐induced electron transfer is formed with a multiplicity that is equal to that of its precursor state due to the conservation of spin angular momentum.…”