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

Abstract

Abstract. Photo-chemically induced dynamic nuclear polarization\n(photo-CIDNP) was used to observe the light-induced disproportionation\nreaction of 6,7,8-trimethyllumazine starting out from its triplet state to\ngenerate a pair of radicals comprising a one-electron reduced and a\none-electron oxidized species. Our evidence is based on the measurement of\ntwo marker proton hyperfine couplings, Aiso (H(6 α )) and\n Aiso (H(8 α )), which we correlated to predictions from density\nfunctional theory. The ratio of these two hyperfine couplings is reversed in\nthe oxidized and the reduced radical species. Observation of the dismutation\nreaction is facilitated by the exceptional C–H acidity of the methyl group\nat position 7 of 6,7,8-trimethyllumazine and the slow proton exchange\nassociated with it, which leads to NMR-distinguishable anionic (TML − )\nand neutral (TMLH) protonation forms.