Marcus Cross-Relationship Probed by Time-Resolved CIDNP

The time-resolved CIDNP method can provide information about degenerate exchange reactions (DEEs) involving short-lived radicals. In the temperature range from 8 to 65 °C, the DEE reactions of the guanosine-5′-monophosphate anion GMP(-H)⁻ with the neutral radical GMP(-H)^•, of the N-acetyl tyrosine anion N-AcTyrO⁻ with a neutral radical N-AcTyrO^•, and of the tyrosine anion TyrO⁻ with a neutral radical TyrO^• were studied. In all the studied cases, the radicals were formed in the reaction of quenching triplet 2,2′-dipyridyl. The reorganization energies were obtained from Arrhenius plots. The rate constant of the reductive electron transfer reaction in the pair GMP(-H)^•/TyrO⁻ was determined at T = 25 °C. Rate constants of the GMP(-H)^• radical reduction reactions with TyrO⁻ and N-AcTyrO⁻ anions calculated by the Marcus cross-relation differ from the experimental ones by two orders of magnitude. The rate constants of several other electron transfer reactions involving GMP(-H)⁻/GMP(-H)^•, N-AcTyrO⁻/N-AcTyrO^•, and TyrO⁻/TyrO^• pairs calculated by cross-relation agree well with the experimental values. The rate of nuclear paramagnetic relaxation was found for the 3,5 and β-protons of TyrO^• and N-AcTyrO^•, the 8-proton of GMP(-H)^•, and the 3,4-protons of DPH^• at each temperature. In all cases, the dependences of the rate of nuclear paramagnetic relaxation on temperature are described by the Arrhenius dependence.