Importance of Acidity on an Energetically Unfavorable Electron-Transfer Reaction-An Extension of the Rehm-Weller Equation. Photoreaction of Triplet 2,4-Pyridinedicarbonitrile with 2-Propanol

Article abstract of DOI:10.1021/ja00269a013

An electron spin resonance study of the radical formed when 2,4-pyridinedicarbonitrile (1) is irradiated in the cavity of an ESR spectrometer has been done and the values of the ?-spin populations on the ring atoms were determined from the measured proton and nitrogen couplings.Radicals were generated by in situ photolysis of deaerated solutions of 1.The rate of radical formation was measured in acetonitrile at 33 deg C with varying amounts of 2-propanol, water, and hydrochloric acid.Quenching and photosensitization studies show the triplet state of 1 to be the photoreactive state. ΔG_et values calculated by the Rehm-Weller equation were found to be endoergic when either the T₁ or the T₂ state of 1 is involved.Evidence is presented which supports a mechanism in which N-H bond formation is coupled to the electron-transfer step, resulting in an exoergic process.N-H bond formation becomes the driving force for the reaction.