Time-Resolved Resonance Raman, Electron Spin Resonance, and ab Initio Molecular Orbital Study of the Structure and Proton Reactivity of 4-Carbamylpyridinyl Radicals

Short-lived pyridinyl radicals (4-carbamylpyridinyl; neutral and cation forms) produced on one-electron reduction of isonicotinamide by e aq −, H • or (CH 3 ) 2 C • OH in aqueous solution have been probed by resonance Raman and ESR spectroscopies. It has been shown that the radical cation formation by direct addition of H • atom to N-protonated isonicotinamide is less efficient (29% yield) than the sequential additions of e aq − and H + . The neutral carbamylpyridinyl radical (1-hydro-4-pyridine amide) protonates in mildly acidic solutions (p K a ∼ 2), in contrast to closed-shell amides which protonate only in highly concentrated aqueous acid solutions. This anomalous property of the amide group conjugated with semireduced pyridine has been explained in terms of ab initio molecular structures which are consistent with resonance Raman frequencies of the radicals and their deuterated isotopomers, and with ESR hyperfine constants. As with stable amides, the proton binds with the oxygen, and not the nitrogen atom, in the radical structure. However, the CO bond is ∼0.02 Å longer in the neutral radical than in isonicotinamide, and there is substantial negative charge on the oxygen atom, which makes this species very reactive toward the proton, as compared to the regular amides.