TIME-RESOLVED RESONANCE RAMAN, ELECTRON-SPIN-RESONANCE, AND AB-INITIOMOLECULAR-ORBITAL STUDY OF THE STRUCTURE AND PROTON REACTIVITY OF 4-CARBAMYLPYRIDINYL RADICALS

Short-lived pyridinyl radicals (4-carbamylpyridinyl; neutral and catio n forms) produced on one-electron reduction of isonicotinamide by e(aq )(-), H-. or (CH3)(2)(COH)-O-. in aqueous solution have been probed by resonance Raman and ESR spectroscopies. It has been shown that the ra dical cation formation by direct addition of H-. atom to N-protonated isonicotinamide is less efficient (29% yield) than the sequential addi tions of e(aq)(-) and H+. The neutral carbamylpyridinyl radical (1-hyd ro-4-pyridine amide) protonates in mildly acidic solutions (pK(a) simi lar to 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 explain ed in terms of ab initio molecular structures which are consistent wit h resonance Raman frequencies of the radicals and their deuterated iso topomers, and with ESR hyperfine constants. As with stable amides, the proton binds with the oxygen, and not the nitrogen atom, in the radic al structure. However, the CO bond is similar to 0.02 Angstrom longer in the neutral radical than in isonicotinamide, and there is substanti al negative charge on the oxygen atom, which makes this species very r eactive toward the proton, as compared to the regular amides.