This paper presents the first molecular spectroscopic study of the structure, bonding, and vibrational modes of a short-lived pyridinyl radical, an important intermediate in a variety of chemical and biochemical reactions, using time-resolved resonance Raman spectroscopy and ab initio SCF molecular orbital calculations. A structural explanation has been provided for the proton reactivity, which plays a fundamental role in the aqueous chemistry of the electron adduct states of nitrogen-heterocyclic aromatic molecules. Three protonation forms of the radical derived from 4-pyridinecarboxylic (isonicotinic) acid have been examined as model systems. Theoretical calculations performed on the electron adduct of the isonicotinate anion show that most of the added charge goes to the ring nitrogen, which explains the rapid protonation of the species at this site in aqueous solution. The resulting pyridinyl radicals have very nearly a quinoid ring structure, as manifest in the unusually high Raman frequency for the ring stretching Wilson mode 8a. In the neutral 4-carboxypyridinyl radical, the frequency of the vibrational mode containing the C=O stretch is similar to 100 cm(-1) lower than in isonicotinic acid, indicating some formal negative charge on the carboxylic group. This partially ionic structure explains the radical protonation at the carbonyl oxygen at moderately low pH (H-0 similar to 0), a chemical behavior which contrasts with that of the aromatic carboxylic acids. It also accounts for a significant increase in the -CO2H proton dissociation constant (pK(a) 6.3) in the radical with respect to that in isonicotinic acid. This study illustrates the relationship between the vibrational structures and the acid-base properties of reactive intermediates, which are often quite different from those of their stable precursors.