TIME-RESOLVED RESONANCE RAMAN-SPECTRA, STRUCTURE, AND PROTON REACTIVITY OF 4-CARBOXYPYRIDINYL RADICALS

This paper presents the first molecular spectroscopic study of the str ucture, bonding, and vibrational modes of a short-lived pyridinyl radi cal, an important intermediate in a variety of chemical and biochemica l reactions, using time-resolved resonance Raman spectroscopy and ab i nitio SCF molecular orbital calculations. A structural explanation has been provided for the proton reactivity, which plays a fundamental ro le in the aqueous chemistry of the electron adduct states of nitrogen- heterocyclic aromatic molecules. Three protonation forms of the radica l derived from 4-pyridinecarboxylic (isonicotinic) acid have been exam ined as model systems. Theoretical calculations performed on the elect ron adduct of the isonicotinate anion show that most of the added char ge 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 vibr ational 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 radi cal protonation at the carbonyl oxygen at moderately low pH (H-0 simil ar to 0), a chemical behavior which contrasts with that of the aromati c carboxylic acids. It also accounts for a significant increase in the -CO2H proton dissociation constant (pK(a) 6.3) in the radical with re spect to that in isonicotinic acid. This study illustrates the relatio nship between the vibrational structures and the acid-base properties of reactive intermediates, which are often quite different from those of their stable precursors.