Spectral characteristics of 2-hydroxynicotinic acid: effects of solvent and acid or base concentrations

Spectral characteristics of 2-hydroxynicotinic acid (2-HNA) have been studied using absorption, fluorescence and fluorescence excitation spectroscopy and time dependent single photon counting spectrofluorimetry. Electronic structure calculations were carried out using semi-empirical AM1 method and hybrid density functional theory (DFT) B3LYP using 6-31 G** basis set and gaussian98 program. All these observations and results suggest that 2-HNA is present as 2-HNA-K-C in all the solvents in ground (S0) and first excited singlet (S1) states. Absence of dual fluorescence and observation of single lifetime in all the solvents and under different conditions supports above conclusion. Monocation (MC) is formed by protonating the exo carbonyl oxygen (MC-K-1) in S0 state and carbonyl oxygen of the acid group in S1 state. It is formed by excited state intramolecular proton transfer (ESIPT) from exo carbonyl oxygen to carbonyl oxygen atom of -COOH group (MC-K-2). Only one monoanion (MA) and one dianion (DA) are formed in S0 and S1 states. pKa for MC-N, N-MA and MA-DA equilibriums have been determined in S0 and S1 states and discussed.