Solvatochromism and prototropism in methyl 6-aminonicotinate: failure to observe amine-imine phototautomerism in solvents

Absorption, fluorescence excitation and fluorescence spectroscopic and time dependence studies on methyl 6-aminonicotinate (6-ANE) have been carried out in different solvents and under different acid-base concentrations. Observance of only one small Stokes shifted fluorescence band and single exponential decay in all the solvents confirms the presence of amino derivative of 6-ANE. AM1 semi-empirical and density functional theory calculations have supported the presence of amino derivative and the absence of imino form of 6-ANE. Monocation (MC) is formed by protonating the pyridine nitrogen atom, both in the ground (S0) and first excited (S1) states. Dication (DC) is formed by protonating the carbonyl group. In polar and protic non-aqueous solvents, behavior of the ionic species is similar to that in aqueous medium. q 2004 Elsevier B.V. All rights reserved