Solvation and the excited-state tautomerization of 7-azaindole and 1-azacarbazole: Computer simulations in water and alcohol solvents

Computer simulations are performed in order to investigate the role hydroxylic solvents play in catalyzing the excited-state tautomerization of 7-azaindole (7-AI) and 1-azacarbazole (1-AC). Classical Monte Carlo and molecular dynamics methods are used to test the idea that reaction rates in these systems are controlled primarily by the fraction of solutes that are "correctly" solvated. Assuming that correct solvation involves formation of a cyclic 1:1 solute-solvent complex, reactive fractions are computed for a series of eight hydroxylic solvents: methanol, ethanol, 1-propanol, 2,2,2-trifluoroethanol, 2-propanol, tert-butyl alcohol, ethylene glycol, and water. In all cases the reactive fractions so calculated are small (<2%) and are of the correct magnitude to account for the relatively slow reaction observed in neat solvents. The underlying cause for these small reactive populations can be rationalized on the basis of the weak hydrogen bonds afforded by a cyclic arrangement. In nearly all cases these fractions correlate nicely with the observed reaction rates, thereby validating the basic picture of the solvent involvement in these reactions developed on the basis of experimental studies.