Indoline: a versatile probe of specific and non-specific solvation forces

Abstract

Electronic and vibrational spectra of the polycyclic heterocycle indoline have been used to explore both specific and nonspecific solvation forces in a wide variety of solvents. Excitation of indoline to its first excited state shows little sensitivity to solvent dielectric properties, but excitation wavelengths are very dependent on a solvent's ability to accept and/or donate hydrogen bonds. In contrast, emission spectra of indoline in different solvents demonstrate classical dependence on solvent polarity in accord with nonspecific solvation mechanisms. The frequency associated with the single NH stretch, ν_NH, also depends sensitively on specific solvation forces. In different solvents ν_NH can vary by more than 100 cm⁻¹ depending on the opportunities that exist for hydrogen bond formation. In order to predict the strength of specific solvent–solute interactions, a semi-empirical scale is developed based on a ν_NH and each solvent's ability to accept hydrogen bonds. This scale was tested with a second solute, indole, and accurately predicted ν_NH of indole in different solvents (and in the gas-phase) to within 10 cm⁻¹ in most cases.