Solvatlon dynamics in N-methylamides

Solvation times in three homologous amides, N-methylformamide, N-methylacetamide, and N-methylpropionamide, have been detd. from measurements of the dynamic Stokes shift of the fluorescence spectra of two-probe solutes, prodan and coumarin 102. Single-particle reorientation times have also been measured in one of these solvents, N-methylformamide, by using NMR methods. The solvation dynamics are compared to two theoretical models, the simple continuum model and the dynamic MSA model. Although neither model predicts the time dependence of the response satisfactorily, the average solvation times observed are close to the solvent longitudinal relaxation time (τ_L) predicted by the simple continuum model. In contrast, the predictions of the dynamical MSA model are approximately 4 times slower than the observed solvation response. The failure of the latter model appears to result from an overestimation of the single-particle reorientation times of these solvents. Esimates of such single-particle times based on the NMR measurements are within a factor of 2 of τ_L. This similarity seems to account for the near equality of average solvation times to τ_L in the amides.