Ultrafast Protonation/Deprotonation Dynamics of N,N-Dimethylacetamide in Hydrochloric Acid As Studied by Raman Band Shape Analysis

Abstract

The protonation/deprotonation dynamics of N,N-dimethylacetamide (DMAA) in hydrochloric acid has been studied with a Raman band shape analysis based on the two-state dynamic exchange model. The observed Raman band shape changes of the symmetric and antisymmetric CC/CN stretch modes with acid concentration have been successfully interpreted in terms of the ultrafast dynamic exchange between DMAA and its protonated form DMAAH+. It is confirmed that the protonation takes place on the oxygen atom rather than on the nitrogen atom. Quantitative information on the exchange dynamics between DMAA and DMAAH+ has been obtained from the curve fitting of the observed band shapes. The protonation process follows the first-order kinetics with a rate constant of k1 = (10.1 ± 0.2) × 1010 s−1 M−1, which is slightly larger than the value expected for a simple diffusion-controlled reaction and which can be explained in terms of the proton transfer through the hydrogen-bonding network. The mean lifetime of the protonated form DMAAH+ is strongly dependent on the acid concentration; it is several picoseconds in concentration lower than 0.5 mol dm−1 but is prolonged to a few tens of picoseconds in concentrations higher than 2 mol dm−1. It is also shown that the static equilibrium model, which corresponds to the slow exchange limit, fails to explain the observed band shape changes with acid concentration and that they can be explained if and only if the effect of ultrafast exchange is taken into account.