NMR spectra of exchanging and reacting systems

One of the most important contributions that NMR has made to chemistry is the insight it has given into the dynamic, time-dependent nature of many systems, particularly those which are at equilibrium or where simply intramolecular motion is involved. Spectroscopy based on higher-frequency radiation, such as classical infrared (IR) or ultraviolet (UV) spectroscopy, has given mostly a static picture because the timescale of many processes is slow relative to the frequency used. However, the lower frequencies used for NMR and the smaller line separations involved, coupled with the small natural linewidths obtained, means that many time-dependent processes affect the spectra profoundly. As an example, we consider the spectroscopic behaviour of ethanol in Fig. 7.1. The proton spectrum of 50% ethanol, HOCH₂CH₃, in CDCl₃ is a methyl triplet due to coupling to the CH2, an OH triplet for the same reason and a doublet of quartets for the methylene protons. Any acidic impurity catalyses interchange of OH protons between molecules: https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315274690/c2783f29-8d32-4362-a870-c79c1f25faae/content/eqn189_1.tif"/>