Mechanistic Studies of Rearrangements and Exchange Reactions by Dynamic NMR Spectroscopy

Abstract

Since the first successful NMR experiments in 1946 it was well appreciated that dynamic processes play an important role in the NMR spectroscopy of bulk matter [1]. Early theories on the dependence of the relaxation parameters T 1 and T 2 on the motions of nuclear spins were successful in explaining the dipolar broadening of the NMR signal in solids and the motional narrowing in liquids [2]. With the discovery of chemical shifts and spin-spin couplings another type of dynamical process affecting the NMR line shape became apparent, the chemical exchange. As a consequence, dynamical NMR studies split into two groups differing not only in the dynamical topics but also in the method of investigation: physical studies of the motion of spins in liquids and solids by measurement of the relaxation times of single resonances and, on the other hand, chemical studies based on band shape analysis of NMR spectra recorded under steady state conditions. The two fields of research lost some of their basic differences with the development of the Fourier transform NMR method [3], which allows the measurement of relaxation times of different resonances at the same time, i.e. the study of differential motional behavior of different parts of molecules, thus providing a new tool in conformational analyses. For example, information can be obtained by this method on the relative importance of overall motions and internal motions [4]. On the other hand, recent theories on band shapes now also allow the extraction of detailed relaxation information from the spectra of coupled spin systems [5]. These new developments make the distinction between relaxation and band shape studies arbitrary and therefore it is justified to classify all the topics mentioned above by the same term “Dynamic NMR Spectroscopy” (DNMR). Reviews on several aspects of DNMR have been given in a book edited recently by Jackman and Cotton[6]. In this report the characteristics of one branch of DNMR will be described, the use of band shape analyses to obtain mechanistic information on rearrangements and chemical exchange processes.