THE STUDY OF DYNAMIC PROCESSES BY NMR

The traditional bases of protein chemistry have been sequencing for primary structure analysis and x-ray diffraction and nuclear magnetic resonance (NMR) spectroscopy for three-dimensional structure determinations. It is clear that a complete understanding of biochemical processes and their regulation will depend on information at the atomic level. Historically, structure determination by x-ray diffraction and NMR, and the characterization of dynamic processes by NMR, have lagged behind the genetic and biochemical characterization of cellular processes. However, new advances in both of these methods promise to accelerate the rate of structure determination and to increase the size and complexity of systems that can be studied by magnetic resonance. 

These exchanges often occur while the system is in macroscopic equilibrium—the sample itself remains the same and the dynamics may be invisible to other teclmiques. It is merely the enviromnent of a given nucleus that changes. Since NMR follows an individual nucleus, it can easily follow these dynamic processes. This is just one of several reasons that the study of chemical exchange by NMR is important. 

About 1950, Hahn s work with appropriately spaced pulses of radiofrequency radiation and a 1948 report by Nicolaas Bloembergen, Purcell, and R. V. Pound set the stage for the study of relaxation processes in NMR and for investigation of dynamic structure of liquids. 

The first applications of NMR to the study of dynamic systems of the platinum group metals appear to have been studies on the rotation about the metal-olefin bond of coordinated olefins, and this process has been investigated by many workers. There are two reasonable orientations of an olefin with respect to the rest of a square planar complex, XXIV and XXV. 

While various NMR techniques have been used for the characterization of reaction products and the profiling of reaction mixtures over time, different techniques have been used to monitor dynamic processes that occur during metathesis reactions. While many intermediate species have not yet been detected by NMR spectroscopy (e.g. 14e ruthenium carbenes, as mentioned above), the study of exchange processes can be used to infer their existence. 

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