Reaction Studies Using NMR

This section contains a brief survey of NMR spectroscopic investigations of chemical reaction kinetics and mechanisms. One of the goals of reaction kinetics studies is to measure the rate of the reaction (or rate constant) - the rate at which the reactants are transformed into the products. Another goal is to determine the elementary steps that constitute a multi-step reaction. Finally, and perhaps the most important goal is to identify transitory intermediate species. NMR, in common with other spectroscopic techniques, is especially valuable in achieving this 

The use of NMR for studying chemical reactions began about 30 years ago. In 1972, Asahi and Mizuta [11] reported a study of the performance of five different types of flow cells used in NMR studies of chemical reactions (a) a straight glass tube, (b) a pipette-type tube, (c) a spiral capillary in a conventional sample tube, (d) a jet in the base of a conventional tube, and (e) a conventional spinning tube with an inlet at the base and an outlet at a height of about 50 mm. The best results were 

Several other organic and inorganic reaction intermediates have been studied using NMR methods. Trahanovsky et al. [12] reported a series of experiments in which they studied unstable molecules, such as benzocyclobutadiene, using flow NMR. Tan and Cocivera [13, 14] studied the reaction of 4-formylpyridine with amino acids, imidazole and D,L-alanylglycine using stopped-flow proton NMR. 

Although limited by sensitivity, chemical reaction monitoring via less sensitive nuclei (such as 13C) has also been reported. In 1987 Albert et al. monitored the electrochemical reaction of 2,4,6-tri-t-butylphenol by continuous flow 13C NMR [4]. More recently, Hunger and Horvath studied the conversion of vapor propan-2-ol (13C labeled) on zeolites using 1H and 13C in situ magic angle spinning (MAS) NMR spectroscopy under continuous-flow conditions [15]. 

The application of NMR to the study of chemical reactions has been expanded to a wide range of experimental conditions, including high pressure and temperatures. In 1993, Funahashi et al. [16] reported the construction of a high pressure 3H NMR probe for stopped-flow measurements at pressures 200 MPa. In the last decade, commercial flow NMR instrumentation and probes have been developed. Currently there are commercially available NMR probes for pressures of 0.1-35 MPa and temperatures of 270-350 K (Bruker) and 0.1-3.0 MPa and 270-400 K (Varian). As reported recently, such probes can be used to perform quantitative studies of complicated reacting multicomponent mixtures [17]. 

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