tailieunhanh - Ebook A complete introduction to modern NMR spectroscopy: Part 2

(BQ) Part 2 book "A complete introduction to modern NMR spectroscopy" has contents: The study of dynamic processes by NMR; electron paramagnetic resonance spectroscopy and chemically induced dynamic nuclear polarization; double resonance techniques and complex pulse sequences, two-dimensional nuclear magnetic resonance, NMR studies of biological important molecules,.and other contents. | 10 THE STUDY OF DYNAMIC PROCESSES BY NMR REVERSIBLE AND IRREVERSIBLE DYNAMIC PROCESSES In chemistry the term dynamic implies changing with time. Dynamic physical and chemical processes are of two basic types irreversible and reversible. Irreversible processes are normally encountered in the context of chemical reactions. When the free energy G of the product s of a reaction is sufficiently lower than the free energy of the reactant s the final equilibrium reaction mixture will comprise essentially all product s and no reactants . For example a free-energy difference as little as 5 kcal mol ca. 21 kj mol means the equilibrium mixture will consist of products at 25 c Eq. . In such cases we can regard the reaction as essentially irreversible that is going in only one direction or going to completion. If the rate of an irreversible reaction is slow enough that there is no significant change in the composition of the reaction mixture during the time it takes to acquire a spectrum the reaction can be monitored by NMR collecting spectra at regular intervals. It is essential that the temperature of the reaction mixture be held constant C to generate precise kinetic data. Subsequent Fourier transformation followed by integration of the appropriate signals allows us to follow either the disappearance of reactant s or the appearance of product s as a function of time. Such composition-versus-time data can then be fit to appropriate rate laws to determine rate constants half-lives and activation parameters for reaction. Because collection of suitable FID data requires a minute or so for a typical1H spectrum the half-life of an irreversible reaction should be at least on the order of several minutes to be monitored by NMR techniques. When studying irreversible reactions by NMR it is often useful to include an internal standard an inert substance added to the reaction mixture at the beginning of the reaction that generates a signal whose intensity is constant