Isotopic substitution, nuclear magnetic

Primary isotope effects are defined as the difference in magnetic shielding of the isotopes of nuclei of the same element. Secondary isotope effect on nuclear shielding are considered as intrinsic isotope effects and equilibrium isotope effects. The former effects are caused by isotopic substitution, the later by conformational changes or shifts in equilibria as a consequence of isotopic substitution. 

As the glucose is metabolized, the metabolic derivatives become enriched with the isotopic carbon label and are detected using advanced technologies such as mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Importantly, incorporation of isotopes into molecules can alter reaction mechanisms including the rate of a reaction-this is termed isotopic substitution and forms the basis of the kinetic isotope effect. 

The natural stable isotope of fluorine, fluorine-19 (19F), with a spin of one-half and a chemical shift range of around 300 ppm, is a sensitive and useful probe in nuclear magnetic resonance (NMR) studies. Fluorine substitution may be a very effective method for studying the fate of bioactive molecules. Since there are few natural fluorinated materials to create background signals, the analyses are freed from the complications often associated with proton NMR spectroscopy (65). An artificially prepared useful short-lived isotope, fluorine-18 (18F), decays by positron emission. Positron emission tomography (PET) is an especially useful non-invasive... 

Experimentally, water exchange rate constants are mainly determined from nuclear magnetic resonance measurements [6, 7]. Other techniques are restricted to very slow reactions (classical kinetic methods using isotopic substitution) or are indirect methods, such as ultrasound absorption, where the rate constants are estimated from complex-formation reactions with sulfate [3]. The microscopic nature of the mechanism of the exchange reaction is not directly accessible by experimental methods. In general, reaction mechanisms can be deduced by experimentally testing the sensitivity of the reaction rate to a variety of chemical and physical parameters such as temperature, pressure, or concentration. 

P. W. Fowler and W. T. Raynes, Mol, Phys., 43, 65 (1981). The Effects of Rotations, Vibrations and Isotopic Substitution on the Electric Dipole Moment, the Magnetizability and the Nuclear Magnetic Shielding of the Water Molecule. 

There are several experimental tools available for the determination of the H-H distance and the degree of the H-H bonding interaction. Neutron diffraction studies provide an accurate measure of the H-H distance. The measurement of the spin-lattice proton relaxation time, Ti, for an tf -V 2 complex or the proton-deuteron couphng constant, Jhd. for the corresponding isotopically substituted rf -WT) complex via H nuclear magnetic resonance (NMR) spectroscopy provides a quantitative measure of the H-H distance. The frequency of the v(H-H) stretching band, as determined by Raman or infrared (IR) spectroscopy of / -H2 complexes provides semiquantitative information about the strength of the H-H interaction. 

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