engleski

Isotope effects in NMR spectra as a structural tool for organic molecules

A survey on recent results and developments in the field of isotope effects in NMR spectroscopy is given in the present article. Isotope effects on nuclear shielding, spin-spin coupling and relaxation time are discussed. The established empirical trends and correlations of isotope effects with molecular parameters help in better understanding of this phenomenon and also provide a wealth of information which can be very useful in organic and analytical chemistry. It is shown that isotope effects in NMR spectra can serve as an efficient tool for studying structure and conformation of organic molecules. Special attention is devoted to unsaturated systems containing one or two phenyl rings connected via different linking groups. In these p-electron molecules long-range isotope effects are operative, providing data not only on structure, but also on mechanism of isotope effect transmission. Thus, the alternation in sign and magnitude of long-range isotope effects is interpreted in terms of subtle charge shifts throughout the molecule as a consequence of the bond shortening upon isotopic substitution, while the linear correlation between long-range isotope effects and molecular torsional angle can serve as a conformational probe for molecules of the related type.

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