NMR DETERMINATION OF ABSOLUTE SITE-SPECIFIC NATURAL ISOTOPE RATIOS OF HYDROGEN IN ORGANIC MOLECULES. ANALYTICAL AND MECHANISTIC APPLICATIONS

Article abstract of DOI:10.1016/S0040-4020(01)96679-7

It has been shown that the "internal" isotope distribution within a given molecular species at the natural abundance level is accessible by a new method, SNIF-nmr, which is based on deuterium NMR.Relative internal factors, R_i/j, have been defined which enable the isotope content of a given site, i, to be compared to that of another molecular site, j, taken as a reference.Several referencing methods intended to provide direct access to relative externals, T_i, and absolute, (D/H), site-specific parameters, are now discussed from both the theoretical and the experimental points of wiev.In the intramolecular referencing method, which involves a time-consuming chemical transformation of the sample, the risk exists of more or less systematic errors resulting from discriminating fractionation effects.However this technique offers, conversely, an interesting way of investigating kinetic isotope effects without the need for specific labelling.In spite of its lower spectral precision the external referencing method has the advantage of being fast and less sensitive to systemetic errors and may be used for direct rough routine determinations of the site-specific isotope contents.More precise results can be obtained, at the price of contaminating the sample, when an intermolecular reference is added and signal heights are used, remembering however that the intensity parameters then have no strict physical meaning in terms of absolute isotope contents.The site-specific parameters, Tⁱ and (D/H)ⁱ thus accessible, provide new information on the mechanisms of the fractionation effects occuring in natural conditions and exaples are considered.