Exploring the relation between shielding constants, resonance frequencies
and magnetic moments of the nuclei we demonstrate that nuclear magnetic
shielding can be directly observed from NMR spectra. In this approach,
the absolute shielding constants of all the nuclei can be related
to a single reference scale, with atomic (3)He as the primary standard.
The accuracy of the data obtained using our method is confirmed comparing
the (1)H and (13)C shielding constants for a series of deuterated
compounds with those determined analyzing the traditional chemical
shifts. Since the use of helium-3 is not in general a practical alternative,
we next transfer the reference standard to the (2)H signals of external
lock solvents, in this way making the method easy and ready for application
with most NMR spectrometers. Finally, we illustrate our new method
with the measurements of the (2/1)H primary isotope effects in several
liquid deuterated solvents.
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