Nuclear Magnetic Moments and NMR Measurements of Shielding
Gas phase experiments are applied together with the calculations of shielding in small molecules for the accurate determination of nuclear magnetic moments. The determination is possible due to the comparison of NMR experiments performed for the proton and another nucleus present in the same molecule. The accurate magnetic moments allow the measurements of shielding when an isolated helium-3 atom is used as the universal reference standard of shielding. Next the reference standard is transferred to the 2H signals of external lock solvents, in this way making the method easy and ready for application with most NMR spectrometers. It is shown that the new method of shielding measurements delivers valuable results, especially for 1H, 2H, 13C, and 15N experiments. In particular it was possible to measure the primary and secondary isotope effects for hydrogen molecules when 1H atoms were substituted by 2H. The application of shielding measurements may be easily extended on liquid and solid chemical compounds. As suggested the shielding measurements can be used for the alternative standardization of NMR spectra, if the appropriate nuclear moments are known with the sufficient accuracy.