No abstract

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.

13 C and 1 H NMR spectra have been measured for 1,2-13 C-acetylene at low density in binary mixtures with xenon and carbon dioxide gases as the solvents. It was found that both the chemical shifts and the 1 J(CC) coupling constant of 13 C 2 H 2 were linearly dependent on the solvent density. The appropriate NMR parameters for an isolated 13 C 2 H 2 molecule were determined, together with the coefficients responsible for the solutesolvent molecular interactions. The 13 C NMR measurements were extended to solutions of 1,2-13 C-acetylene in liquid solvents. All of the 13 C gas-to-solution shifts were found to be positive (deshielding effects), and the 1 J(CC) coupling constant in liquid acetone was diminished by more than 5% in comparison with the value of 174.78 Hz observed in the gas phase.