The effect of weak intermolecular interactions on the nuclear magnetic resonance shielding constant in N2

Searching for the Best Values of NMR Shielding and Spin-Spin Coupling Parameters: CH4-nFn Series of Molecules as the Example

Attempts at the theoretical interpretation of NMR spectra have a very long and fascinating history. Present quantum chemical calculations of shielding and indirect spin-spin couplings permit modeling NMR spectra when small, isolated molecules are studied. Similar data are also available from NMR experiments if investigations are performed in the gas phase. An interesting set of molecules is formed when a methane molecule is sequentially substituted by fluorine atoms-CH4-nFn, where n = 0, 1, 2, 3, or 4. The small molecules contain up to three magnetic nuclei, each with a one-half spin number. The spectral parameters of CH4-nFn can be easily observed in the gas phase and calculated with high accuracy using the most advanced ab initio methods of quantum chemistry. However, the presence of fluorine atoms makes the calculations of shielding and spin-spin coupling constants extremely demanding. Appropriate experimental 19F NMR parameters are good but also require some further improvements. Therefore, there is a real need for the comparison of existing NMR measurements with available state-of-the-art theoretical results for a better understanding of actual limits in the determination of the best shielding and spin-spin coupling values, and CH4-nFn molecules are used here as the exceptionally important case.

NMR shielding in helium-3 atoms modified by gaseous nitrogen and oxygen

Helium-3 nuclear magnetic resonance (³ He NMR) measurements were carried out for the gaseous mixtures of helium-3 with pure nitrogen and synthetic air as the solvents. It was found that ³ He shielding is linearly dependent on solvent density up to approx. 6 mol/L. At higher density of the gaseous solvent, the change of ³ He shielding is nonlinear and especially distinct when helium-3 atoms can interact with two O₂ molecules. The interaction with paramagnetic oxygen molecules can induce two kinds of ³ He shielding changes: (1) due to the isotropic Fermi contact interaction and (2) from the dipolar magnetic interaction between unpaired O₂ electrons and ³ He nuclear magnetic dipole moment. The two paramagnetic effects in helium-3 shielding cannot be experimentally separated, although for such small molecular objects, they could be presumably modeled by advanced theoretical calculations.