Nuclear magnetic moments from NMR spectra—Experimental gas phase studies and nuclear shielding calculations

Abstract

NMR spectra of gaseous compounds and quantum chemical calculations are combined to determine new accurate values of magnetic dipole moments for a series of nuclei. We have analyzed shielding constants, resonance frequencies, and nuclear magnetic moments for a group of simple molecules. The chemical shifts and resonance frequencies are measured at 300 K and extrapolated to the zero-density limit in order to remove all the intermolecular effects from the experimental parameters. The absolute shielding constants in the studied molecules are obtained from ab initio calculations. Assuming the proton magnetic moment as the reference, we determine the nuclear magnetic moments of 13 C,

Theoretical modeling of 13C NMR chemical shifts—How to use the calculation results

“…The most precise calculations are performed for isolated molecules. At the same time, in the condensed phases and even in the gas phase intermolecular interactions are present (16). Inclusion of these effects into the theoretical model using the same level of theory as for the remaining calculations is simply not feasible at present.…”

A Brief Review on Bioactive Compounds from Species

“…where ν, µ and I are the resonance frequencies in a constant external magnetic field, the nuclear magnetic moments [3,13] and the spin numbers of helium-3 and another observed nucleus (X), respectively. Let us add that the resonance frequencies are strictly proportional to nuclear magnetic shielding in a constant magnetic field.…”

“…First, let us consider only one important problem in nuclear MR spectroscopy. From a theoretical view point it is obvious that any MR measurement performed for a macroscopic sample is disturbed by intermolecular interactions and temperature [3]. It occurs for solids, liquids and gases as well though these effects are considerably smaller in the gas phase than in solids or liquids.…”

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“…Nevertheless, the absolute shielding constant is important as a benchmark for quantum-chemical methods for calculating NMR parameters [4][5][6][7][8] as well as for the determination of nuclear magnetic moments. 9,10 The magnetic moment of a nucleus may also interact with the magnetic moment induced by the molecular rotation arising from the small decoupling of the rotation of the electrons from the rotation of the nuclear framework. 11 This interaction is observable as the hyperfine structure in highresolution rotational microwave spectra and is described by the nuclear spin-rotation tensor.…”

“…For use in comparing chemical shifts obtained experimentally with those obtained using nonrelativistic chemical shielding calculations, this represents a consistent procedure. However, if we, for instance, want to determine the nuclear magnetic moment of a nucleus, 10 such an approach is not appropriate, as it neglects the fact that Eq. (1) is not valid when a full relativistic description of the spin-rotation and nuclear magnetic shielding constants is considered.…”