Magnetic field-induced effects on NMR properties

Area of Science:

• Physical Chemistry
• Nuclear Magnetic Resonance Spectroscopy
• Quantum Chemistry

Background:

• Nuclear Magnetic Resonance (NMR) observables, including spin-spin coupling (J), nuclear shielding (σ), and quadrupole coupling (q), are fundamentally dependent on the applied magnetic field.
• This magnetic field dependence can manifest directly, through deformation of the molecular electronic cloud, or indirectly, via anisotropic molecular orientation distributions influenced by magnetic susceptibility.

Purpose of the Study:

• To investigate and quantify the indirect magnetic field dependence of one-bond Hydrogen-Carbon (¹H-¹³C) and Deuterium-Carbon (²H-¹³C) spin-spin couplings (J couplings) and Deuterium (²H) quadrupole coupling.
• To determine the susceptibility anisotropy, ²H quadrupole coupling constant, asymmetry parameter, and extrapolated one-bond CH and CD coupling constants at vanishing field strength using 1,3,5-D₃-benzene as a model system.

Main Methods:

• Experimental NMR measurements were conducted on 1,3,5-D₃-benzene at four distinct magnetic fields: 4.7, 9.4, 14.1, and 18.8 Tesla.
• A joint fitting procedure was employed to analyze data acquired at different field strengths, enabling the extraction of key physical parameters.
• The study focused on characterizing the indirect magnetic field effect, distinguishing it from direct effects.

Main Results:

• The indirect magnetic field effect was found to be significant even at magnetic fields commonly utilized in modern NMR spectrometers.
• Experimental values for susceptibility anisotropy, ²H quadrupole coupling constant, asymmetry parameter, and one-bond CH and CD coupling constants extrapolated to zero field were successfully determined.
• The determined field-induced contributions were observed to surpass typical error margins for coupling constants, and a primary isotope effect on the one-bond CH coupling constant was indicated.

Conclusions:

• The indirect magnetic field effect plays a substantial role in NMR measurements, influencing key spectroscopic parameters.
• Increasing magnetic field strengths in NMR spectroscopy will lead to more pronounced indirect, and potentially direct, magnetic field effects.
• Accurate determination of NMR parameters requires careful consideration and correction for magnetic field-dependent phenomena, especially at high fields.