Unexpected geometrical effects on paramagnetic spin-orbit and spin-dipolar 2J(FF) couplings

Area of Science:

• Quantum Chemistry
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Computational Chemistry

Background:

• The second-rank tensor character of paramagnetic spin-orbit and spin-dipolar contributions to nuclear spin-spin coupling constants is typically disregarded in isotropic phase NMR measurements.
• This simplification may overlook crucial orientation-dependent effects influencing coupling constants.

Purpose of the Study:

• To investigate the influence of the second-rank tensor character on nuclear spin-spin coupling constants in the isotropic phase.
• To demonstrate the strong dependence of isotropic (2)J(FF) couplings on the relative orientation of C-F bonds and tensor eigenvectors.

Main Methods:

• Qualitative predictions using the polarization propagator formalism at the Random Phase Approximation (RPA) level.
• Corroboration through high-level ab initio spin-spin coupling calculations (SOPPA(CCSD)/EPR-III//MP2/EPR-III) in a model system.
• Note: RPA level calculations were not performed in this work; predictions were theoretical.

Main Results:

• Isotropic (2)J(FF) couplings exhibit a significant dependence on the relative orientation of the involved C-F bonds and the eigenvectors of the relevant tensors.
• The study provides a theoretical framework and computational evidence for this orientation-dependent phenomenon.
• The findings suggest that orientation effects, often ignored, play a critical role in determining coupling constants.

Conclusions:

• The orientation of molecular fragments, specifically C-F bonds, critically affects nuclear spin-spin coupling constants in the isotropic phase.
• This work challenges the conventional assumption of orientation independence for these couplings.
• The observed properties are expected to extend to the second-rank nuclear magnetic shielding tensor, suggesting broader implications for NMR analysis.