Reducing Exact Two-Component Theory for NMR Couplings to a One-Component Approach: Efficiency and Accuracy

Area of Science:

• Quantum chemistry
• Relativistic effects in spectroscopy
• Nuclear magnetic resonance (NMR)

Background:

• The exact two-component (X2C) formalism accurately describes relativistic effects in NMR spin-spin coupling constants.
• Computational cost of X2C methods can be prohibitive for large molecular systems.

Purpose of the Study:

• To develop a computationally efficient one-component approximation of the X2C formalism for NMR spin-spin coupling constants.
• To partition the first-order response into physically meaningful contributions (Fermi-contact, spin-dipole, paramagnetic spin-orbit).

Main Methods:

• Reduction of the complex X2C formalism to a scalar one-component ansatz.
• Integration with modern density functional theory (DFT) including current density response.
• Application to a large tin compound (137 atoms).

Main Results:

• Significant reduction in computational demands (8-24 times).
• Demonstration of the partitioning of the response term into real (FC+SD) and imaginary (PSO) parts.
• Assessment of the ansatz's accuracy for compounds containing Sn, Pb, Pd, and Pt.

Conclusions:

• The one-component ansatz offers a computationally feasible alternative to X2C for NMR coupling constants.
• The approach is generally accurate but shows limitations for systems with heavy halogens bonded to tin.