Comparing Vibrationally Averaged Nuclear Shielding Constants by Quantum Diffusion Monte Carlo and Second-Order...

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Accurate computation of nuclear shielding constants is crucial for interpreting NMR spectroscopy.
• Molecules with large-amplitude motions, such as internal rotations, pose challenges for traditional computational methods.
• Second-order perturbation theory (PT) may not be sufficient for these complex systems.

Purpose of the Study:

• To compute vibrationally averaged isotropic nuclear shielding constants (⟨σ⟩) for H2O, O3, and HCOOH using quantum diffusion Monte Carlo (QDMC).
• To compare QDMC results with second-order PT to assess the adequacy of PT for molecules with large-amplitude motions.
• To develop and validate a method for constructing shielding constant surfaces using modified Shepard's interpolation.

Main Methods:

• Modified Shepard's interpolation was used to construct potential energy surfaces.
• Quantum diffusion Monte Carlo (QDMC) was employed to compute ⟨σ⟩.
• Shielding constant surfaces were constructed and utilized within the QDMC framework.

Main Results:

• QDMC calculations provided accurate ⟨σ⟩ values for H2O, O3, and HCOOH.
• Significant differences were observed between QDMC and second-order PT for HCOOH, particularly for hydrogens and the carbonyl oxygen.
• The developed method for constructing shielding constant surfaces reproduced ab initio results with high accuracy.

Conclusions:

• Quantum diffusion Monte Carlo is a superior method for calculating nuclear shielding constants in molecules exhibiting large-amplitude motions.
• Second-order perturbation theory is inadequate for obtaining experimental-quality ⟨σ⟩ for molecules like HCOOH.
• The novel approach to constructing shielding constant surfaces can reduce the need for expensive ab initio calculations.