Ab Initio Molecular Dynamics Study of Quadrupolar Spin Relaxation in an Ionic Liquid

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Nuclear spin relaxation is crucial for understanding ionic liquid dynamics.
• Ab initio molecular dynamics (aiMD) and density functional theory (DFT) are powerful tools for simulating molecular behavior.
• Accurate calculation of electric field gradients (EFG) is essential for predicting quadrupolar relaxation.

Purpose of the Study:

• To implement and validate a methodology for studying quadrupolar nuclear spin relaxation in ionic liquids.
• To investigate the dynamic behavior of ions in ionic liquids using aiMD and DFT.
• To determine the accuracy and efficiency of different solvation models for EFG calculations.

Main Methods:

• Employed ab initio molecular dynamics (aiMD) with DFT for ionic liquid simulations.
• Calculated electric field gradients (EFG) using quantum mechanically treated clusters with solvation models.
• Assessed the impact of coordination shell size on EFG calculation accuracy.
• Performed long simulation times ( > 330 ps) and ensemble averaging for converged relaxation rates.

Main Results:

• A simplified EFG calculation model using one coordination shell showed results within 4% of the two-shell model.
• The study confirmed that 2H relaxation of deuterated ethylammonium nitrate occurs in the extreme narrowing regime at 11.7 T.
• The calculated 2H relaxation rate (13.3 ± 1.0 Hz) was approximately 60% higher than experimental values.
• The developed methodology demonstrated comparable or superior accuracy to previous methods.

Conclusions:

• The implemented methodology provides a robust framework for studying quadrupolar relaxation in ionic liquids.
• The single-shell solvation model offers a computationally efficient alternative for EFG calculations.
• Further refinement may be needed to fully reconcile calculated and experimental relaxation rates.