A comparison of calculated NMR shielding probes

Area of Science:

• Computational Chemistry
• Nuclear Magnetic Resonance (NMR) Spectroscopy

Background:

• Hydrogen nuclei near anisotropic functional groups experience magnetic shielding/deshielding.
• This phenomenon arises from magnetic anisotropy and intramolecular effects.
• Accurate calculation of these effects is crucial for understanding molecular structure and dynamics.

Purpose of the Study:

• To investigate computational probes for magnetic shielding surfaces near anisotropic functional groups.
• To compare computational predictions with experimental observations.
• To determine the most efficient and accurate computational method.

Main Methods:

• Utilized Gaussian 03 software with the GIAO-HF method.
• Calculated isotropic shielding values and NMR shielding increments.
• Employed various computational probes (methane, H2, H atom, He atom, ghost atom) over test molecules (ethene, ethyne, benzene, HCN).
• Examined single point vs. constrained geometry-optimized calculations and probe orientation effects.

Main Results:

• Ab initio methods can calculate magnetic shielding effects with reasonable accuracy.
• Single point calculations with a diatomic hydrogen probe oriented perpendicular to the pi bond yielded optimal, economical results.
• Atomic charges of probes were analyzed to understand probe-test molecule interactions.

Conclusions:

• Computational chemistry provides a reliable method for studying magnetic shielding near anisotropic functional groups.
• Diatomic hydrogen as a probe in single point calculations offers an efficient approach.
• This method aids in predicting and understanding NMR shielding increments in various molecular systems.