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Zero-point vibrational effects on proton shieldings: functional-group contributions from ab initio calculations
K Ruud1, P O Astrand, P R Taylor
1San Diego Supercomputer Center and Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive MC0505, La Jolla, California 92093-0505, USA.
Zero-point vibrational motion significantly impacts nuclear magnetic shielding constants in organic molecules. These vibrational corrections are crucial for accurate chemical shift calculations and are transferable for hydrogen atoms.
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Molecular Spectroscopy
Background:
- Nuclear magnetic shielding constants are essential for interpreting NMR spectra.
- Vibrational motion can influence electronic properties, including magnetic shielding.
- Previous studies often neglected or approximated vibrational effects.
Purpose of the Study:
- To investigate the impact of zero-point vibrational motion on nuclear magnetic shielding constants in organic molecules.
- To quantify vibrational corrections, including anharmonic and harmonic contributions.
- To assess the transferability of these corrections across different molecules.
Main Methods:
- Ab initio calculations of potential energy surfaces and property surfaces.
- Inclusion of anharmonic contributions from the potential energy surface.
- Inclusion of harmonic contributions from the curvature of the property surface.
- Systematic study on a large set of organic molecules.
Main Results:
- Vibrational corrections to hydrogen shielding constants can be substantial (+0.50 to -0.70 ppm).
- Ignoring these effects can lead to chemical shift errors exceeding 1 ppm.
- Vibrational corrections for hydrogen shieldings demonstrate significant transferability between molecules.
- No similar transferability was observed for carbon, nitrogen, or oxygen shielding constants.
Conclusions:
- Zero-point vibrational motion effects are critical and cannot be neglected for accurate NMR chemical shift predictions.
- Transferable vibrational corrections for hydrogen shieldings can be tabulated and utilized.
- Further research is needed for understanding vibrational effects on other nuclei.
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