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A solid-state nitrogen-15 NMR and ab initio study of nitrobenzenes
Glenn H Penner1, Guy M Bernard, Roderick E Wasylishen
1Department of Chemistry, University of Guelph, Ontario, Canada N1G 2W1. gpenner@uoguelph.ca
The Journal of Organic Chemistry
|May 24, 2003
Summary
Researchers explored nitrogen chemical shifts in nitrobenzene derivatives using solid-state NMR and computational methods. They found that substituent effects on chemical shift tensor components cancel out, resulting in a narrow range of observed isotropic shifts.
Area of Science:
- Physical Chemistry
- Solid-State Nuclear Magnetic Resonance (NMR) Spectroscopy
- Computational Chemistry
Background:
- Nitrobenzene derivatives exhibit a surprisingly narrow range of isotropic nitrogen chemical shifts.
- Understanding the factors influencing these shifts is crucial for interpreting NMR data in organic chemistry.
Purpose of the Study:
- To investigate the reasons behind the limited variation in isotropic nitrogen chemical shifts in substituted nitrobenzenes.
- To correlate chemical shift tensor components with electronic properties of para-substituents.
Main Methods:
- Solid-state Nitrogen-15 (15N) Chemical Shift (CS) tensor measurements using NMR spectroscopy.
- Ab initio molecular orbital (MO) and Density Functional Theory (DFT) calculations (HF, MP2, B3LYP/6-311G) to determine CS tensor components.
- Analysis of principal components (delta(ii)) and their orientation relative to the nitro group.
Main Results:
- Experimental measurement of 15N CS tensor components for various nitrobenzene derivatives.
- Significant variations in individual CS tensor components (delta(11), delta(22), delta(33)) were observed with para-substitution.
- These variations largely cancel each other, leading to a small overall range (3-4 ppm) for the isotropic chemical shift (delta(iso)).
- DFT calculations (B3LYP) showed good agreement with experimental data, revealing trends in tensor components related to substituent electron-withdrawing ability.
- The orientation of the CS tensor was found to be similar to that in the benzoate anion.
Conclusions:
- The narrow range of isotropic nitrogen chemical shifts in nitrobenzene derivatives is a result of the near cancellation of opposing changes in the chemical shift tensor components.
- Computational methods, particularly DFT with the B3LYP functional, are effective in predicting these tensor components and their behavior.
- The orientation of the nitrogen chemical shift tensor provides insights into the electronic structure of the nitro group.