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Computation of through-space NMR shielding effects in aromatic ring pi-stacked complexes
Ned H Martin1, Ruth M Floyd, H Lee Woodcock
1Department of Chemistry and Biochemistry, University of North Carolina Wilmington, 601 S. College Road, Wilmington, NC 28403-5932, USA. martinn@uncw.edu
Journal of Molecular Graphics & Modelling
|November 21, 2007
Summary
Aromatic compounds form stacked dimers, influencing NMR shielding. Complexation shields nearby hydrogen probes, with effects explained by pi-cloud polarization.
Area of Science:
- Computational chemistry
- Nuclear Magnetic Resonance (NMR) spectroscopy
- Supramolecular chemistry
Background:
- Aromatic compounds can form dimeric complexes in solution.
- Substituted aromatics often form parallel-stacked complexes, either aligned or offset.
- Understanding these interactions is key to predicting molecular behavior in solution.
Purpose of the Study:
- To calculate NMR isotropic shielding values for a pi-stacked complex of 1,3,5-trimethylbenzene and 1,3,5-trinitrobenzene.
- To investigate the effect of complexation on NMR shielding of hydrogen probes.
- To interpret the results in terms of pi-cloud polarization and its NMR consequences.
Main Methods:
- Employed the HF-GIAO method in Gaussian 03 for calculations.
- Calculated NMR isotropic shielding values for hydrogen probes at various positions.
- Subtracted individual shielding values from complex values to determine complexation effects.
Main Results:
- Complexation causes each aromatic ring to shield the other.
- NMR shielding of a hydrogen probe molecule increases on both sides of the pi-stacked complex.
- Shielding is slightly greater on the side nearest 1,3,5-trimethylbenzene.
Conclusions:
- Complexation-induced pi-cloud polarization significantly impacts NMR shielding.
- The study provides insights into the NMR consequences of aromatic complexation.
- NMR shielding calculations on N-phenylpyrrole dimer were compared to experimental data to estimate dimer concentration.
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