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Published on: June 10, 2018
A solid-state 39K and 13C NMR study of polymeric potassium metallocenes
Cory M Widdifield1, Robert W Schurko
1Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4.
The Journal of Physical Chemistry. A
|July 13, 2006
Summary
This study presents the first solid-state 39K NMR spectra for organometallic complexes CpK and Cp*K. These techniques reveal significant quadrupolar interactions and bent structures around potassium atoms, offering insights into molecular dynamics.
Area of Science:
- Solid-state NMR spectroscopy
- Organometallic chemistry
- Materials science
Background:
- Nuclear Magnetic Resonance (NMR) is a powerful technique for elucidating molecular structure.
- Solid-state NMR is crucial for studying insoluble or non-crystalline materials like polymers.
- Potassium-39 (39K) NMR is challenging due to its quadrupolar nature, requiring specialized techniques.
Purpose of the Study:
- To obtain the first solid-state 39K NMR spectra of organometallic complexes CpK and Cp*K.
- To investigate the quadrupolar interactions and structural properties of these potassium-containing metallocenes.
- To explore temperature-dependent structural changes and identify impurities using advanced NMR methods.
Main Methods:
- Quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) and double frequency sweep (DFS)/QCPMG pulse sequences for 39K NMR.
- Piecewise QCPMG techniques for high signal-to-noise ratio spectra of broad transitions.
- 13C Cross-Polarization Magic Angle Spinning (CP/MAS) NMR for carbon chemical shielding anisotropy (CSA).
- Variable-temperature (VT) 39K NMR experiments.
- Ab initio calculations of carbon CSA and 39K electric-field gradient (EFG).
Main Results:
- Successful acquisition of solid-state 39K NMR spectra for CpK and Cp*K.
- Significant quadrupolar interactions (CQ) and asymmetry parameters (ηQ) indicating bent structures around potassium nuclei.
- Temperature-dependent changes in quadrupolar parameters for CpK, linked to bond distance and angle alterations.
- Quantification of carbon CSA at Cp' ring carbons using 13C CP/MAS NMR.
- Correlation between experimental NMR parameters and molecular structure through ab initio calculations.
Conclusions:
- 39K DFS/QCPMG and 13C CP/MAS NMR are effective for probing molecular structure in organometallic complexes.
- These NMR techniques are valuable for studying temperature-dependent structural dynamics.
- The methods can identify structural features like bent geometries and detect impurities.
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