Molecular and spin dynamics in solid state nuclear magnetic resonance spectroscopy

Abstract

Nuclear Magnetic Resonance (NMR) spectroscopic data often involve spatial and spin dynamics and interference phenomena between the two. Experimental results are presented which demonstrate several of these phenomena and exploitation of these anomalies to acquire and improve knowledge regarding relevant chemistry. In some cases numerical analysis of such data are illustrated for obtaining quantitative rates of molecular dynamics. First, the dynamics of methyls bonded to the metals Tungsten and Tantalum are examined. In the case of Tungsten. an interference between methyl dynamics and proton decoupling is studied, whereas in the Tantalum instance, interference between molecular dynamics and Magic Angle Spinning (MAS) is examined. Agostic interactions had been hypothesized to exist in both compounds, but the unusual molecular dynamics are explainable without invoking a relation to such interactions. Further, no evidence is found to support the existence agostic bonds from examination of X-ray crystal data. Next. the spin dynamics in high resolution solid state proton systems diluted by deuterium is examined. In this case techniques such as Rotational Resonance and Radio-Frequency driven Dipolar Recoupling (RFDR) are employed to counteract the averaging of dipole couplings induced by MAS. The numerical analysis of such data in systems in which internuclear distances of interest are not identical but rather statistically distributed is discussed. Finally, a technique which may improve the sensitivity of NMR experiments is demonstrated. Dynamic Nuclear Polarization (DNP) is adapted to solid state MAS experiments and shown to enhance the signal intensity of a biomolecule, T4-lysozyme, by a factor of ~50. The necessary conditions for application of this technique to solid state NMR is discussed.