Solid-State Nuclear Magnetic Resonance Spectroscopy
Monitoring Molecular Packing and Dynamic Processes in Bulk Material
Solid-state NMR spectroscopy is a versatile method to study local molecllular packing as well as dynamic molecular process in bulk material. Compared to the much more common solution NMR spectroscopy, which povides the chemical structure of isolated dissolved molecules in great detail, the capability of solid state NMR spectroscopy to analyze chemical structures is limited. Due to the lack of rapid isotropic Brownian motion of molecules in the solid-state, anisotropic NMR interactions are not averaged to a well defined isotropic value, so that the information of the chemical structure is overlayed with the information on the local orientation of the individual molecules in the magnetic field and the local arrangement of neighboring molecules in close spatial proximity.
Fast sample rotation at the magic angle (MAS) can average out most of the anisotropic NMR interactions and improve the very short T2 relaxation times. Combining MAS with tailored recoupling pulse sequences, which selectively reintroduce anisotropic NMR interaction in a time controlled manner, allows us to the utilize the otherwise obscuring local NMR interactions to gain information about local spatial proximities and molecular orientations. This local packing information is crucial for the functional properties of the materials for e.g. molecular electronics applications. Nevertheless, the chemical shift resolution for strongly coupled 1H site in the solid state is strongly limited and most our solid-state NMR studies try to combine the intense, relatively fast relaxing 1H polarization with better resolved NMR nuclei like 13C, 31P or 29Si via CP-MAS or other polarization transfer methods.
In many functional materials, however, not only the local packing on the molecular level determines the functional properties. Solid-state NMR is particular well suited to monitor dynamic processes on the molecular level on nano second to seconds and even longer time scales. A broad variety of one and two dimensional methods monitoring the NMR line shape and chemical exchange processes can provide a detailed insight in dynamic processes in bulk materials, and thus help to understand and improve the functional properties of novel materials.
The solid-state NMR group at MPIP provides both, simple routine CP-MAS NMR chemical characterization of unsoluble materials as well as complete studies of local packing or molecular dynamics processes.