TOM PRATUM
Education
BS Chemistry University of Puget Sound, 1979
PhD, Physical Chemistry, UC Berkeley, 1983
Postdocs: University of British Columbia, Physics, 1984; University of Washington, Chemistry 1985
Research Interests
Solid and liquid state NMR in both its theory and in application development.
Why Solid State NMR?
• Sometimes it is simply because you can't dissolve your sample in a solvent.
• Sometimes it is because by dissolving the sample in a solvent, you change its' structure.
• Sometimes it is because you can observe things in a solid state NMR spectrum that cannot be observed in a liquid state NMR spectrum.
A bit of information is given in the table below..... more will follow...... and I have included some examples.
The fact that certain terms in the table above are directly observed in solids, and indirectly observed in liquids (e.g. via relaxation effects), leads to NMR spectra of solids often appearing vastly different than those of liquids. In particular, since a number of terms lead to line structure in solids, solid state NMR spectra are often much wider (cover a larger frequency range) than those in liquids.
Note that the form of the terms in the table above needs to be modified before we can actually consider them in detail. The biggest term is the Zeeman interaction with the static magnetic field, and the eigenstates at equilibrium must be eigenstates of the Zeeman Hamiltonian. However, some terms, as given in the table above, will lead to transitions among Zeeman eigenstates - a non-equilibrium situation.
For example, if the direct dipole interaction were expanded, we would see some operators that lead to transitions (e.g. raising and lowering operators). These operators make up the non-secular part of the Hamiltonian (if they occur between nuclei with much different resonance frequencies) and must be truncated. Our truncated Hamiltonian will look fairly complex and is not shown here, but will have additional angular dependencies with respect to the static field. These angular dependencies lead to a distribution of resonance frequencies in a powdered solid, and are a primary source of the linebroadening observed.
Additionally, a number of terms may overlap • for example, the heteronuclear dipolar terms may be much larger than the chemical shift terms, causing the two to overlap in the NMR spectrum.
Commonly, line narrowing techniques are used to reduce the line structure effects in solids. These almost always involve averaging either the spin, or spatial parts of the Hamiltonian with respect to the magic angle - the angle at which 3cos2θ - 1 = 0 (θ = 54.7°). To average the spatial part of the Hamiltonian we use magic angle spinning (MAS).
In MAS we spin the sample about the magic angle with respect to the static magnetic field. The spinning rate must be large relative to the line structure we are trying to average. The usually means is must be 2 - 20 kHz (note that liquid samples are spun at about 20 Hz). MAS is the most commonly used line narrowing technique at least in part because it is technically very difficult to effectively average the spin part of the Hamiltonian (especially using your run-of-the-mill commercial NMR spectrometer).