The Dybowski Research Group

Nuclear Magnetic Resonance Spectroscopy

Group Members

Cecil Dybowski

Ph.D., University of Texas at Austin

Shi Bai

Ph.D., Brigham Young University

Jaclyn Catalano

Ph.D., Columbia University

Fahri Alkan

B. S., Bilkent University

Sean Holmes

B. S., Washington and Jefferson College

Anna Murphy

B. S., California University of Pennsylvania

Yao Yao

B. S., Wuhan University

Research in the Dybowski group provides insight into fundamental NMR properties of materials, the effects of electronic configuration, and the relationship of spectroscopic parameters to physical structure. The information provides a basis for understanding the structure and functional relationships in complex systems such as pure crystalline solids, mixed solids, and the surface phase on heterogeneous catalysts. The studies conducted in the group range from theoretical underpinnings of NMR spectroscopy to applications of the technology to understanding materials problems as diverse as the action of heterogeneous catalysts, the degeneration of art objects, the response to stress of solids, and the association of ions in solution.

NMR Applied to Materials at Surfaces

Ø Early work on the NMR of materials at surfaces centered on identifying species formed during heterogeneous catalysis or interaction with a surface.

1. NMR of adsorbed hydrogen and carbon monoxide on supported-metal catalysts demonstrated that the formation of a hydride-like species could be stopped by the adsorption of carbon monoxide, but that once formed, subsequent addition of carbon monoxide did not reverse its formation.

2. Structures formed when various osmium carbonyl compounds are put in contact with magnesium oxide from their C-13 NMR spectroscopy.

3. Methanol on ZSM-5 catalysts, both immediately after adsorption, and following heating to promote reaction. The latter demonstrate the formation of dimethyl ether at the surface. The analysis of relaxation times of various species at the surface gives clues to the dynamics of the methanol molecule in these environments.

4. Xenon-129 NMR to study the NMR parameters of the gas adsorbed in various zeolites. The spectroscopy has delineated the nature of xenon-zeolite, xenon-xenon and xenon-coadsorbate interactions when xenon is adsorbed in zeolites of various structures.

5. Ion-molecule complexes in zeolites to determine enthaplies of formation of molecules like phenanthroline in zeolitic structures.

NMR Chemical Shielding of Spin-½ Heavy Metals in Solids

Ø Text Box: Almost every element has at least one isotope amenable to investigation with NMR spectroscopy. Recently our group has studied the NMR spectroscopy of heavy-metal spin-½ nuclei in solid materials, such as ²⁰⁷Pb, ¹¹⁹Sn, ¹¹³Cd, and ¹⁹⁹Hg. We have investigated the tensorial properties of the chemical shift in these materials. Example magic-angle-spinning spectra (theoretical and experimental) of HgCl are shown to the right at two different spinning speeds. Analysis gives information on the chemical shift tensor. The table shows data for various lead-containing materials. Measurements show that, for example, Pb-207 NMR parameters are strongly affected by the local environment of the lead ion.

Ø Recently, we have begun to predict the chemical-shift parameters from density-functional calculations on model clusters that represent the solid-state structures on which we have done measurements. These calculations for heavy-metal spins require that one include the effects of relativity on the electronic structure. The results are demonstrated by the figure below, which compares the predicted isotropic chemical shift of solid lead dihalides versus the experimentally measured shielding. Such state-of-the art calculations provide the connection between changes in electronic state and the observed NMR parameters that makes NMR such a useful barometer of electronic state.

Ø Complexation of lead ion by materials such as 1,10-phenanthroline and thiourea also alters the NMR chemical shift parameters. We use the specificity of NMR parameters of lead in these solids to determine speciation of lead, a useful means of analyzing lead-contaminated soils, for example.

Chemical Shielding in Ionic Solutions

Ø In solution, materials like Pb(NO₃)₂ form ions. For these materials, the situation is even more complex. In addition to the solvated ions, one may find the formation of ion complexes. The exchange between ionic forms in solution is reflected in concentration- and temperature-dependent NMR chemical shifts.

Pb²⁺ (aq) + NO₃^- (aq) = Pb(NO₃)⁺ (aq)

Studies of the chemical shift in solution as a function of concentration and temperature allow one to extract thermodynamic information on the dynamic equilibrium among these ionic species in solution.

Temperature-dependent NMR Processes in Solids

Ø In many lead-containing solids, the chemical shift is strongly temperature dependent. This provides a means of determining the temperature in the NMR probe, for example. We have demonstrated several different ways of using this phenomenon as a thermometer, as have others, particularly for magic-angle-spinning experiments. Recently, we have showed that a simple measurement of the chemical shift of the "peak" in the spectrum allows one to determine the temperature in a probe without spinning. The temperature variation of this point is given by the simple equation:

d (T) = - {3670.6 +/- 1.0) ppm + {0.666 +/- 0.003 ppm/K} T

Ø In recent studies, we have correlated this temperature dependence with thermal expansion of the lattice, to show that the observed NMR changes are evidence of the dependence of electronic state on structural properties such as the unit cell dimension. It is clear from these results, both experimental and theoretical, that the observed changes in NMR parameters reflect significant changes in the electronic structure of the solid material. For example, in the figure is shown the dependence of the isotropic chemical shift of PbMoO4 as a function of the cube root of the unit cell volume, as determined from the temperature dependence of of the NMR parameter and the unit cell volume determined by X-ray analysis. As one can see, the isotropic shift is directly proportional to the lattice parameter. The NMR chemical shift is a function of the electronic wave function, so that over this small variation of the lattice parameter, this suggests that the integrals over the electronic wave functions are linear functions of the interatomic spacing.

Relaxation in Heavy-Metal Systems

Ø NMR relaxation times reflect the dynamics in a material. It has long been known that random processes induce spin-lattice relaxation of spins. The nature and efficiency of spin-lattice relaxation depends on the mechanism of coupling with other degrees of freedom of the lattice. For spin-1/2 nuclei there are numerous mechanisms by which coupling may allow energy transfer to induce relaxation. We have examined T₁ for several lead-containing materials as a function of temperature and magnetic field to determine the nature of relaxation in these materials, as shown in the figure. For example, relaxation in Pb(NO₃)₂ is quite efficient, with short relaxation times for a crystalline solid containing a reasonably rare spin-1/2 nucleus. The temperature dependence found for all of the lead-containing materials investigated so far is of the form

(1/T₁ a T²)

and there is a unique lack of dependence on the magnetic field strength. Similarly, as seen in the figure, the relaxation rates of PbMoO₄ and PbCl₂ show the same sort of dependence on temperature and lack of dependence on the field strength.

Ø We have recently developed a theory of relaxation to explain these observations. The theory predicts that relaxation results from coupling to phonons through modulation of a spin-rotation coupling. The temperature dependence implies a Raman coupling of the spins to the phonon bath. The mechanism is seen for lead-containing materials, for tin in SnF₂, and for thallium in certain compounds, but it is not seen to be a major mechanism for the relaxation of ¹¹³Cd in CdMoO₄.

Ø Recent measurements of relaxation in SnF₂ demonstrate that the spin-phonon mechanism also affects the ¹¹⁹Sn relaxation at temperatures below about 373 K, but that a second mechanism becomes competitive at higher temperatures. The second mechanism appears to be a thermally activated process which we tentatively associate with the onset of fluoride hopping in the matrix.

Prediction of NMR Chemical Shielding of Heavy Nuclei in Solids

Ø Text Box: Chemical shielding at unique sites are useful in identifying nuclear sites. The chemical shielding can be predicted from the electronic states, including all of the various tensor components, as integrals over the electronic state. There are many different programs available to predict the chemical shielding in molecular materials containing light nuclei such as carbon-13, using a variety of techniques.

Ø To predict the chemical shielding of heavy nuclei such as Pb-207 and Hg-199 in solids requires one to confront issues that are not as important for light nuclei. First, many of the electrons associated with these heavy atoms are moving at sufficiently high speeds that the neglect of relativistic effects (which is usually assumed for light nuclei) is not justified. Second, in the solid state, the structures formed are often not “molecular” in the sense that a simple organic structure is. Thus, the calculation of electronic properties such as the NMR chemical shielding requires one to consider the extended structure of the solid.

Ø In our laboratory, we predict chemical shieldings of nuclei like Pb-207 and Hg-199 using density functional theory augmented with the zero order regular approximation (ZORA) to take into account approximately the relativistic nature of the electrons. In addition, the calculations mostly involve truncations of the solid structure (to be manageable for calculation). We attempt to learn what properties the clusters we use as models of the solid environment are important in allowing an accurate prediction of the NMR properties. For example, the figure shows three different clusters representing the local environment of the lead atom in PbF₂. Calculations on these clusters show that extended arrays of atoms have to be used in order to represent accurately the local electronic environment at the lead site.

Studies of Reactions of Lead Soaps

Ø Lead soaps are frequently found in paintings of a particular time period because of the utility of lead salts as pigments. Over time, reactions produce deterioration of the paintings. In some cases, there is loss of paint, the formation of protrusions, or the creation of areas of transparency, all of which destroy the integrity of the artwork. In these joint studies with the Metropolitan Museum of Art, we study the structure, molecular dynamics, and phase behavior of lead carboxylates with solid-state 207Pb NMR spectroscopy, as well as with proton and carbon NMR spectroscopy. By understanding the chemistry in these model systems, we hope to address the chemistry underlying a major problem with maintenance of these objects.

Cecil Dybowski, 1998 - 2011.
Last Updated: November 18, 2011
URL of this document: http://www.udel.edu/dybowski/research.htm