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. Our group has studied the chemical shift tensors of several heavy-metal spin-½ nuclei in solid materials, such as ²⁰⁷Pb, ¹¹⁹Sn, ¹¹³Cd, and ¹⁹⁹Hg. Example magic-angle-spinning ¹⁹⁹Hg NMR spectra (theoretical and experimental) of HgCl are shown to the right at two different spinning speeds. Analysis of the sideband pattern on the band shape gives the principal elements of the chemical shift tensor. The table shows the most recent data for various lead-containing materials determined by our group and its collaborators. Measurements show that, for example, Pb-207 NMR parameters are strongly affected by the local environment of the lead ion.

Ø The NMR chemical shift is a function of the electronic state of a system. We have begun to predict the chemical-shift parameters from density-functional calculations on model clusters representing 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. Some results are discussed below. 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 lead NMR parameters 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 Shifts in Solids Containing Heavy Nuclei

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

Ø We have correlated the temperature dependence of the chemical shift tensor elements 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, a plot of the isotropic chemical shift versus lattice parameter for a material like PbMoO₄ shows the direct proportionality between these two quantities. 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.

Ø The variation with temperature is not unique to the ²⁰⁷Pb resonance. For example, our recent measurement of the temperature variation of the chemical shift of tetraethylammonium sodium tetracyanomercurate show that the isotropic position of the ¹⁹⁹Hg resonance varies linearly with temperature from room temperature to 50^oC. It has the following form

d_Hg (T) = - 381.01 ppm - {0.1781 ppm/K} T

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 developed a semiclassical 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 is 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 Related to Processes in Masterworks of Art

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.

Ø Examples of ²⁰⁷Pb spectra of lead oleate, one of the soaps one might expect to see in these materials, are shown in the figure. The NMR chemical-shielding anisotropy is rather large.

Ø Spectra of these materials cannot be determined by usual procedures. One may determine the spikelet spectrum, using WURST pulses for the inversion to insure inversion over a wide spectral range. However, because the initiating pulse may not be uniform over the entire range, one takes spectra in pieces. The WURST-CPMG spectrum shown is a sum of three spectra, taken with different offsets.

Ø The excitation of the entire spectrum may also be done with a DANTE sequence coherent with the magic-angle-spinning procedure. The spectrum shown is a DANTE-2, which gives only the even sideband pattern. From this pattern, one can determine all elements of the shielding tensor.

Ø Similar spectra are seen for lead azelate and lead heptanoate, whereas the spectra of lead palmitate and lead stearate have a much smaller chemical shielding anisotropy. It should be possible from lead NMR to distinguish these latter two materials from the others, but it is not possible to distinguish lead palmitate from lead stearate by ²⁰⁷Pb NMR spectroscopy.

Cecil Dybowski, 1998 - 2013.
Last Updated: March 29, 2013
URL of this document: http://www.udel.edu/dybowski/research.htm