Solid-State and Solution-State Nuclear Magnetic Resonance Spectroscopy

The research in Professor Dybowski's group provides insight into fundamental NMR properties of materials, electronic configuration, and physical structure. The information can provide 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.

NMR Applied to Materials at Surfaces

Ø Early work on the NMR of materials at surfaces centered on identifying species formed during adsorption of hydrogen and carbon monoxide on supported-metal catalysts. We 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.

Ø In another set of studies, we identified structures formed when various osmium carbonyl compounds are put in contact with magnesium oxide from their C-13 NMR spectroscopy. These results allowed us, in this collaboration with Professor Bruce Gates, to follow the sequence of transformations in producing so-called "third-generation" catalysts, in which organometallic structures are tethered to an oxide support.

Ø Our group has also investigated methanol as it sits in ZSM-5 catalysts by carbon NMR spectroscopy, 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.

Ø NMR spectroscopy of adsorbed xenon-129 has been an experiment for which our laboratory has become nationally and internationally recognized. We have systematically investigated how the properties of zeolites are reflectd in Xe-129 NMR parameters of the adsorbed gas. The spectroscopy has delineated the nature of xenon-zeolite, xenon-xenon and xenon-coadsorbate interactions when xenon is adsorbed in zeolites of various structures.

Ø The study of ion-molecule complexes in zeolites with NMR spectroscopy has allowed us to determine enthaplies of formation of molecules like phenanthroline in zeolitic structures.

NMR Chemical Shielding of Spin-½ Heavy Metals in Solids

Ø 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. An example spectrum is shown to the right, from which one extracts information on the chemical shift tensor, as seen in the table. Measurements show that, for example, Pb-207 NMR parameters are strongly affected by the local envrionment 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.

Lead Chemical-Shift Parameters of Solids

Compound	Isotropic Shift [ppm]	Span [ppm]	Skew
Lead sulfate	-3607.7	566.0	0.36
Lead nitrate	-3491.6	54	1.00
alpha-Lead fluoride	-2666.4	470	0.58
Lead carbonate	-2622.4	764	0.56
Lead chromate	-2236.2	858	-0.09
Lead molybdate	-2004.9	186.8	-1.00
Lead chloride	-1714.9	533	0.52
Lead thiocyanate	-1592.6	1275	-0.58
Lead titanate	-1395.1	1130	1.00
Lead zirconate (a)	-1349.5	870	0.74
Lead zirconate (b)	-994.7	1434	0.74
Lead bromide	-980.7	699	0.58
Lead hydroxychloride	-705.8	2341	0.57
Lead hydroxybromide	-638.8	2101	0.63
Lead hydroxyiodide	-546.1	1726	0.83
Lead iodide	-29.1	0.0	0.00
Lead sulfide	113.2	0.0	0.00
Minium, Pb₃O₄	804.	3088	0.72
beta-Lead Oxide	1526.7	3820	0.97
alpha-Lead Oxide	1930.	3300	1.00
PbO₂	5206	2473	0.05

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.

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.

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 at least three different lead-containing materials, 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.

Cecil Dybowski, 1998 - 2007.
Last Updated: July 13, 2007
URL of this document: http://www.udel.edu/dybowski/research.htm