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The Dybowski Research Group

Materials Characterization


 

 

 

Group Members

Cecil Dybowski

Ph.D., University of Texas at Austin

Shi Bai

Ph.D., Brigham Young 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 the properties of materials.  Much of the early work has been NMR analysis of materials through studies of magnetic shielding and relaxation.  More recently, we have used other characterization tools such as FTIR and X-ray analysis, specifically synchrotron X-ray analysis, to address these issues.  The information provides a basis for understanding the structure and function in these complex systems.  The studies include computational chemistry to predict spectroscopic properties of materials, usually with density functional theory.

 

With our long-standing interest in the topic, we develop the theory of NMR spin-lattice relaxation in solids, such as materials containing heavy nuclei.  Most theoretical studies in our group, such as the unusual relaxation mechanism in Pb-207 in solids, are motivated by unusual results from experimental measurements.

Materials at Surfaces

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

1.      The observation of a hydride-like species on the surface of a supported-rhodium catalyst by adsorption of hydrogen, and how it interacts with carbon monoxide

 

2.      Structures formed when various osmium carbonyl compounds are put in contact with magnesium oxide.

 

3.      The structure and dynamics of adsorbed methanol on ZSM-5 catalysts, both immediately after adsorption, and following heating to promote reaction.

 

4.      The NMR spectroscopy of xenon gas sorbed in various zeolites delineates the nature of xenon-zeolite, xenon-xenon and xenon-coadsorbate interactions.

 

5.      Study of ion-molecule complexes in zeolites and exchange with free organic materials.

 

6.      ESR studies of radicals formed at the surface of supported-metal catalysts.

 

Our laboratory rarely studies surface reactions at the present time.

 

 

Experimental NMR Chemical Shifts of Spin-1/2 Heavy Metals in Solids

 

Text Box:  Almost every element has at least one isotope amenable to investigation with NMR spectroscopy.  Our group has studied, both experimentally and theoretically, the chemical-shift tensors of several heavy-metal spin-½ nuclei in solid materials, 207Pb, 119Sn, 113Cd, and 199Hg.  Example magic-angle-spinning 199Hg NMR spectra (predicted and experimental) of HgCl are shown below at two different spinning speeds.  The table shows Pb-207 data for various pure lead-containing solids determined by our group from spectra such as those in the figure.   Measurements show that, for example, Pb-207 NMR parameters are strongly affected by the local environment of the lead ion.  Complexation of lead ion by materials such as 1,10-phenanthroline and thiourea, for example, alters the NMR chemical-shift parameters, which can be used to specify lead co-ordination in solids.

 

Magnetic Shielding in Ionic Solutions

Text Box: Lead Chemical-Shift Parameters of Solid Materials
Compound	Isotropic Shift [ppm]	Span [ppm]	Skew
Lead sulfate	-3608	566	0.37
Lead nitrate	-3491	54	1.00
Lead selenate	-2849	512	-1.00
Lead niobate	-2844	221	-0.70
Lead meta-tantalate	-2724	510	-1.00
alpha-Lead fluoride	-2666	470	0.58
Lead carbonate	-2622	764	0.56
Lead sulfite	-2463	446	0.80
Lead meta-vanadate	-2295	517	-0.15
Lead chromate	-2284	884	-0.10
Lead molybdate	-2012	182	-1.00
Lead tungstate	-2003	198	-1.00
Lead chloride	-1715	533	0.52
Lead thiocyanate	-1593	1275	-0.58
Lead titanate	-1409	1139	1.00
Lead zirconate (a)	-1401	860	0.65
Minium, Pb3O4 (Pb4+)	-1091	123	1.00
Lead zirconate (b)	  -1055	1172	0.95
Lead bromide	  -981	699	0.58
Lead hydroxychloride	  -706	2341	0.57
Lead hydroxybromide	  -639	2101	0.63
Lead hydroxyiodide	  -546	1726	0.83
Lead iodide	    -29	0.0	0.00
Lead sulfide	   113	0.0	0.00
Minium, Pb3O4 (Pb2+)	   804	3088	0.72
beta-Lead oxide	 1527	3820	0.97
alpha-Lead oxide	 1930	3300	1.00
PbO2	 5206.	2473	0.05


In solution, materials like Pb(NO3)2 form several different ionic species.  In addition to solvated ions, one finds ion complexes.  The rapid exchange between ionic forms in solution is reflected in concentration- and temperature-dependent NMR chemical shifts.  An example of this process is

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

Experimental studies of the NMR 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 Pb-207 Chemical Shifts in Solids

In many lead-containing solids, the chemical shift is strongly temperature-dependent.  This characteristic, for examle, provides a means of determining the temperature in the NMR probe directly.  We have demonstrated means to use this phenomenon as a thermometer in NMR spectroscopic experiments.  For example, the temperature variation of the “peak” of a Pb(NO3)2 powder pattern is given by the simple equation.

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

The origin of the temperature variation has been investigated.  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.  One may derive, for example, a prediction of the NMR properties as a function of the unit cell dimension that connects these two parameter theoretically, through the variation of the solid-state wave function by a perturbation expansion.

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 50oC in the following manner

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

 

Theory of Relaxation in Heavy-Metal Systems

NMR relaxation reflects molecular dynamics around the nuclear spin’s site.  By conventional theory, it has long been known that random rotational dynamics induce spin-lattice relaxation of spins, as has been known since the late 1940s.  The nature and efficiency of spin-lattice relaxation depends on the mechanism of coupling with other degrees of freedom of the material.  For spin-1/2 nuclei there are numerous mechanisms by which coupling may allow energy transfer. 

 

We discovered unusual experimental T1 dependences on temperature and field in several pure lead-containing materials (shown in the figure).  In each case, the result was

1/T1  a  T2

with no dependence on magnetic field, as seen in the figure for Pb(NO3)2, PbMoO4 and PbCl2.  This form is unusual, and is not what one might predict by “normal” or “standard” theories of relaxation which assume random isotropic rotational motion to modulate the coupling of spins with their surroundings.

 

We have developed a semiclassical theory of relaxation that explains these observations.  The model upon which the theory is based involves interaction with phonons through modulation of the spin-rotation coupling.  In addition to Pb-207 relaxation, the mechanism has also been experimentally seen for tin in SnF2 at sufficiently low temperature, for thallium in certain compounds, and – recently in our laboratory – for Hg-containing solids.  Interestingly, this mechanism does not seem to be a major mechanism for the relaxation of 113Cd in CdMoO4, for which relaxation by interaction with remote paramagnetic centers seems to dominate the relaxation behavior.  The theory predicts that the mechanism should only be effective for materials with large spin-rotation coupling constants, as is often the case for heavy nuclei.

 

We also have a strong interest in understanding the nature of spin-lattice relaxation in solids such as semiconductors, where several competing processes may exert strong effects on it. 

 

 

Computational Prediction of Magnetic Shielding of Heavy Nuclei in Solids

 

Magnetic shielding is the major characteristic of NMR spectra that is useful in identifying nuclear sites.  The shielding arises from coupling to the magnetic fields produced by electrons.  Thus, it gives an indirect measure of the electronic states.  We use density functional theory to calculate electronic states and thereby predict the magnetic shielding of nuclei in solids.  To predict the shielding of heavy nuclei such as Pb-207 and Hg-199 in solids requires one to confront issues not as important for light nuclei.  First, electrons associated with heavy nuclei must be treated as relativistic particles.  Second, in the solid state, the structures formed are often not “molecular” in the sense that a simple organic structure is.  Thus, calculation of electronic properties such as the magnetic shielding requires one to consider the extended structure of the solid.  In our laboratory, we predict the full chemical shielding tensor 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.  The calculations to connect NMR parameters with structure require the use of cluster models of the solid structure (to be manageable for calculation) developed in our laboratory.  The studies have resulted in a systematic method for calculation of NMR parameters when a structure is known that achieves agreement with experiment to within about 2%.  For example, in the figure the calculated principal components of the magnetic-shielding tensors of a suite of Hg-containing materials are correlated with the experimentally measured chemical shifts.  The dashed line is the trend that should be followed if the calculation absolutely reproduces the experimental data.  The red line is the best-fit linear correlation between these two sets of parameters.  As one can see, the experimental and calculated quantities agree within experimental error.

 

The results suggest that, with current available computational resources and reasonably large cluster models, one can fairly quantitatively predict NMR parameters of these heavy nuclei.  Network solids require special considerations in constructing the clusters to ensure agreement with experiment.  We have developed models to allow these calculations for both molecular and network solids.

 

 

 

 

Computational Prediction of Magnetic Shielding for Organic Solids

 

The C-13 magnetic shielding of nuclei in solid phases may also be predicted from approximation of the solid structure with cluster models and density functional theory.  For light elements like carbon, the effects of relativity are usually minimal (although this must always be checked), which simplifies the computational methodology for these materials.  We have investigated a wide variety of carbon resonances representing various electronic environments.  The use of appropriate large-cluster models results in quantitative agreement with experimentally determined NMR chemical shifts, as seen in the figure.  The different colors represent carbons in various types of environments: aliphatic, aromatic, and carboxyl environments.  Note that all types of carbon obey a single relationship between calculated and experimental parameters.  A comparison of these data to predicted shieldings from an isolated-molecule model demonstrates that, although a great deal of the chemical shielding for organic materials arises from electrons associated with the molecule in which the nucleus resides, the effects of other molecules cannot be neglected in prediction of solid-state shielding effects.  As with the heavy-nucleus-containing materials, clusters having appropriate symmetry elements and sufficiently large size are required to ensure agreement with experiment.  Typically, the appropriate model cluster contains greater than 13-15 molecules, appropriately chosen so as to represent the environment.

 

In the limit of large clusters, the mean error in calculation of tensor components is less than 2 ppm, sufficient to allow calculational results to aid in assignment of spectra.

 

Text Box:

 

 

 

 

Studies of the Formation of Lead Soaps in Masterworks of Art

 

Lead soaps, salts of lead ions with fatty acids, are frequently found in paintings from the 14th through the 20th century.  Over time, reactions between free fatty acids in the binder and lead (or other ions) from pigments produce these unwanted materials, and degrade the paintings.  In some cases, there is the formation of protrusions, or the creation of areas of transparency.  In joint studies with the Metropolitan Museum of Art, we have begun to use NMR, FTIR, and other techniques to study these effects in model paint systems, in the hope that our analyses may lead to a better understanding of the chemistry, thereby permitting the development of measures to stop of slow down these reactions on these extremely valuable cultural artifacts.  Recently, we have extended the studies to include synchrotron X-ray techniques (such as XANES and XRF).

 

Pb-207 spectroscopy of these model materials is extremely challenging for the spectroscopists because of the very wide resonance lines in the spectrum.  Spectra of several pure materials obtained with the so-called WURST-CPMG NMR technique show a “spikelet” spectrum which mimics the continuous Pb-207 band shape of the material. (See the figure.) After an initial phase of identifying the lead signatures of possible materials in a film, we have begun studying paint films that model the slow chemistry in such paintings.  Spectrum (e) is of the lead-tin yellow type I pigment, which shows the presence of impurities.  Spectra (a) – (c) are of various lead fatty-acid esters. Spectrum (d) is of lead carbonate, an impurity in lead white, a common paint pigment. The lead-tin yellow type I pigment has two unique sites for lead.  The spectrum of lead tin yellow, type I, also shows an impurity of minium, the starting material for making the pigment, at the level of a few percent.

 

Studies in our laboratory show that reaction to produce lead soaps occur when free acid is allowed to contact paint films.  The process in such a complex system has many parts.  Currently, we are studying the diffusion of free acids in paint films, and the porosity and tortuosity of those films, as those factors affect the reaction. 

 


Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: (c) Cecil Dybowski, 1998 - 2016.
Last Updated: May 25, 2015.
URL of this document: http://www.udel.edu/dybowski/research.htm