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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

 

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 experimental studies 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 of solids to stress, and the association of ions in solution.

Materials at Surfaces

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

1.      Adsorbed hydrogen and carbon monoxide on supported-metal catalysts

NMR detected the formation of a hydride-like surface species, the production of which could be stopped by the adsorption of carbon monoxide.

 

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

 

3.      Methanol on ZSM-5 catalysts, both immediately after adsorption, and following heating to promote reaction. ‘

Demonstrating 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.

Delineates the nature of xenon-zeolite, xenon-xenon and xenon-coadsorbate interactions when xenon is adsorbed in various zeolite structures.

 

5.      Ion-molecule complexes in zeolites

Determines enthaplies of exchange of molecules like phenanthroline in zeolitic structures.

 

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

 

Our laboratory rarely studies these materials at the present time.

 

 

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


NMR Chemical Shifts 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 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    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-containing materials, for example.

 

Magnetic Shielding in Ionic Solutions

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

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

Ø    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 means to use this phenomenon as a thermometer, particularly for magic-angle-spinning experiments.  Recently, we have shown 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:

             dPb (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, 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 50oC.  It has the following form

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

 

Relaxation in Heavy-Metal Systems

Ø  NMR relaxation times reflect the molecular 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 this energy transfer.  We have examined T1 for several lead-containing materials as a function of temperature and magnetic field to determine the nature of relaxation, 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)

 

as is seen in the figure for Pb(NO3)2, PbMoO4 and PbCl2 analyzed at several magnetic field strengths.  There is a unique lack of dependence of the relaxation rate on the magnetic field strength for all of these materials.

 

Ø  We have developed a semiclassical theory of relaxation to explain these observations.  The theory predicts that relaxation results from interaction with phonons through modulation of the spin-rotation coupling.  The temperature dependence implies the existence of a Raman coupling of the spins to the phonon bath.  In addition to Pb-207 relaxation, the mechanism has also seen for tin in SnF2 at sufficiently low temperature, for thallium in certain compounds, and – recently in our laboratory – in Hg-containing solids.

 

Ø  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. 

 

 

 

Prediction of NMR Magnetic Shielding of Heavy Nuclei in Solids

 

Ø  Magnetic shielding at unique sites is useful in identifying nuclear sites.  The shielding can be predicted if one knows the form of the electronic states.  There are many different calculational approaches to predict the shielding in 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 are 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 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).  The tensor data are compared to experimental chemical-shift tensor components to determine the quality of the calculation.  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.

Ø  The results suggest that, with current available computational resources and the reasonably large cluster models, one can quantitatively predict NMR parameters of these heavy nuclei by these cluster methods.

Ø  In an extension of this technique, we have shown how a cluster-based model for a network solid may also give essential agreement with experimental results.  The extension involved a bond valence method for approximating the local bonds effects at terminal atoms of the cluster.

 

 

 

 

Prediction of NMR Magnetic Shielding for Organic Solids

 

Ø  The C-13 magnetic shielding of nuclei in solid phases may also be predicted from approximation of the solid structure with cluster models.

Ø  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.

Ø  We have investigated a wide variety of carbon resonances representing various electronic environments.

Ø  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.

Ø  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.

Ø  It was observed that agreement of isotropic shielding with isotropic shift does not necessarily imply that the wave functions are correct.  Comparison of individual principal components of the tensor (as shown in the figure) is a much more stringent test of the reliability of any model structure.

Ø  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.

Ø  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.

Ø  Unlike other calculations, the results of these large-cluster calculations (figure at the right) show a single linear relation between chemical shielding and experimental chemical shift, as should be expected.

 

 

 

Studies of the Formation of Lead Soaps Related to Processes in Masterworks of Art

Text Box:

Ø  Lead soaps, salts of lead ions with fatty acids, are frequently found in paintings from the 14th through the 20th century.  It is apparent that, over time, reactions between fatty acids and lead 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, specifically Pb-207 NMR) 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 these reactions on these extremely valuable cultural artifacts.  We also use proton and carbon NMR spectroscopy and FTIR spectroscopy, as well as XANES, to answer these questions.

Ø  Pb-207 spectroscopy of these model materials is extremely challenging for the spectroscopists because of the very wide resonance lines in the spectrum.  See the figure to the left for an example of lead oleate.

Ø  Spectra of these 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.    

Ø  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.  We have seen that the presence of free palmitic acid on a film containing a pigment in linseed oil shows by NMR spectroscopy that lead palmitate is being produced.  A film with no excess palmitic acid shows no reaction over periods of months.  Reactions are sufficiently slow that the relative amount of lead palmitate presnet can be monitored as a function of time by NMR spectroscopy.

Ø  The figure shows several Pb-207 spectra relevant to the study of these art objects, taken with the WURST-CPMG technique.  Spectrum (e) is of the lead-tin yellow type I pigment.  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.

 

 

 

 

 

 


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