Structural Studies of Alkali Modified Silicate Glasses Using Si-29 Magic-Angle Spinning, Magic-Angle Turning, and J-Resolved PIETA NMR Spectroscopy
May 2013 to August 2013
Jay Baltisberger
My work during the summer of 2013 had four major projects being conducted by three students
(Pyae Phyo, Mikiyas Assefa, Anna Tribble) at three different locations (Berea College, the Ohio State
University, and the CEMHTI in Orleans France). This required quite a lot of Skype sessions to make
sure everyone was keeping organized and on-track and the usefulness of the internet allowed this to
proceed quite nicely. The first project (involving primarily Pyae Phyo) was a continuation of the work
we did last summer and during my sabbatical to study the magic-angle flipping (MAF) spectra for
alkali modified silicate glasses (example shown in figure 1). The second project (involving all three
students) was to measure J-resolved spectra (primarily using the phase-incremented echo train
acquisition, PIETA, method) for 31P sites
in phosphate glasses (modified by zinc,
lead, and barium) as well as 29Si in
silicates (both with and without 29Si
isotopic labels). These experiments were
run in all three locations on various
samples. The third project (involving
Mikiyas Assefa and Pyae Phyo) attempted
to develop software and pulse sequences to
do spinning sideband analysis of both
phosphate and silicate glasses (sample spectrum shown in figure 1). Some of these were the same
samples we had previously studied with MAF while others were new samples prepared at Berea
College by my research assistants. The fourth project (mostly studied by Anna Tribble) attempted to
demonstrate the problems associated with a Carr-Purcell-Meiboom-Gill (CMPG) echo train acquisition
when used in a variety of situation from oil-well logging to magnetic resonance imaging to relaxation
dispersion in biomolecules. Each project had quite lofty goals and many of these were met, as I will
discuss individually in the attached appendices.
!
Figure 1: Comparison of MAF-CPMG (left) and MAT-PIETA (right) for a K2Si4O9 glass, showing that both experiments may be used to extract comparable chemical shift tensor information.
Appendix A: MAF Experiments and Simulations
As stated previously, the first project was running MAF experiments at OSU and simulating the
resulting datasets with software I have developed for our OS X based Apple computers. The samples
were primarily alkali modified silicate glasses with nominal formulas of M2O • x SiO2, where M is any
of the alkali metals (Li, Na, K, Rb, or Cs) and the x determined the relative abundance of metal doping
and ranged from about 1.5 to 6. This represents about 15 different MAF spectra that were simulated
(see figure 2) and the resulting NMR parameters have been used to develop a structural model for
these glasses.
The basic story seems to be that silicate tetrahedra of the glass are connected to each other in
long chains which then are interconnected via bridging oxygen sites between these chains. Some of
these long chains can loop onto themselves and form ring structures as well according to the literature
but our results do not distinguish these rings from the chains, calling into question if the ring structures
are real or not.
The other feature is the distribution
of the alkali cations around these chains to
act as charge balance. Our spectra suggest
that these coordination environments are
relatively ordered (when cations are mixed,
the glasses tend to have all mixed sites
without regions rich or poor in one of the
cations) and occur in steps starting with low
coordination numbers (4 or 5) when the
cations are small (Li or Na) and increasing
in coordination number (7 to 10) when the
cations get larger. We are in the process of
building computer models to examine how
these features may constrain the various
local geometries seen in a silicate glass.
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Figure 2: Example of MAF data (top) and simulation (middle) along with residuals (bottom) for a Cs2O•2SiO2 glass.
Appendix B: J-Coupling in Glasses
The second project to measure J-
coupling parameters is a continuation of work
that Pyae Phyo did as a labor student during the
2012-2013 academic year (sample spectrum
shown in figure 3). We really focussed on
synthesizing new samples (mostly the previous
work was on Zn2P2O7 glasses only) that included
both zinc, lead, and barium cations. The PIETA
expereiments were run at Berea College on our
own NMR spectrometer, some at OSU in the
Grandinetti lab (where we have access to a
much faster spinning speed module), and some
in France (where we have access to a much larger magnetic field instrument). The work on the
phosphate glasses indicates that the modifying cation (surprisingly) is quite influencial on the observed
average J-coupling between adjacent phosphorus atoms linked by a bridging oxygen. It seems the
electron density on the modifying cation must strongly influence the electron density in the P–O–P
bonds to the extent that the J-coupling changes from an average around 9 Hz in the glasses with a
smaller zinc cation to almost 18 Hz in the glasses with a large lead cation (seen in figure 4). It is
possible that some of this variation also comes from changes in the P–O–P bond angle and we are now
working on experiments that might be able to
separate out the P–O–P bond angle directly from
a dipolar coupling measurement. Our hope is
that by measuring these J–coupling distributions
we may ultimately use these to uncover the
longer range structural motifs present in these
phosphate glasses.
The work on the silicate glasses is more
difficult since the 29Si isotope is only 4.7%
abundant. This means our overall signal is
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Figure 3. 2D PIETA spectrum of Zn2.25P2O7.25 glass showing the doublet structure for the Q(1) sites (10 to 30 ppm) and singlet for the Q(0) sites (-10 to 10 ppm). Each slice may be individually fit to extract a correlation between isotropic shift (vertical) and J-coupling (horizontal).
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Figure 4: PIETA spectrum of a lead phosphate glass showing the sharp singlet peak (top) for the isolated orthophosphate units and the broader doublet (bottom) for the pyrophosphate units.
reduced by about a factor of ten, in addition the lower NMR frequency for this nucleus makes it
inherently less sensitive than 31P. Work on isotopically enriched silicate glasses proved to us that we
could in fact see these J-coupling distributions (which are similar in size to those for phosphate
glasses) yet since most silicates are extensively network linked, the coupling patterns were quite
complicated to analyze. By using a natural abundance sample we could simplify the resulting J-
coupling patterns and extract average J-couplings for silicate glasses. This is a very low sensitivity
experiment that was only possible in the highest
magnetic field instrument that we had access to
in Orleans, France. This work on the 850 MHz
NMR spectrometer produced a singularly
remarkable spectrum that clearly shows coupled
silicon sites in amorphous silica (SiO2). We are
particularly excited about this spectrum (shown
in figure 5) because it is the first time the
distribution of J-coupling parameters for isolated
silicon pairs has been observed at natural
abundance. This distribution is directly related to the distribution of Si–O–Si bond angles that have
been measured indirectly in many other experiments and has been the source of a great deal of debate
in the glass literature. The SiO2 sample represents kind of a “grandaddy” of them all because it is the
starting point for almost every other technologically important glass that is in use today. As we work
to analyze this data, we hope to compare the implications of this J-coupling distribution to other
approaches to measuring the local structure in this important glass. While this does represent nearly 2
days of signal-averaging on a $15M instrument, it does also prove that this kind of work can become
routine in time and applicable to many kinds of samples, especially when appropriately doped with
paramagnetic materials (which this one was not).
Figure 5: PIETA spectrum of amorphous SiO2 showing the sharp singlet peak for the silicate sites that have no 29Si neighbors while the broad doublet pattern underneath represents the approximately 10% of the sample where a pair of 29Si sites are seen together.
Appendix C: Sideband Simulations
The third project was again
related to work that I performed on my
sabbatical at OSU as well as
continuation of work that Pyae Phyo
started last summer when we began
performing MAT and PASS
experiments usint PIETA on silicate
glass samples. This year we also began to run similar experiments on phosphate glasses. In addition
we modified a sideband scaling pulse sequence (XCS) developed at the National High Field Magnet
Laboratory by Gan and coworkers to give us a chemcial shift anisotropy (CSA) correlation where the
CSA produced a sideband pattern that was scaled by a factor of six (so the sidebands appear at
multiples of 2.0 kHz even though we were spinning at 12.0 kHz). This has a number of advantages as
well as a few disadvantages. It is useful to spin rapidly for a phosphate glass since the magic-angle
spinning at slow speeds does not average the homonuclear dipolar couplings. Sadly, at high speeds we
lose the sideband information so combining slow and fast spinning in one experiment is quite useful.
Adding the PIETA approach to the XCS proved relatively simple and effective. The efforts to
modify our existing sideband simulations from one-dimensional into two-dimensional simulation
programs proved more difficult since Mikiyas
Assefa had to learn C computer language from
the ground up. I would have liked to have
gotten farther on this work but the
programming seemed to just take a greater
effort than we could do remotely coding the
project together. Mikiyas did gain a lot of C
programming skills but was unable to finish
this portion of the project. We have a good
start though and are hopeful we can manage to
get this running by next summer.
τ/3
2τ/3 − 3π/4 τ/3 − π τ/3 − π τ/3 − π + t1 τ/3 − π τ/3 − π 2τ/3 − π τ − π/2 − t1
τ/3 τ/3 τ/3 τ/3 τ/3 τ/3 τ/3 τ/3 τ/3 τ/3 τ/3
Figure 6: The eXtended Chemical shift Scaling (XCS) pulse sequence showing the added 180 pulse (red) that allows for scaling of the spinning speed in the anisotropic dimension by a factor of 6.
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Figure 6: The eXtended Chemical shift Scaling (XCS) spectrum for Zn2.25P2O7.25 glass. The major sites are an orthophosphate (top) and pyrophosphate (bottom) with both showing extensive sidebands. This has led us to interpret the data using a model proposed by Czjzek.
Appendix D: CPMG Pulse Effectiveness
The final project was performed by Anna Tribble primarily at Berea College using the Spin
Evolution program to examine the effects of both pulse length and offset errors on the CPMG and
PIETA experiments. These are the two major problems that we encounter when we run these
experiments in real world applications that I mentioned earlier. What Anna was able to show is that
the CPMG sequence tends to produce most of the signals as stimulated (rather than spin) echoes when
the pulse offset or length errors are large. This is significant because the evolution of stimulated
echoes is very different than spin echoes when you acquire a train of echoes using an approach like
CPMG or PIETA. In the case of J-coupling, the CPMG essentially loses the coupling completely and
produces a single narrow spike rather than the appropriate doublet, triplet, or higher multiplet structure.
In the case of relaxation dispersion, the CPMG problems mean that the relaxation seen is dominated by
longitudinal (T1) rather than transverse (T2) processes. This implies that the use of CPMG to measure
exchange processes (like the motion of a folding protein) is in fact flawed, yet there are now probably
hundreds of papers that use CPMG for relaxation dispersion in this way to measure folding timescales.
We believe we have found a solution but ran out of time this summer to run the NMR experiments
needed to demonstrate that the CPMG may be replaced with a PIETA approach to fix the problem.
The same issue arises in the use of CPMG in oil-well logging applications where the pulses are by
nature highly offset in frequency and incorrect in length due to the inhomogenous environment
encountered by a borehole spectrometer. We are in collaboration with researchers at OSU, Brazil, and
Saudi Arabia to discuss a possible PIETA solution, though this may again involve a steep uphill climb
to convince companies like Schlumberger and Halliburton that they have a problem in the
experimental setup they have used in literally thousands of oil well logs pulled in the last 15 years.
Appendix E: Conclusion and Future
I think we have made very strong progress on all of these projects. The ties between the
Grandinetti laboratory at OSU and my students have grown stronger as we continue to work on
collaborations. It is likely that my students will work at OSU in future summers with the Grandinetti
lab, depending on funding. We will have at least six more papers coming out of the work (nominally
the topics will be: Cs silicate glass, Alkali silicate glass, dp-Echo, MQMAS-PIETA, CPMG
Effectiveness, and J-coupling Distributions) we have done with OSU (and France as well since
Professor Grandinetti worked with me at the same time there). The work at France promises as well to
build a strong collaboration and the principal investigators there (Franck Fayon, Pierre Florian, and
Michael Deschamps) are interested in having me come back soon. I was very pleased with the
progress made by all three students. I think they may all end up going to graduate school in chemistry
in coming years and have learned a lot of basic research skills from these projects.
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