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.

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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.