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= Solid State NMR Characterization of Materials = Solid State NMR Characterization of Materials: Dipolar and Quadrupolar Nuclei NMR

Spring 2023 MSE 551 Xiaoxi Ji

Introduction
NMR spectroscopy has become a very powerful analytical tool for the characterization of materials. One of the advantages of NMR over other analytical techniques is that it is very element specific. While IR spectroscopy, for example, simultaneously measures all contributions of all vibrations from all elements in the material under study at each wavelength, NMR spectroscopy selects out only the nuclei being probed. All chemical and structural information about this nuclei can then be studied independently. If other nuclei in the sample under study are NMR active, then these can also be studied in turn and as a result yield significant information about the sample. Another advantage of the technique is that NMR is often non-destructive and requires a small amount of material often in very convenient powder form.

There are a few disadvantages and these mainly result from the NMR characteristics of the nuclei, rather than from the sample itself. For example, not every nuclei is NMR active and for some that are their low naturally occurring abundance makes NMR of these nuclei very difficult. For example, oxygen is one of the most abundant of all elements, yet the combined factors of its low abundance, 17O is only 0.037% abundant, and its low sensitivity, 0.029 compared to 1H NMR, make oxygen NMR almost impossible without significant isotope enrichment and extremely sensitive NMR spectrometers. Another complicating factor is that some important nuclei also have high spin numbers which generate large numbers of Zeeman energy levels and this increases the complexity of the NMR spectra of these nuclei. 27Al with nuclear spin 3/2 is such a nuclei.

Another disadvantage is that NMR can be a very expensive analytical technique. New modern FT-NMRs can be in excess of $500K and require full time dedicated laboratory technicians to run and maintain the spectrometer. The technique itself can also be quite complex and difficult to master. As a result, a common situation is for users not to use the NMR themselves, but rather for them to submit samples to the NMR lab and have them run for them.

In this laboratory, we will get a glimpse of solid state NMR by running a few samples and collecting some spectra on known samples. We will be using 31P since it is reasonably abundant, has a good mid-range frequency of 162 MHz at 9.4 T and has good sensitivity. In the next laboratory section, we will examine the quadrupole nucleus 11B and compare the NMR spectra of this quadrupole nucleus to that of the 31P and 29Si dipolar nuclei.

Outcomes from the Laboratory
The laboratory report for this lab will consist of answering the following questions either during the lab or shortly thereafter.

1. What is the field of the magnet used in the lab in _____________ Tesla, __________ Gauss?
9.4 Tesla, 94000 Gauss.

==== 2.    In this field, what is the resonance frequency of __________ 31P, ____________ 29Si, ________________ 27Al, _______________ 13C and _______________ 11B, ______________1H in the magnetic field used in this laboratory? ==== take the equation:

ν = μB0/hI,

μz = 5.05095 × 10-27 JT-1, h = 6.626 × 10-34 Js,

μ values other than 27Al are from lecture slides.

==== 3.    What is the static line width of the 31P NMR signal in the Na2HPO4 standard sample in ________________ ppm, ______________ Hz, _________________ nm, ________________ microns, __________________ cm-1, and ___________________ eV? ==== Take the peak's full width at half maximum, we get 60.85 ppm.

4.    What were the rotation speeds that were used to spin the sample? ______________ Hz, ______________ Hz, ______________ Hz, ______________ Hz.
For 1H, 25000Hz;

For 31P, 25000Hz;

For 11B, 50000Hz and 25000Hz.



5.    How does the line width change with spinning speed?  What are the linewidths at the sample spinning speeds used?
For the two 11B spectra with different spinning rate, line width slightly decreases as the spinning speed doubles.

6.    What was the pulse width used to collect spectra?_____________________.
For 1H, 10 ppm.

For 11B, 40ppm, 20ppm, and 10ppm were used.

7.    What is effect of using pulses of shorter and longer width?
Generally, short pulse tp leads to high frequencies, and longer tp gives low frequencies.

It can be observed from the graph above. Both curves are from 11B NMR and the blue curve with longer duration time(40ppm) shift to lower frequency and the orange one (10ppm) shows up in higher frequency.

8.    What was the delay time between the pulses used to collect the spectra?_________________.
For 11B spectra, 400ppm was used for spinning rate of 50000; 130ppm was used for spinning rate of 25000.

9.    What is the advantage of using smaller and longer delays between pulses?
By using longer delay time, more excitation and relaxation occurs and it's easier to obtain strong enough signal. In the graph above, blue curve with longer delay time (400ppm) has better signal-noise ratio since enough signal was built to give separated peaks. However, shorter delay time can significantly decrease the experiment lasting time, which is always a goal for solid state NMR.

10.    How many individual spectra were collected and were co-added?_______________
For 31P spectra 64 scans are collected.

For 1H, 8 scans.

For 31P with cross polarization, 8 scans.

For 11B, 32 and 256 scans were collected in two individual experiments. 8 scans are used for neutation experiment,

11.    What would the effect be of using smaller and larger numbers of co-added spectra?
Easy to see that the peak signal of 11B increases and the signal to noise ratio gets improved. Generally using larger numbers of co-added spectra will lead to higher S/N, which means better sensitivity.

12.    Was there any evidence of saturation of the signal?___________________.  If so, how did this affect the signal.
No. No saturation was observed.

13.    How did the signal change as the delay time between pulses was changed?
In the two 11B spectra with different delay time, the first with a delay time of about 400 ppm shows better signal-noise ratio, which is due to a higher signal collection during longer delay time. Since delay time is the dependent on the relaxation rate, a longer delay time allows more excitation and relaxation processes.

==== 14.    If not, what would be a maximum spin lattice relaxation time for the 31P nucleus in this sample?__________________. What would be a maximum spin lattice relaxation time for the 11B nucleus in this sample? ==== For T1 relaxation, generally maximum spin lattice relaxation time requires a time of 5 x T1. For the 31P sample in this experiment with duration time of 150ppm, the maximum T1 relaxation time should be 30ppm. The 11B spectra with spinning rate of 50000 should have a maximum T1 of 80ppm, while the one with 25000 spinning rate should have a T1 of 25ppm.

15.    What is the nature of the 31P absorption line, is it a single line or are there multiple lines?
For the 31P cross polarization spin echo NMR spectra, it represents the P bonded with three Butyl functional groups and Pt. Butyl, as four-carbon chains, is a electron rice group that pull electron density to P nuclei. The cross polarization of 1H and 31P causes the peak to spilt into three. As we can see from pure P, there is only one peak. Generally the left and right peaks indicate the spin up and down, with a ratio of 50:50.

==== 16.    Look up and find the 11B NMR spectra of these two materials and compare them to the spectra we collected. Are they similar? Are they different? Are the differences what you would expect if there were differences in how the spectra were collected? What field, spinning speed, and pulse sequences were used to collect the literature spectra? Are they different than what we used? If so how are they different and do these differences account for the differences, if any, you see in the spectra? ====

The spectral was collected at 240 MHZ, with a spinning rate of 30kHz, and 25 kHZ RH field was used for excitation. Since a higher field was used, one can expect a better resolution and sensitivity from the literature NMR condition.

17.    What are some of your conclusions about the use of solid state NMR as a characterization technique compared to others you have studied in this course?
Generally signal of one single scan is very weak; which is due to the low abundance of the detected nuclei. So a large number of scans are used to bring an acceptable signal to noise ratio. And the process of solid state NMR can be slow. The signal is taken as distance from the standard chemical to define the 0 ppm. Also the peak is very narrow.

One advantage is that NMR can detect the isotopes. What really matters is that NMR spectroscopy can indicate the chemical environment around the nuclei, thereby it is possible to determine the atomic structures and bonding information.