Lab 5: Isotope Ratio via NMR
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Measurement of the Isotopic Ratio of 10B/11B in NaBH4 by 1H NMR
Create a 5% (approximate) solution of NaBH4 in D2O:
Weigh out 50 mg solid NaBH4 into a vial. Carefully mix the NaBH4 with 1 ml of D2O. Stir until all solids dissolve. The solution may be initially cloudy, but it will eventually clear up. (Note: NaBH4 reacts with D2O to create hydrogen gas.)
Take a 0.7 ml aliquot of the 5% NaBH4/D2O solution and transfer it to an NMR tube. Take a thumb tack or a push pin, and gently pierce a hole through the cap of the NMR tube so that any H2 gas created may vent out and not create pressure in the NMR tube, which may potentially cause the NMR tube to break.
After transferring the solution, cap the tube with the pierced NMR tube cap. The sample is now ready for data acquisition.
Follow the directions to acquire a NMR spectrum given in the short NMR instrumentmanual you were provided. Be sure to load standard parameters, lock, and shim Z1 and Z2 before you acquire the spectrum. From the spectrum and the known spin quantum numbers of 10B and 11B identify the peaks in the spectrum. Integrate the peaks to determine the 10B to 11B ratio.
Integrate the peaks three times and take the average ratio.
Boron isotopic ratio (5). Accepted values and typical methods to obtain it.
Using a vector model, explain how an NMR signal arises (5).
On a more quantum-mechanical level, what gives rise to an NMR signal?
A nucleus has an intrinsic spin angular momentum and an associated magnetic dipole.
What will happen if this nucleus is placed in a magnetic field? Write out the spin
Hamiltonian that will give you the energy of such a nucleus arising from its spin.
What is the corresponding Schrodinger equation and what are its eigenvalues? Are
the spin energy levels always separated? When does the separation arise? Describe
the Zeeman effect. (9)
Based on the expression for the energy associated with the nuclear spin, why are
stronger magnetic fields preferred over weaker ones, especially for structural
studies of complex molecules? (2)
Assume two interacting 1H nuclei, draw the energy levels arising from their
interaction and the allowed transitions. (5)
Description of instrument What exactly is meant when you speak of a 300-MHz
NMR instrument? What is the magnetic field for the 300 MHz instrument? (3)
From where does peak splitting arise? What is the interaction that produces this
splitting? Write out the Hamiltonian that takes into account the effect of this
How many peaks are you expecting to see in a proton spectrum of these two 1H
What is the population difference at 298 K between the energy levels for the two
interacting 1H nuclei in the above question? (10)
Why is it important that the two populations (above) are not exactly the same? (5)
What would happen to the population difference if the magnetic field strength
Method and Instrumentation
Sample Preparation (2)
Description and definition of steps in acquiring an NMR spectrum (5). Why do you a) spin the
sample, b) lock the signal, and c) shim the magnetic coils?
You used an FT-NMR machine. That is, the spectrum that you got is already processed data.
What is the raw data that is actually obtained during data acquisition? What is it called?
What is the data processing technique that converted this raw data to your frequency
Data and Results (25)
Calculation of isotopic ratio (3)
Based on typical instrument performance, peak integration has an inherent 5-10% error (instrumental error estimate). What is the final error associated with your calculated ratio using this assumed error? Calculate for both the lower and higher range of the errors (i.e.
assuming 5% and 10% error in integration and let’s identify them as Error1a and Error1b).(12)
A possible way to calculate errors for this experiment is to treat each peak as a data point (Error 2). Grouping the two different sets of protons according to which Boron isotope they’re
attached, you can calculate for the average area and the std. dev. of the area for each set.
See Table 3 Chapter 2 of your text for errors from a small set of samples. Using these
values, what is the calculated isotopic ratio and error? (10)
How well does the calculated isotopic ratio agree with the accepted values? Do they agree within experimental error? (4)
Compare Error1 and Error2. What can you say about the way you performed this experiment? Is it safe to assume that the errors are purely instrumental? Or were other factors involved?
Assuming that errors were purely instrumental, what can you conclude from Error1a and Error1b? (7)
Why isn’t NMR a “standard method” for doing these sorts of analyses? Can it be used for all types of nuclei? (5)
What are the sources of errors for an NMR experiment starting from sample prep to data processing? And how will these errors specifically affect the final result? (4)
Formatting, Grammar, Style (5)
Note on grading: This grading checklist should serve as a guideline as to the minimum requirements for the lab report. Don’t limit yourselves to merely answering the questions (though they should be answered).