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When a nucleus moves between two (sometimes more) chemical states we have Dynamic NMR. Common examples are: the equilibrium between conformers; complexes that can have two different structures, etc. 'Chemical Exchange' is the expression that encompasses everything. The principle is so general, that the kind of chemical bonds that are created and destroyed is not relevant. What matters is the rate of exchange between the two states. If the exchange is slow, you see nothing in 1-D, but you see a cross-peak in the EXSY spectrum (another name for the more familiar NOESY). If the exchange is a little faster, you see two broad signals in 1-D. Warm the sample and the exchange becomes faster and faster: you see a single signal, but quite a broad one. At higher rates the single signal is so sharp that we don't mention Dynamic NMR anymore.
More exactly, the appearance of the spectrum depends both on the rate of exchange and on the difference (in Hz) between the two peaks. If we increase the magnetic field, the effect is similar to cooling.

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To calculate the rate, we perform Lineshape Analysis, that is we compare a simulated spectrum with the experiment. At the fastest and slowest extremes, even a drastic change in the rate has little effect on the spectrum. At coalescence, instead, even a small change in the rate has a dramatic effect. This is when the signal is the broadest and when the rate of exchange can be calculated with the highest accuracy.
A few years ago I wrote a tutorial on a complex between a ligand with two nitrogens and a platinum ion. The ion could move between the two nitrogens. There were also six hydrogens in the molecule: A exchanging with A', B with B' and C with C'. The very nice thing was that the three frequency differences (A-A', B-B', C-C') had different values. We could therefore see three temperatures of coalescence and measuring the exchange rate was easy. The similarity between the simulation (black) and the experiment (red) was really OK:

In the first days of DNMR, acronym for dynamic NMR, only the singlets were studied. Not only the signals were stronger, but also easy to understand. Simple formulas, valid at coelescence only, were used instead of lineshape analysis. Partly because of this simplicity, results were not consistent. In the late 60s Binsch showed that coupled systems, just because they were very complicated, were also a much more accurate probe than singlets. Binsch also wrote the theory to simulate the coupled systems and the first computer program for the task, called 'DNMR'.

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A great expert, today, is prof. Alex Bain. His suite of programs (open source) is called MEXICO. Another great expert is Hans Reich, who wrote WinDNMR. If you want a cheap alternative, I have written iNMR. You can try the Windows version for free for two months.

Labels: Chemical Exchange, DNMR, Dynamic NMR, Lineshape Analysis

Sample Preparation

To prepare a sample for NMR spectroscopy, dissolve 10-50 mg of the sample in about a mL of a deuterated solvent, usually CDCl₃. FT-NMR instruments require that samples be run in a solvent containing deuterium because the instrument locks on the resonance of deuterium to achieve field-frequency stabilization. Deuterated solvents must be extremely pure and are expensive, with CDCl₃ being the least expensive (because it only has one deuterium atom) as well as the most versatile.

The dissolved sample is then transferred to an NMR tube. NMR tubes are special-purpose glass tubes that are manufactured to certain specifications of high-quality glass. In the instrument, the tubes are spun rapidly, thus the tubes must be balanced so that they both spin evenly and do not break.

All solid material must be removed from the solution before it is placed in the NMR tube. Suspended or solid particles cause broadening of the absorption peaks in the spectrum. After you prepare your sample, measure the depth of the liquid in the NMR tube; it should be about 6 cm: adjust the volume of your sample tube as necessary.

A sample run in CDCl₃ will always show a peak at 7.26 ppm because deuterochloroform is never 100% pure and a tiny amount of residual ordinary chloroform, CHCl₃, is in the sample. This peak is used to calibrate the spectrum during the workup process.

Acquiring an NMR Spectrum

The NMR facility in the Chemistry Department at CU Boulder is under the direction of Dr. Rich Shoemaker. To operate one of the NMR instruments in this facility, you must be trained by Dr. Shoemaker. The Lab Coordinator and all of the TAs have been trained, but students are only trained if they join a research lab in the Chemistry Department. Therefore, if you want to have an NMR sample run, you will need to ask your TA or the Coordinator to run it for you (you can probably go to the facility with them to watch the process). The NMR website has instruction manuals for the various instruments in the NMR facility.

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Work-up of the Spectrum

After acquiring the raw NMR data, the instrument operator will save the spectrum on the NMR lab computer as a FID file. How to install cricket 07 in windows 10. The FID can then be worked up (changed into the familiar NMR spectrum format) at one of several computers/workstations in the NMR lab or elsewhere. The NMR website has instruction manuals for NMR data processing in the NMR facility.

Alternatively, you can ask the coordinator to transfer the FID file to your own computer (or a computer in the teaching labs area) and you can work up the spectrum using a program called NUTS. Acorn NMR, Inc., offers this NMR data processing software for PC and Mac operating systems. You can download free, functioning demo software and manuals on how to use this software on the Acorn NUTS web site.

Common Problems

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Solubility: If your sample is not soluble in chloroform, other deuterated solvents are available, such as deuterium oxide (D₂O) or deuterobenzene (C₆D₆).

Contaminants and impurities: Often a sample contains a trace amount of a solvent used in the preparation of the compound. Solvents are organic compounds too and will show up in the NMR spectrum. Usually, solvents are only present in trace amounts, showing small peaks. For your reference, a table of the ¹H-NMR peaks of common solvents is presented below. The data in this table is both from an article in the Journal of Organic Chemistry ('NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities', J. Org. Chem. 1997, 62, 7512-7515) and from data collected by running the various solvents on the Varian 300 NMR instrument in the CU Boulder NMR facility.

Also remember from above that the most common NMR solvent, CDCl₃, will itself show up as a peak due to residual ordinary chloroform at 7.24 ppm.

¹H-NMR peaks of common solvents when run in CDCl₃

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Peaks of the most common solvents are listed below. For a more complete list, consult the paper.

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Solvent	Structure	Proton	Mult.	Chem. Shift	1H-NMR spectrum
Water	H2O	H2O	s	1.56	Water
Acetone	CH3COCH3	CH3	s	2.17	Acetone
Dichloromethane	CH2Cl2	CH2	s	5.30	Methylene chloride
Diethyl ether	O(CH2CH3)2	CH2	t	1.21	Diethyl ether
CH3	q	3.48
Ethanol	CH3CH2OH	CH3	t	1.25	Ethanol
CH2	q	3.72
OH	s	1.3-1.5
Ethyl acetate	CH3COOCH2CH3	CH3CO	s	2.05	Ethyl acetate
OCH2CH3	q	4.12
OCH2CH3	t	1.26
Hexanes	C6H14, mixture of isomers	CH3	t	0.88	Hexanes
CH2	m	1.26
Methanol	CH3OH	CH3	s	3.49	Methanol
OH	s	1.1-1.6