Dynamic NMR
Many spectra acquired in routine NMR spectroscopy, on close inspection, show signs of chemical exchange.
Probably half of the spectra acquired in solution have this characteristic.
Common examples are labile protons, tertiary amides, aliphatic rings and heterocycles.
The effect of exchange on simple 1D spectra, acquired at equilibrium, is to broaden lines.
It may be useful to simulate these spectra on the computer,
either for curiosity or to measure the activation energy of the process.
You start by defining two or more chemical sites (often different conformations or configurations
of the same molecule)
and then proceed to simulate the exchange between them.
To fully understand this chapter you must already know how to simulate a
static spin system with iNMR.
If the command “Dynamic” remains grayed out, it means that not all the declared systems are equivalent. Note, also, that the X approximation cannot be used in conjunction with dynamic NMR.
The calculation time can increase tenfold for each nucleus you add.
To simulate DMF and other molecules containing methyl singlets, declare them as single isolated protons!
When each system contains 6 or more nuclei, the calculation can be extremely time-consuming.
In very rare cases (for particular combinations of parameters) the system degenerates
(mathematically speaking) and the results are wrong.
You can detect this problem from a reduction in the total area of the spectrum.
It is enough to increase or decrease the exchange constant(s) by 0.01 to remove the degeneracy.
When you increase the exchange rate you are simulating the heating of the sample.
If you are trying to fit an experimental spectrum,
it is natural to expect a drift of the chemical shifts upon such heating.
You have to change the chemical shifts manually, because iNMR will not do it for you.
Above coalescence, you will see a single peak and two chemical shift labels under it.
How can you drag the labels together?
It is highly recommended to activate the menu option “Lock Equal Letters”.
In this way you are sure that the chemical shift difference, between the exchanging sites, remains constant.
Relations Between the Parameters
iNMR automatically generates as many k parameters as possible. For example, if you define 4 systems, iNMR will generate 6 parameters of type kp→q, if you define n systems, iNMR will generate n(n-1)/2 such parameters. In real cases, however, the exchange under observation can be described with only a few rate constants (or a single one). The other parameters are either zero or equal to the first rate. If you know, for example, that k3→4 = k1→2, look into the sidebar for the line corresponding to k34. Substitute its numerical value with k12. Initially nothing happens, but the next time you change the value of k12, the same value will be assigned to the linked parameter k34.
If, instead, you know that the rates are proportional but not equal, for example: k3→4 = 1.25 k1→2, add a symbol, like: k12x. Any symbol is valid. Now open the dialog Simulate > Your Constants and define x = 1.25. You can define 100 different symbols.
Step increments
When the rate of exchange becomes high (> 100 sec-1), and the step parameter is below 100, the latter is interpreted as a percentage increment, not as an absolute increment. This is very advantageous, because you can use the little arrows for the whole range of possible values, without changing the parameter “step”. Just remember that step must be below 100.
If you need some practice, you are invited to read the web tutorial.
Related Topics
Web Tutorials
Simulate Chemical Exchange with iNMR
Reference
Binsch, G., J. Am. Chem. Soc., 91, 1304-1309 (1969)