Spin Simulation

NOTE: The NMRpredict Desktop plugin must be installed (a license for this plugin is not needed).

This module of MestReNova is an efficient simulator for high resolution NMR spectra which can be used by following the menu 'View/Tables/Spin Simulation (or Prediction/Spin Simulation') which will display the applicable dialog box:

Once there, we can easily simulate a spectrum by entering the desired values. For example, 3 protons (3 spins Groups) called A, B and C, with chemical shifts 1, 5 and 7 ppm respectively. Their coupling constants were J(AB) = 3Hz, J(BC) = 5.5Hz and J(CA) = 3.4Hz. Finally, select the number of points and the spectral limits and click on the 'New Simulation' button to generate the corresponding spectrum:

Once you have simulated the spectrum, you can modify any value in the table and obtain the new spectrum by clicking on the 'Recalculate' button . Click on the 'delete' button to remove a specific row from the table, or click on the 'clear all' button to start from the scratch. You can introduce 'Dipolar' or 'Quadrupolar' coupling constants and change the 'number of nuclei', 'Spin' and the 'Line Width' values by clicking and entering the desired values on the corresponding boxes. Finally, click on the ‘Recalculate’ button to update the changes in the spectrum.

Click&drag the assignment label to change the chemical shift of any signal of the spin-simulated spectrum:

Select 'New as Superimposed' over an experimental spectrum to stack the synthetic spectrum with an experimental one:

If parameters of experimental spectrum and simulated spectrum are not the same, when you try to generate the calculated spectrum as superimposed a message will be displayed:

Clicking on Yes, the Spectrum Properties parameters will be modified to match with the experimental ones.

Here you can see another example comparing an experimental dataset with the simulated one:
 

You can see below the simulated spectrum of the  'orto-dichlorobencene'; which is a AA'BB' system:

How can I simulate an A3BXY system?

Here you can see an example of an A3BXY system:

To get the table above just follow this procedure:

1. Firstly, create a system with 4 spin groups. Please bear in mind that A3 is a unique spin group with 3 magnetic equivalent spins, so you will need to type N=3 in the label A..

2. Then, type the desired values for the other nuclei (B, C, D) with N = 1 for all of them (enter also the desired chemical shift value). Please bear in mind that Mnova does not need to know if the coupling is strong or weak, because Mnova makes the quantic calculations by using a rigorous method taking into account the chemical shift and the coupling constants

3. Once you have entered all the values and selected the Larmor Frequency and number of digital points, Linewidth for each nuclei, etc..., just click on the 'New Simulation' button to get the synthesized spectrum.

How can I add new systems?

Click on the 'Add System' button to add several systems. A new system will be a fragment of the molecule that can be simulated separately since its protons do not interact with the rest. In other words, a set of nuclei that are related through their coupling constants. Once you have added a new system, you can change the population of the system by using the 'Population' edit box.

As an example, consider the case of a small molecule, ethyl acrylate CH2CHCOOC2H5

As you can see, a standard simulation of the 1H-NMR spectrum of this molecule would require eight frequencies to be introduced, one for each proton. Moreover, you should introduce nine coupling constants as well. However, even a brief study of this spin system suggests us a more elegant solution.  

Why not treat this spin system as two 'subsystems'? One 'subsystem', corresponding to the ethyl group, is composed of five protons with only two distinctive frequencies labeled a (the methyl group) and b (the methylene group), and only one coupling constant, Jab. Other 'subsystem' is composed of the three remaining protons (those of the vinyl group), with three frequencies, c, d and e, and three coupling constants, Jcd, Jce and Jde. This decomposition of the spin system in two subsystems is possible because they are not connected in any way. The decomposition of a spin system into subsystems can be carried out in many molecules, and is, for example, the conceptual basis of the NMR spectroscopic data tables.

You can see below, the synthesized spectrum of the ethyl acrylate; after having created two different systems; system A (vinyl protons) in red, and system B (ethyl group) in green:

This decomposition method represents a great advance in terms of calculation speed, because the complexity of the calculation grows exponentially with the number of spins (nuclei) considered. However, we can perform even faster calculations and achieve greater flexibility going one step further.  

Why not introduce only five frequencies, and tell the program how many protons correspond to each frequency? The introduction of magnetic equivalence allows the calculation to be performed using five spin groups, labeled from a to e, with their corresponding frequencies. The two first spin groups, a and b, have three and two protons respectively, while the other three spin groups have only one proton each. Of course, we need then only four coupling constants, Jab, Jcd, Jce and Jde. This process can be carried out in many molecules.