Tutorial: How to Measure the Coupling Constants from a DQF-COSY
To repeat this tutorial you need iNMR 3.3.1 and this sample spectrum. Install iNMR, unpack the spectrum, then open it with the program. You will see a complex multiplet. Select the region around it with the mouse. Also create an horizontal line with alt-click. Drag the line to select a row corresponding to the maximum intensity of the multiplet. Here is what you should see:
Choose the menu command: Simulate > Multiplet. The selected signal will be copied into a new window (the simulation module). From now on, we'll be working exclusively into this module. Drag the horizontal red line (or the horizontal blue line) upwards to reveal the negative part of the multiplet.
- black = experimental spectrum.
- red = simulation.
We can count 4 positive peaks and 4 negative peaks. It's reasonable to assume that there are 3 couplings
of different magnitude. (because 23 = 8).
We need to estimate the magnitude of these couplings, one by one.
The little vertical light blue line on top of the blue label works like an handle. Drag the doublet in correspondence to the first two peaks on the left. Change the constant to 6 Hz. Here is what you should get:
Memorize the number 6 and move on to the next coupling, between the first positive peak and the first negative peak. To simulate it, select “Antiphase d.” from the menu. Move the red doublet slightly to the right and increase the coupling. Here, 10 Hz seems to be a good estimate. Memorize this number too.
The last coupling can be measured in the same way, as shown by the picture below.
Now move the doublet at the center, introduce three couplings by acting on the three pop-up menus, and type the three values of the coupling constants we already know. After clicking “Apply” your simulation should look like the one here below:
Note how the positive red peaks fall at the same positions as the positive black peaks and how the negative red peaks correspond to the negative black peaks. This is an important precondition, otherwise the automatic fitting will fail to converge. Now click on the “FIT” icon.
Here is the final result! Read the values of the coupling constants in the top part of the window.