Thursday, 5 March 2009

NMR and the Chemist’s Illusion

Stan has just posted a nice entry in which he uses the aromatic region of Strychnine to discourse about the different effects in the NMR spectrum (in terms of resolution and multiplicity) produced when the magnetic field frequency is changed. In particular, I like his description of the ‘Chemist’s Illusion’ and as a chemist, I would like to illustrate, just with a picture, what this illusion is all about.

In the picture below, I have synthesized the ABCD spin system corresponding to the aromatic region of Strychnine at different fields (we don’t own a 1500 MHz spectrometer and we don’t expect to get one for Mestrelab in the short- or mid-term :-) ). It can be appreciated that as we move to higher fields, the multiplets appear to be more separated (this is an illusion: their chemical shifts, in ppm, are exactly the same!) and get more resolved and more first-order like.

Below I’m showing an expansion of the right most multiplets:

Another interesting and well known example is represented by an AA’BB’ spin system (for example, o-diclhrobenzene) . Again, as we go to higher fields, the apparent multiplets separation looks larger, although the multiplet fine structure remains virtually unchanged. In other words, in these systems, second order effects will always exist regardless of the magnetic field. When the magnetic field is increased, it will be possible to get a larger chemical shift difference between the AA’ and the BB’ groups, but not between A and A’ or B and B’ (it’s always zero), so that the highest simplification one can achieve by increasing the magnetic field is to move from an AA’BB’ group to an AA’XX’ group which is a second order spin system too.


old swan said...

If you use a ppm scale you have the illusion that the peaks remain at the same position. If you place your spectra on the SAME Hz scale, they will be quite different!
I agree that there is too much confusion in our field, but please don't increase it with these artificial divisions between "chemists" and "true spectroscopists"... It's not your fault, it's more Stan's. He has written that chemists' spectra are (today) underdigitized. Alas, he forgot to give the recipe, which is so easy: set the acquisition time 5 times longer than the apparent T2.
If, for example, you set the acquisition time to 3 seconds, the number of points will be proportional to the magnetic field, there will be no problem of under-digitization, etc... Excuse me if I posted the reply to Stan's blog on yours!
Anyway, the "recipe" above is, indirectly, the answer to your post too: if the same spectrum, doubling the field, requires the double of points, it is twice more resolved. Isn't it?

stan said...

Hi, old Swan, this is Stan.
Nice to hear from you, and of course you are right! The problem is just that MOST spectra at or above 400 MHz that come my way are under underdigitized. It is not my fault; it just happens. And they mostly stop at 32K points so I though that maybe they just don't have enough RAM.
And another thing: I do not want to be impolite to the chemists. I think that it is absolutely correct if the primary display for chemists is on the ppm scale. In which case the multiplets appear to shrink when the field increases. I called it "chemist's illusion" but it was not meant in a critical way - within that kind of context, I would do the same.
Btw, Carlos' 100 MHz ODCB has a terrible resolution. Fortunately it is simulated. Otherwise, such an instrument would be unsellable :-))
Cheers, Stan

old swan said...

Thank you, Stan for the explanation. Now I can tell you the whole story. There is a minority of chemists who don't like to read the labels under the ppm scale. For this reason they print all their spectra from 0 to 10 ppm, standard. After many years of inspecting spectra in this way, they are really able to read them even if the scale is not printed. If, instead, you show them a spectrum from 1 to 5 ppm, they can't read it, even if the scale is printed. Anyway, they are a minority.
There are, instead, many chemists that like to expand the regions of interest, like to navigate into the spectrum with a computer, to measure the coupling constants on the monitor, etc.. after many years they have arrived to understand the difference between 60 and 600 MHz. There are even a few chemists that know very well that shimming a 600 MHz is more difficult than shimming a 60 MHz!

old swan said...

...and I forgot to say the most important thing. Bruker users think in terms of "TD", therefore it's natural that they tend to use the same TD whichever the magnetic field. Varian users, instead, set the "AT", which is in units of seconds. They can't fall into the same trap. These are the real categories.