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.