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What would be the directions of ring current-induced magnetic fields in purine-like heterocycles in a strong magnetic field?
Would be the antiparallel state be a default state for these substances in the absence of strong magnetic field at warm temperatures due to heat motion?
This is an interesting problem, I think it is best for a moment before I get stuck in that we considered a slightly different compound. I will consider it with an external magnetic field applied to it. Napthalene, now napthalene has two aromatic hydrogen environments in its NMR spectrum. These are very similar in chloroform to benzene in terms of chemical shift.
This makes me think that the ring currents go in the same direction in this compound.
It may be better to consider the ring current as flowing along the perimeter of the aromatic system rather than the loops formed by the six membered rings.
It is important when considering aromatic rings to escape being trapped by six membered rings. A lot of aromatic molecules have six membered rings but there are plenty of five and seven membered aromatic rings. It is more important to understand that a pi system which is cyclic and has 4n+2 pi electrons is the defining thing which makes a ring system aromatic.
If napthalene had ring currents going in different directions I would expect that if the interconversion of the sites (by ring current reversal or flipping of the molecule) was slow then I would expect to see four different chemical shifts in the proton spectrum.
If this exchange of sites was fast then I would expect to see two sites which are each the average of the two sites. I would also expect the chemical shifts of the napthelene to be very different to that of benzene.
If we allow ourselves a little diversion for a moment, the cyclopropanes have a rather rare proton chemical shift, this is becuase the C-C bonding in cyclopropane has some pi character and the hydrogen nuclei are above the ring in such a place that the ring current has the opposite effect on the NMR shift to the magnetic field due to the ring current in benzene.
Now if we move onto purine, in case anyone is not sure what the shape of it is then here it is.
If we look at the literature (S2-75) in the supporting literature of Thomas H. Graham, Wensheng Liu, and Dong-Ming Shen, Org. Lett., 2011, 13 (23), pp 6232–6235 DOI: 10.1021/ol2026813. This compound in DMSO has three very similar proton NMR shifts. This suggests that the ring current goes the same way around in both rings. These NMR shifts are all in the aromatic area and are further left (higher ppm) than benzene.
