One of the most impressive pieces of evidence (to me at least) that we really understand what electrons are doing in organic molecules are the ring currents. Recall that the pi electrons in benzene are delocalized above and below the planar ring determined by the 6 carbon atoms.
How do we know this? When a magnetic field is applied the electrons in the ring cloud circulate to oppose the field. So what? Well if you can place a C – H bond above the ring, the induced current will shield it. Such molecules are known, and the new edition of Clayden (p. 278) shows the NMR spectra showing [ 7 ] paracyclophane which is benzene with 7 CH2’s linked to the 1 and 4 positions of benzene, so that the hydrogens of the 4th CH2 is directly over the ring (7 CH2’s aren’t long enough for it to be anywhere else). Similarly, [ 18 ] Annulene has 6 hydrogens inside the armoatic ring — and these hydrogens are even more deshielded. Interestingly building larger and larger annulenes, as shown that aromaticity decreases with increasing size, vanishing for systems with more than 30 pi electrons (diameter 13 Angstroms), probably because planarity of the carbons becomes less and less possible, breaking up the cloud.
This brings us to Nature vol. 541 pp. 200 – 203 ’17 which describes a remarkable molecule with 6 porphyins in a ring hooked together by diyne linkers. The diameter of the circle is 24 Angstroms. Benzene and [ 18 ] Annulene have all the carbons in a plane, but the picture of the molecule given in the paper does not. Each of the porphyrins is planar of course, but each plane is tangent to the circle of porphyrins.
Also discussed is the fact that ‘anti-aromatic’ ring currents exist, in which they circulate to enhance rather than diminish the imposed magnetic field. The molecule can be switched between the aromatic and anti-aromatic states by its oxidation level. When it has 78 electrons ( 18 * 4 ) + 2 in the ring (with a charge of + 6) it is aromatic. When it has 80 elections with a + 4 charge it is anti-aromatic — further confirmation of the Huckel rule (as if it was needed).
On a historical note reference #27 is to a paper of Marty Gouterman in 1961, who was teaching grad students in chemistry in the spring of 1961. He was an excellent teacher. Here he is at the University of Washington — http://faculty.washington.edu/goutermn/