_What is anti-aromaticity?_ Some molecules look less symmetric than might be, they appear to be distorted out of some "idealised" symmetry. For example this molecule composed out of 4 carbon atoms and 4 hydrogen atoms that looks like a rectangle with spikes at its vertices (1/n)

One could ask: Why doesn't it have the shape of a square (with spikes)? The answer is that its because sometimes the most symmetric shape cannot possibly be the most favourable one. A simple example for this would be the case of 1 or 2 negatively charged balls in between (2/n)

two walls. In the case of one ball we cannot keep the high symmetry, for the most stable position of the ball, which would be the exact middle, rather the ball has two decide in between two equally good positions, either on the right or on the left wall: (3/n)

Exactly this can happen in molecules and the effect is called Jahn-Teller effect or Jahn-Teller distortion, after the guys who first explained this. One can call it also spontaneous symmetry breaking, though the effect does not happen as a real process (4/n).

In general the JT "effect" is much less of an effect as the theory behind can be seen as a general mathematical theory of molecular structure. Accordingly any molecule that hasn't arranged its atoms in the highest symmetrical way possible, is a sort of JT case (5/n).

So there are plenty of examples for a JT effect of the one or the other kind. One component of the phenomenon of "antiaromaticty" is exactly a motive symmetrybreaking or JT. And a reformulation of JT is that the symmetrical structure is less stable or instable comp. with (6/n)

an "idealized" higher symmetry of the molecule. Now we come to the second component of the phenomenon, which is a physical property that appears if we put the molecule in a magnetic field. In such an experiment there are two major possibilities for what can happen (7/n)

Either the molecule is sucked into the field (kind of attracted) or it is pushed out of the field. Normal molecules such as the frog here (OK a frog is a huge molecule ...) are pushed out of the magnetic field (8/n)

This "normal" can be specified its in 99,9% the case when the molcule contains an even number of electrons. But now it can happen, very rarely though, that even a molecule with an even number of electrons is sucked into the field. This is called closed shell paramagnetism (9/n)

Now it happens that in a relatively large fraction of cases of closed shell paramagnetic molecules that the come with a certain type of JT distortion. The best example is already in the C4H4 molecule from above. Chemists have noted that coincidence for a long time and (10/n)

case by case clear explanations have been found for why and how the two effects are correlated in certain molecules. On the other side we are still missing a fully general theory of antiaromaticity. (The more intriguing implication of the existence of such a theory is (11/n)

that it most likely would automatically imply a full general theory of aromaticity at the same time, since the two effects are exactly antipodal, and that's kind of a "holly grail" in chemistry) (12/n)

It happened that I have discovered by incidence not a general "theory" for antiaromaticity but a concise argument for the general interdependence of the two pheonmena for the cases when both effects are already determined by the symmetry of the molecule itself and alone. (13/n)

This argument is:In certain symmetries(think "shapes" of molecules)JTdistortion implies paramagnetism such that all "distorted" molecules in this symmetry must also be sucked into the magnetic http://field.By  this notion antiaromaticity becomes a consequence (14/n)

of an underlying structure rather than a coincidence of two phenomena of uncertain or only occasionally clarified connection. (15/n)

For those of you who are a bit into theoretical chemistry, here are the very symmetries, called K2, for which the two effects are logically/mathematically connected (with additional informations) (16/n)

And (almost) finally a diagram that gives an overview about the whole argument: (17/n)

The argument has some pretty trivial aspects if you know a bit quantum mechanics, however there is one thing hidden that is far from trivial, this is connected with the fact that in many AA molecules you have non-degenerate high symmetric ground states and consequentially (18/n)

One has to analyse second order ("pseudo") JT distortions in such configurations. Then you are left with the question what is the peculiar "Physics" of these states and JT distortions that characterise the AA cases? (19/n)

The answer to this is that they have symmetry related degenerate AND partially occupied natural orbitals. They are a (subset of) biradicals in fact. Thats it (20/n)