Antiaromatic compounds atomic orbitals

A monocyclic compound that is planar or nearly so has one Ip orbital on each atom of the ring, and has 4n tt electrons in the cyclic arrangement of overlapping Ip orbitals, where n is an integer. Antiaromatic compounds are especially unstable. 

The electron spin resonance (ESR) spectra of the radical ions of 230 indicate there are no large deviations from the free-electron g value that would have been expected had the 3d orbitals of the sulfur atom played an important part in influencing the spin density of the molecule. Consequently, structure 230 may not be the main contributor to the electronic structure of the compound. Such stability in this compound could be attributed to the inertness of the NSN group and the presence of the aromatic naphthalene ring. However, the H-NMR chemical shifts (8 = 4.45 ppm) suggest the compound is antiaromatic. The compound is therefore referred to as an ambiguous aromatic compound (78JA1235). 

A compound is classified as being antiaromatic if it fulfills the first criterion for aromaticity but does not fulfill the second. In other words, it must be a planar, cyclic compound with an unintermpted ring of p orbital-bearing atoms, and the tt cloud must contain an even number of pairs of tt electrons. Hiickel would say that the tt cloud must... 

In contrast to furan, pyrrole, and thiophene, the heteroatom in borole does not contain an electron pair that can participate in cyclic delocalization. The p orbital in the sp -hybridized boron atom (see Figure 1-17) is empty and the molecule is isoelectronic with the antiaromatic cyclopentadienyl cation (Section 15-7). Indeed, the parent compound borole is not known. 

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