Cyclobutadiene antiaromatic molecule

Antiaromatic molecules are kinetically unstable, and aromatic molecules are kinetically stable (Scheme 13). In cyclobutadiene, the n orbitals can be combined out of phase and the n orbitals can be combined in phase. Cyclobutadiene is kinetically unstable toward electron donors and acceptors. In benzene, all neighboring pairs of n orbitals cannot be combined out of phase, and all neighboring pairs of 71 orbitals cannot be combined in phase. Benzene is kinetically stable toward donors and acceptors. 

The kinetic stabilities and the donor-acceptor properties of cyclic conjugated molecules [68] have been described (Scheme 12) in the theoretical subsection (Sect. 2.2.2) to be controlled by the phase property. There is a parallelism between the thermodynamic and kinetic stabilities. An aromatic molecule, benzene, is kinetically stable, and an antiaromatic molecule, cyclobutadiene, is kinetically unstable (Scheme 13). 

Fig. 9 illustrates that the two acetylenic systems become nearly parallel at C1-C6 distances close to 3 A where the constructive overlap of the re-orbital with one of the re -nodes is compensated by a destructive overlap with the other rc -node (Fig. 9, bottom). From a conceptual point of view, the properties of the in-plane re-system at the 3 A threshold bear a striking resemblance to the interaction of the two re-bonds in D2h cyclobutadiene where the re-re interaction is zero and the re-re repulsion is considerable, thus accounting for the extreme instability of this antiaromatic molecule.41 Even more relevant is a comparison with the TS of the symmetry forbidden thermal [2S + 2S] cycloaddition (Fig. 10) which prompted us to call this region antiaromatic .42... 

Whereas for the Dih structure of cyclobutadiene the 50-5, splitting calculated using a multireference double (MRD) Cl treatment is 46 kcal/ mol, for (62) this value is about 89 kcal/mol [87AG(E)170], which also exceeds the value of the S0-S, splitting for (63) (Fig. 2). Ab initio calculations show (89JA6140) that 1,3,2,4-diazadiboretidine (62) has, unlike cyclobutadiene, a rhombic structure without bond length alternation in the ring, which is characteristic of the antiaromatic molecules. 

Planar cyclic conjugated species less stable than corresponding acyclic unsaturated species are called antiaromatic. They have 4n tt electrons. 1,3-Cyclobutadiene (n = 1), for which one can write two equivalent contributing structures, is an extremely unstable antiaromatic molecule. This shows that the ability to write equivalent contributing structures is not sufficient to predict stability. 

Benzene and other aromatics alike are stable molecules, while cyclobutadiene and other antiaromatic molecules are unstable molecules.27-76 Similarly, allylic species are stable intermediates and possess significant rotational barriers. It may appear as a contradiction that, for example, the tr-component of benzene can be distortive but it still endows the molecule with special stability or that the distortive jr-component of allyl radical can lead to a rotational barrier. We would like to show in this section that these stability patterns derive from the vertical resonance energy which is expressed as a special stability because for most experimental probes (in eluding substitution reactions) the o-frame restricts the molecule to small distortion167 in which the vertical resonance energy is still appreciable, as shown schematically in Figure 5. 

The 1,3,2,4-diazadiboretiidine, being isoelectronic to cyclobutadiene, is supposed to be an example of inorganic antiaromatic molecule. Although the 1,3,2,4-... 

Thus, both square cyclobutadiene and planar cyclooctatetraene are antiaromatic. Antiaromatic molecules are destabilized by delocalization of their rr electrons. Consequently, both cyclobutadiene and cyclooctatetraene adopt structures that minimize the delocalization of these electrons. 

At present, then, aromaticity is best defined in terms of stability derived from the delocalization of bonding electrons. An aromatic molecule is characterized by appreciable stabilization relative to a noncyclic polyene. An antiaromatic molecule is one that is destabilized relative to a polyene model, and the term nonaromatic can be applied to molecules for which the calculated energy and energy of the polyene model are comparable. Cyclobutadiene, with an estimated destabilization energy of 15-20 kcal/mol, is a good example of an antiaromatic species. 

When the accuracy of HMO theory is improved by introducing inter-electronic repulsion, the degeneracy of the highest occupied orbitals is broken in both aromatic and antiaromatic molecules. As a result, even cyclobutadiene turns out to have a plane-rectangular singlet ground-state. [36]... 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

In contrast to aromatic molecules which have An + 2 n electrons, cyclobutadiene and cyclooctatetraene do not have An + 2 7r electrons and are not aromatic. In fact, diese molecules, which contain An jt electrons (n is an integer), are less stable than die planar model compounds and are termed antiaromatic. Bodi of these molecules adopt shapes that minimize interactions of die n orbitals. 

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