Antiaromatic molecules

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. 

Scheme 13 Kinetic instability of antiaromatic molecules and kinetic stability of aromatic molecules...

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... 

One can argue that the presence of the in-plane C3-C4 bond and the 6-electron out-of-plane 7t-system renders the whole molecule non-antiaromatic. Therefore, we use the quotation marks for the term antiaromatic region . However, use of this term is justified because the lack of the n-n stabilization along the strong n-n repulsion provides an appealing analogy to antiaromatic molecules... 

Structural indices constructed in this fashion are, in essence, phenomenological, and one is entitled to ask whether the specific features in the geometry of the aromatic and antiaromatic molecules are indeed determined, and if so, to what degree, by the cyclic electron (bond) delocalization. 

Distinguishing Characteristics in the Geometry of Aromatic and Antiaromatic Molecules... 

Whereas the benzene molecule possesses a structure of D6A symmetry with equal lengths of the CC bonds, for acyclic polyenes alternation of bond lengths is a characteristic [87JCS(P2)S1]. For antiaromatic molecules, alternation is even more pronounced and unlike the aromatic molecules, a high-symmetry structure of the lowest singlet state of the antiaromatic molecules does not correspond to a minimum on the PES. For... 

By contrast, in the case of antiaromatic molecules with N = An the contribution is positive (paramagnetic ring current). 

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. 

Hovis, J. S. and Hamers, R. J. Structure and bonding of ordered organic monolayers of 1,3,5,7-cyclooctatetraene on the Si(001) surface Surface cycloaddition chemistry of an antiaromatic molecule. Journal of Physical Chemistry 102, 687 (1998). 

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. 

Pentalene, 22, is another antiaromatic molecule known to undergo bond-shift equilibrium with a barrier of approximately 4 kcal/mol.182 A second-order perturbation Hiickel treatment100 by Heilbronner and Shaik183 indicated that linking pentalene into bipen-talene, 23, should cause one of the moieties to undergo delocalization or at least lower the barrier for achievement of the local D2h situation. Many antiaromatic species seem to behave in the same manner.184... 

Antiaromatic molecules have, as a rule, very low-lying triplet states, ca. 10 kcal/mol or less above the ground state.89 187 Furthermore, since the a-frame limits the distortivity of the antiaromatic molecule, the triplet excitation is constrained to remain small. Consequently, the instability of antiaromatic species is a kinetic instability associated with availability of low-lying triplet states. A related argument has been used recently by Zilberg and Haas.188... 

The heightened reactivity of an antiaromatic molecule, with n-electrons on /7-centers, is not rooted in... 

C. The Kekule Mode in the Twin States of Antiaromatic Molecules... 

The abbreviation TRE comes from topological resonance energy , as this measure of the overall cyclic conjugation in a polycyclic conjugated molecule was originally named [35,44], The TRE-concept was introduced in 1976 independently by Aihara [34] and by Milun, Trinajstic and the present author [35, 44, 45], Aromatic conjugated molecules have positive whereas antiaromatic molecules have negative TRE-values. 

Nevertheless, the NICS values appear to readily classify standard molecules into three discrete categories. Aromatic molecules possess NICS values that are negative. The values at the center of the six-member rings of benzene and naphthalene and anthracene are -9.7 and -9.9, respectively. Charged aromatic molecules also have negative NICS values the values for cyclopentadienyl anion and tropy-lium cation are -14.3 and -7.6 ppm, respectively. Nonaromatic compounds like cyclohexane and adamantane have NICS values near zero. Lastly, antiaromatic molecules such as cyclopentadiene and planar Z>4 cyclooctatetrane have NICS values that are positive, 27.6 and 30.1 ppm, respectively. 

The bond separation energy is quite negative (—37.9 kcal/mol with 6-31G ), but less so than other antiaromatic molecules such as oxirene. This may partly be due to the distortion of the triangle noted above and also to a stabilizing interaction between the polar bonds. The total energy of oxazirine is predicted (by 6-31G ) to lie 97.3 kcal/mol above that of the most stable HCNO isomer, isocyanic acid. )... 

Scheme 1 Bifurcation values of ELF and ELF functions for some aromatic and antiaromatic molecules...

The application of two photon spectroscopy to antiaromatic molecules is exemplified by a study of biphenylene in which both 2Ag and 28, states have been... 

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