Cyclobutadiene distortion

Hiberty, P. C. The Distortive Tendencies of Delocalized n Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153, 27-40 (1990). 

To achieve non-zero 7ta—7tb conjugation, the pi NBOs of 18 may polarize in opposite directions, leading to a wavefunction of lower symmetry than the nuclear framework. Alternatively, the nuclear framework may distort to diamond-like D2h geometry. However, each such distortion destabilizes what is already a highly unfavorable Lewis-structure wavefunction, so cyclobutadiene is expected to remain highly destabilized relative to other possible polyene topologies. 

Butterfly. While Tl2Te22- has a (2)(1) + (2)(4) + 2 = 12 skeletal electron count isoelectronic and isolobal with neutral cyclobutadiene, it undergoes a different Jahn-Teller-like distortion to the butterfly structure discussed by Bums and Corbett [81]. 

Distortion Energies and QMREs (in kcal/mol) for Benzene, Cyclobutadiene, and Their Heteroanalogs According to Calculations (76JA2750 87JA363 88IC2219) ... 

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... 

Hiberty PC (1990) The Distortive Tendencies of Delocalized tt Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153 27-40 Hintz S, Heidbreder A, Mattay J (1996) Radical Ion Cyclizations. 177 77-124 Hirao T (1996) Selective Transformations of Small Ring Compounds in Redox Reactions. 178 99-148... 

The actual geometry of a conjugated system is determined by a fine balance of the distortive r-component and the symmetrizing force of the o-frame.12 4 This o— -balance and opposition is summarized schematically in Figure 4. In allylic species and in benzene, the cr-frames win and the species remain symmetric, with equal C—C distances and with delocalized -electrons. In contrast, in cyclobutadiene the a-resistance is insufficient and the structure relaxes to a rectangular geometry. 

Table 2. jt- and a- Distortion Energies (AE, AE ) of Analogues of Cyclobutadiene, and 3jtjf Excitation Energies of a. T-Bond (AEst)" ... 

Another trend which is apparent from the data is that the final geometry is obtained as a fine balance between the a-resistance to distortion and the T-dis-tortivity. Thus, hexagonal benzene is a D h hexagon because its (/-resistance exceeds the. T-distortivity by a significant margin. In contrast, N6 is almost indifferent to the distortion because its T-component is more distortive while its c/-frame is less resistant to the distortion than those of benzene. No doubt, the o-lone pair repulsion affects the force constants of the o-frame in N6. Similarly, while cyclobutadiene and N4 are clearly distortive as a whole, the P4 and Si4H4 squares are pretty indifferent to distortion. This is because the T-distortivity gets very small and the... 

Clearly, as already summarized in Figure 4, benzene and allyl radical are quite special in having a combination of strong o-resistance which overcomes a moderately distortive Tr-component. On the contrary, in cyclobutadiene the fine balance is tipped in favor of the r-distortivity. However, this balance is fragile and can be tampered with. Even substituted cyclobutadienes have been considered at times to be effectively square.144-146 As we shall see later, manipulation of the o-resistance by proper substitution of benzene enables its r-distortivity to become manifest. Heilbronner s expression13 of the fragility of the jt o-balance for benzene and allyl radical is perhaps a most appropriate one If by an act of God all the spf—spf o-bond force constants were reduced to half... 

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. 

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