Cyclobutadiene geometry

High level molecular orbital calculations of cyclobutadiene itself and experimen tally measured bond distances of a stable highly substituted derivative both reveal a pat tern of alternating short and long bonds characteristic of a rectangular rather than square geometry... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

The total 7t-eIectron energy of benzene is 6a -I- 8)3, corresponding to a DE of 2)3. Cyclobutadiene is predicted to have a triplet ground state (for a square geometry) and zero... 

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. 

Table 15.9. Energies and derivatives of energies relating to the stability of the square geometry of cyclobutadiene.

Cyclobutadiene has been shown to have a rectangular geometry by competitive trapping of the two valence tautomeric 1,2-dideuteriocyclobutadienes using methyl... 

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. 

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... 

Figure 18. (a) jt—a-Interplay diagrams for the twin states of /7-electron—/7-center antiaromatic species. (b,c) Calculated geometries for the twin states of cyclobutadiene and cyclooctatetraene (from refs 164a, 177, 188, and 219). 

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