Cyclobutadiene rectangular

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... 

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

The rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. With 6-3IG calculations, for example, cyclobutadiene is found to have a negative resonance energy of—54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. G2 and MP4/G-31(d,p) calculations arrive at an antiaromatic destabilization energy of about 42kcal/mol. ... 

The reaction of o-diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with p-benzoquinone yields A as the exclusive product. With tetracyanoethylene, however, B and C are formed in a 1 7 ratio. Discuss these results, and explain how they relate to the question of the square versus rectangular shape of cyclobutadiene. 

Cyclobutadiene is a rectangular molecule with two shorter double bonds and two longer single bonds. Why do the following structures not represent resonance forms ... 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

Later in 1978, Masamune reported the first matrix Fourier transform IR spectrum of cyclobutadiene that now allowed a direct comparison of the observed spectrum of 1 with the calculated spectra. In Figure 6, Masamune s experimental spectrum is compared with the computed IR spectra (STO-4G and 4-3IG basis sets). It is seen that both calculated spectra, which were performed with relatively small basis sets, are in good qualitative agreement with the observed spectrum of 1. One could therefore conclude that the ground state structure of cyclobutadiene is indeed rectangular. 

One question remained to be resolved regarding the structure of cyclobutadiene. Chapman reported in 1973 that in an attempt to separately prepare the two dideuterio cyclobutadienes ([1,2- H2]-1 and [1,4- H2]-1) from the two dideuterio 2-pyrones ([3,6- H2]-2 and [5,6- H2]-2), both yielded the same IR spectrum. Of course if cyclobutadiene were square only a single IR spectrum would have been expected, while if it were rectangular one would have expected different IR spectra for the two dideuterio cyclobutadienes. The IR spectra of the two dideuterio cyclobutadienes were computed, and indeed they were found to be quite different from each other. When a 1 1 mixture of the two calculated spectra was compared with Chapman s spectrum (see Figure 8) with inclusion of additional bands later observed by Michl, it was immediately obvious that Chapman... 

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

Show the products expected from the Diels-Alder reactions. Use orbital interaction theory to develop a bonding scheme for rectangular cyclobutadiene and explain why rectangular cyclobutadiene may be exceptionally reactive as a diene in Diels-Alder reactions. 

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. 

The first unequivocal assignment of rectangular singlet cyclobutadiene by theory was reported by Buenker and Peyerimhoff, see Buenker, R. J. Peyerimhoff, S. D. J Chem. Phys. 1968, 48, 354. 

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