Square cyclobutadiene

Stabilizing resonances also occur in other systems. Some well-known ones are the allyl radical and square cyclobutadiene. It has been shown that in these cases, the ground-state wave function is constructed from the out-of-phase combination of the two components [24,30]. In Section HI, it is shown that this is also a necessary result of Pauli s principle and the permutational symmetry of the polyelectronic wave function When the number of electron pairs exchanged in a two-state system is even, the ground state is the out-of-phase combination [28]. Three electrons may be considered as two electron pairs, one of which is half-populated. When both electron pahs are fully populated, an antiaromatic system arises ("Section HI). 

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... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

FIGURE 11 13 Frost s circle and the TT molecular orbitals of (a) square cyclobutadiene (b) ben zene and (c) planar cyclooctatetraene... 

Section 11 19 An additional requirement for aromaticity is that the number of rr elec Irons m conjugated planar monocyclic species must be equal to An + 2 where n is an integer This is called Huckel s rule Benzene with six TT electrons satisfies Huckel s rule for n = 1 Square cyclobutadiene (four TT electrons) and planar cyclooctatetraene (eight rr electrons) do not Both are examples of systems with An rr electrons and are antiaromatic... 

For some systems a single determinant (SCFcalculation) is insufficient to describe the electronic wave function. For example, square cyclobutadiene and twisted ethylene require at least two configurations to describe their ground states. To allow several configurations to be used, a multi-electron configuration interaction technique has been implemented in HyperChem. 

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... 

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. 

The pattern of orbital energies in Figure 11.13 provides a convincing explanation for why benzene is aromatic while square cyclobutadiene and planar- cyclooctatetraene are not. We start by counting tt electrons cyclobutadiene has four, benzene six, and cyclooctatetraene has eight. These tt electrons are assigned to MOs in accordance with the usual rules—lowest energy orbitals first, a maximum of two electrons per orbital. 

It is clear that simple cyclobutadienes, which could easily adopt a square planar shape if that would result in aromatic stabilization, do not in fact do so and are not aromatic. The high reactivity of these compounds is not caused merely by steric strain, since the strain should be no greater than that of simple cyclopropenes, which are known compounds. It is probably caused by antiaromaticity. ... 

The unfused cyclobutadiene system is stable in complexes with metals (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is no aromatic quartet. In fact, these cyclobutadiene-metal complexes can be looked upon as systems containing an aromatic duet. The ring is square planar, the compounds undergo aromatic substitution, and NMR spectra of monosubstituted derivatives show that the C-2 and C-4 protons are equivalent. ... 

With application of reasonable values for trapping parameters and AS2, it was possible to bracket the enthalpy and entropy of activation for isomerization of cyclobutadiene. Hence, A/Zj was estimated to fall between 1.6 and lOkcal/mol, where the upper limit was consistent with theoretical predictions for square-planar cyclobutadiene. Most surprising, though, was the conclusion that AS for automeriza-tion must lie between -17 and -32cal/(molK), based on the AS values normally observed for Diels-Alder reactions as a model for AS2. ... 

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. 

Square. Bi42-, Se42+, and Te42+ are isoelectronic and isolobal with the delocalized planar cyclobutadiene dianions. There are 14 skeletal electrons [e.g., for Bi42- (4)(3) + 2 = 14] corresponding to 8 electrons for the 4 c-bonds and 6 electrons for the 71-bonding (see Chapter 2.7.2.1). 

Crystal structure data15 indicate that in the vast majority of (cyclobutadiene)metal complexes (4) the cyclobutadiene ligand is approximately square-planar with nearly equal C—C bond distances (ca 1.46 A) and bond angles of ca 90°. Within a given complex the cyclobutadiene carbon-to-metal distances are roughly equal. 

Dill). A hypothetical square-planar anion 64114 (cf the isoelectronic cyclobutadiene dianion 0464 ") would have four o-bonding MO s (of symmetries Aig, Big, and Bg) and three 7r-bonding MO s (of symmetries Am and Eu), which would be stabilized and modified in symmetry, but not added to, if 2 6H + units were brought up the fourfold axis to complete the octahedron of 68Hb . Significantly, the neutral arachno-borane 64640 actually has a C v arrangement of its 4 skeletal boron... 

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... 

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

It turns out that cyclobutadiene is not a perfect square (two bonds are longer than the others), but it is essentially planar. Not surprisingly, it is very unstable and dimerizes extremely readily. It only exists at very low temperatures either in a matrix with an inert solvent (where the molecules are kept apart), or at room temperature as an inclusion compound in a suitable host molecule. Azacyclobutadiene (azete) is also extremely unstable, for similar reasons. 

There is another important effect which influences the ordering of the electronic states. Square cyclobutadiene is an organic molecule... 

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