Jahn-Teller distortion cyclobutadiene

Note that in fact cyclobutadiene does not have degenerate, singly-occupied molecular orbitals, as a Jahn-Teller type (actually a pseudo-Jahn-Teller) distortion lowers its symmetry from square to rectangular and leads to a closed-shell paired-electron molecule [4]. 

The second is the existence of the dication of variously substituted cyclobutadienes Ab initio quantum chemical calculations indicate that at least the parent dication, 42, is significantly non-planar (43). This is suggestive of some 1,3-bonding although one would hesitate to describe this ion as bicyclobutane-1,3-diium (44) vide infra). Perhaps even more reluctantly, one may describe this species as the dication of tetrahedrane (45) where both electrons have been removed from the same C-C bond. Indeed, the closed shell (or singlet) dication of tetrahedrane will lack strict tetrahedral symmetry because of unequal orbital occupancies and resulting Jahn-Teller distortion It is presumably this C-C bond... 

The e2 state factorises to 1A, + 1 Bt + 3A2 + 1B2 so that there is in fact no orbitally degenerate state and the distortion is thus really of the Renner type. Strictly, therefore, a true Jahn-Teller distortion will occur in cyclobutadiene cation and anion but not in the neutral species (W. N. Lipscomb, personal communication, 1978). 

Resonance contributors and resonance hybrids for benzene (top), carbonate ion (middle), and cyclobutadiene (bottom), which actually adopts a rectangular structure due to Jahn-Teller distortion... 

Fig. 2.14 The four Huckel n molecular orbitals and energies of square cyclobutadiene (a) and the consequences of a Jahn-Teller distortion on the two frontier orbitals fz and jp3 (b). The fillings of the molecular orbitals are indicated.

The Mj" (M = Li, Na, K, Rb, Cs) anions are examples of a-antiaromatic systems with 4 o-electrons [100]. Singlet Mj" anions at the symmetry have the la/ le electron configuration and their triangular structures undergo Jahn-Teller distortion. Indeed, Mj anions are linear. Two valence MOs can be localized into two 2c-2e bonds and the linear structure of Mj" can be considered as a classical structure. That is similar to the antiaromatic cyclobutadiene, where canonical MOs can be localized into two double and two single carbon-carbon bonds, producing a classical structure. 

Consider the consequences of a Jahn-Teller distortion in each cyclobutadiene unit in poly(l,3-cyclobutadiene) that introduces long and short bonds into the ring (giving a Peierls distortion to the polymer). Sketch how this would change the band structure developed in Exercise 3. 

Figure 2. Calculated geometries of Jahn—Teller-distorted structures of cyclobutadiene radical cation. (Reprinted with permission from ref 15i. Copyright 1995 Elsevier Science.)...

B2g. Thus, Fq = b g<8)0 g and Fq = b g< )b2g, and the molecule is susceptible to a pseudo Jahn-Teller distortion along a b g or 02g coordinate. As shown for the D4h A4 system in Appendix III, there are no normal modes of 02g symmetry but the b g motion takes the square H4 to a rectangle, a result anticipated by the orbital correlation diagram in Figure 5.5. This very same state of affairs occurs in the tt system of square cyclobutadiene. 

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]. 

We have implicitly assumed that cyclobutadiene is square. If it distorts and adopts a rectangular structure, its frontier orbitals become nondegenerate (Jahn-Teller effect) but the orbital separation will remain small and the molecule will still be highly reactive. 

Fig. 5-4 rc-Electron energy-levels of cyclobutadiene (a) in its ideal , symmetrical form, in which all carbon-carbon bond-lengths are equal, and (b) in its rectangular form, caused by a pseudo Jahn-Teller or Renner distortion. 

An important aim of the book is then to show how common orbital situations arise throughout the whole chemical spectrum. For example, there are isomorphisms between the electronic structure of CHj, Fe(CO)4, and Ni(PRj)s and between the Jahn-Teller instability in cyclobutadiene and the Peierls distortion in solids, These relationships will be highlighted, and to a certain extent, we have chosen problems that allow us to make such theoretical connections across the traditional boundaries between the subdisciplines of chemistry,... 

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