Triplet state cyclobutadiene

Although aUcenes react rapidly with singlet oxygen, reaction with triplet oxygen is normally slow. Exceptions are very electron-rich aUcenes such as tetraaminoethylenes , and normal aUcenes with low-lying triplet states such as cyclobutadienes andketenes . 

We can next consider, as an annulene other than benzene, the very unstable enigmatic cyclobutadiene, see Refs. 94 and 95, which was first prepared some 40 years ago. The 13C NMR spectra from doubly labelled 13C molecules 97 suggest that the two valence isomeric forms square and rectangular, interconvert rapidly even at ca. 25 K. The energies and some NMR parameters of the two forms have been estimated recently from theory,98 as have those of benzene in its ground state—for comparison. More work is needed to establish exactly where the electron singlet and triplet states occur. 

Straightforward applications of the spin-coupled approach confirm the relative energies and the preferred geometries of cyclobutadiene in these singlet and triplet states [11], without the requirement for multiconfigurational descriptions,... 

Existence of cyclobutadiene as the simplest cyclic hydrocarbon with conjugated double bonds was predicted by Hiickel in the 1930s. Until recently, however, all attempts to prepare this compound ended in failure. In 1956 Longuet-Higgins deduced 166) that coordination of cyclobutadiene with a transition metal should stabilize its unstable triplet state. 

Molecule (I) is stable (singlet ground state), while molecule (II) is unstable (triplet ground state). Cyclobutadienes are unstable regardless of the substituents. There are a number of experimental supports for this general conclusion 113,123). 

Other examples of NVET include the triplet energy acceptors 1,3,5,7-cyclooctate-traene,116 which has a puckered structure in the ground state but relaxes to a planar structure in the triplet state, and a cyclobutadiene derivative,117 which changes from a rectangular to a square structure. 

Wirz, J., Krebs, A., Schmalstieg, H., Angliker, H., Electron Structur. The Electronic Triplet State of a Peralkylated Cyclobutadiene, Angew. Chem. Int. Ed. Engl. 1981, 20, 192 193. 

The cyclobutadiene in the triplet state has the square symmetry (the two adjacent C-C bonds of equal length), while in the ground state the molecule is rectangular. Therefore, if one takes Eq. (8.95) for the first C-C bond and then for the second one, we should get flie same length in the triplet state (all values calculated by the DPT method see Chapter 11). Let us insert the bond lengths for the first C-C bond ... 

In support of the prediction that cyclobutadiene is likely to have a triplet ground state, Skell and Petersen 87) have recently examined the reactions of the species obtained by gas-phase reaction of 3,4-dichloro-tetramethylcyclobutene (X) with potassium. They found that the dehalo-genated species (XI) behaved as a molecule in the triplet state would be expected to, particularly in its reaction with triplet methylene to give l,2-dimethyl-3,4-dimethylenecyclobutene (XII) and hence proposed this as the first direct evidence for the existence of a < clobutadiene as an intermediate. 

Osmocene is a photoactive compound. Photolysis of osmocene in n-hexane led to metallic osmium by reductive elimination. In the LF excited triplet state, osmocene undergoes a distortion by bending, which facilitates the transfer of a CH group between the cyclopentadienyl ligands. The resulting intermediate is an osmium complex with benzene and cyclobutadiene ligands, that decomposes to osmium metal, benzene and cyclobutadiene. ... 

The calculated DE, is zero. Therefore simple molecular orbital theory predicts (apparently correctly) that cyclobutadiene should have no resonance energy and, if Hund s rule is applicable, a lowest diradical (triplet) state. 

C4H4 is predicted by the 4m -l- 2 rule to have a triplet ground state. Cyclobutadiene, C4H4, first synthesized in 1965, is a highly reactive compound that dimerizes at temperatures... 

In some case, the photolysis products of dienes and trienes depends on the excited state in which photochemical cyclisation occurs (singlet or triplet). To populate triplet state, suitable triplet sensitizer are used if required. The. 3-Methylene-l, 5-hexadiene gives a cyclobutadiene derivative through singlet excited state and tricyclic ring compound from triplet excited state. This difference of photochemical process is because of life-time of the excited states. Higher the life-time of reacting intermediate more will be possibility to form more stable radicals. 

In fact, some addition reactions of the liberated cyclobutadiene suggest it to be a diene with a rectangular, singlet state structure rather than a delocalized aromatic ring with a square, triplet state structure. 

If one of the double bonds of the cyclobutadiene triplet state were reduced, the product would be the triplet state of cyclobutene, having a high energy. NICS calculations by Schleyer et al. found shielding values characteristic of aromatic species.Thus, by any criterion, the triplet state of cyclobutadiene has aromatic character. 

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