Spectroscopy of Cyclobutadiene

Valence bond descriptions of benzene and cyclobutadiene and their counterparts with localized bonds

“…angular geometry [37]. The resonance energy is large, as is the TRE of Ϫ7.85 kcal/mol with respect to IV.3.…”

Comment on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

“…The challenge provided by anti-aromatic character and intrinsic instability of CBD inspired numerous experimental studies (4)(5)(6)(7)(8). The photochemical transformation of a-pyrone (9, 10) (Scheme 1) confined within a matrix emerged as a promising route to this elusive molecule without protection provided by bulky substituents or transition metals (11).…”

Cyclobutadiene: The Antiaromatic Paradigm?

“…[29] According to these calculations, rectangular CBD has two near-UV electric-dipole-forbidden transitions that correspond to the excitation of one (1 1 A g !1 1 B 1g ) or two electrons (1 1 A g ! 2 1 A g ) from the HOMO to the LUMO.…”

“…Experiment has so far only provided rough estimates and limits. From trapping studies in solution, Whitman and Carpenter concluded that the barrier must lie between 6.7 and ca 40 kJ mol À1 , [29] while Orendt et al deduced from IR and NMR measurements in cryogenic matrices that the rate constant of interconversion must be higher than 10 3 at 25 K, [30] which translates into a free energy of activation of less than 4 kJ mol À1 . However, the best high-level ab initio calculations predict the barrier to lie between 27 and 32 kJ mol À1 .…”

“…[31,32] These two facts can only be reconciled if one invokes heavy-atom tunnelling. In 1983 Carpenter had in fact done exactly that to account for the negative entropy of activation which he had deduced from his experiments, [29] and he concluded that the tunnelling mechanism would actually dominate the automerization process at temperatures below 0 8C. [33] Although this proposal originally met with a great deal of scepticism, it has not been dispelled to date.…”