Photolysis of Cyclobutadiene

It is clear from Fig. 10.4 that thermal [ 2 -h 2]-cycloaddition of acetylene, like that of ethylene (Fig. 6.2), is forbidden in T 2h by a mismatch between a bi and a 63 orbital. The same conclusion applies to the reverse reaction, thermal fragmentation of cyclobutadiene (CBD) to two acetylene molecules [18]. Here too the 625 correspondence between cr (63 ) and +(5iti) that formally allows the thermal reaction is geometrically unsuitable. The HOMOs and LUMOs of the reactant and product-pair are in cross-correspondence under an displacement, that also induces a correspondence between J (i ) and The two 

C2H2 fragments then only have to twist about the (72-axis as they recede to form two acetylene molecules, one in the closed shell ground-state and the other as an excited singlet. 

Does the possible formation of the acetylene pair in Ti have to be taken into account There are two reasons why it need not be. First, at the orbital level of approximation, the electrons in the crossing orbitals have to be considered separately. Each is transferred from a u to a MO a spin-flip would require 

The symmetry analysis is consistent with the experimental results reported by Chapman et al. [19]. Photolysis of CBD in an argon matrix produces a pair of caged acetylene molecules that diffuse apart on warming to 35°K. If the matrix is thawed without being irradiated, CBD does not fall apart but dimerizes to syn-tricyclo[4.2.0.0] octa-3,7-diene, as described in Section 7.3.1. 

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Cyclobutadiene has been observed directly by kinetic mass spectrometry in the flash photolysis of cyclobutadiene iron tricarbonyl 513). 

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