Chemical Sensitization Singlet Dioxygen

We limit ourselves here to a single example, due to Turro et al. [16]. In it, ground-state 62 is apparently converted to 2 by interaction with a strained acetylene, with which it then reacts to produce an a-diketone in a chemiluminescent excited state  

Exceptions would be the approach of an atom or axially symmetric molecule along the internuclear axis of O2 (— Coov) or the orthogonal approach of two O2 molecules to one another (— D2d)- 

The detailed symmetry-analysis by Trindle and the author [17] of the parent reaction is summarized concisely in Fig. 10.3. Two 0 atoms, two C atoms and two H atoms provide twenty-two valence electrons altogether. Ten of these six in the CC and CH bonds of ethylene and four oxygen lone-pair electrons - need not be considered explicitly, because the five orbitals housing them retain their symmetry properties across the diagram. Fig. 10.3 makes do with six MOs for the closed shell dioxetene intermediate these are supplemented by one more in the reactants and product, where open shell states have to be taken into account. The ordering of the orbitals in the three species is intuitive, but - apart from the upper two, that determine the nature of the lowest excited states - it is immaterial. 

Reading Fig. 10.3 from left to right, we consider first the cycloaddition of 02 to the strained acetylene. For the purpose of the analysis, the reactants are assumed to approach one another in the coplanar [7r25-f- 2 ] orientation, in which the degeneracy of the tt orbitals of O2 is split by the approaching acetylene. Favorable interaction with the vacant tt MOs of the alkyne should stabilize both, but better overlap with the in-plane component is expected to push the one labeled 62 below its partner and select it as the doubly-occupied HOMO. The only orbital mismatch with the dioxetene intermediate is between an a  

If the approaching O2 molecule is in its triplet ground-state, its ir orbitals -each singly occupied - are split as described above. If the loose complex formed is sufficiently long-lived, ISC of its Tz bi) component is allowed and the singlet complex can then either dissociate or collapse to the dioxetene. 

---