ABSTRACT
Molecular oxygen (O2) was independently discovered by Joseph Priestly and Carl Wilhelm Scheele in the late eighteenth century [1]. It can react at sulfur and with nearly every other element under a variety of conditions to produce oxides [2]. e €rst photooxygenations (photochemically initiated incorporation of oxygen into molecules) were reported as early as 1867 [3], well before the electronic character of oxygen was understood. It was not until 1924 that G. N. Lewis attributed the paramagnetism of oxygen to a structure with unpaired electrons [4,5]. is discovery was rapidly followed in 1928 by R. S. Mulliken’s suggestion that the electronic con€guration of oxygen, [( ) ( ) ( ) ( ) ( ) ( ) ( ) ],* * *σ σ σ σ σ pi pi1 1 2 2 2 2 2s s s s p p p2 2 2 2 2 4 2 leads to three states in order of increasing energy, 3Σ g− ,1Δg, and 1Σ g+ since the ≠2p* orbital is only half €lled [6,7]. Finally, it was not until the early 1930s that Kautsky and coworkers [8,9] reported that the metastable 1Δg state of oxygen was involved in many of these photooxygenations. It is now well appreciated that photooxygenations can involve both the ground-state triplet, 3Σ g− , and the metastable 1Δg oxygen. On the other hand, the higher energy 1Σ g+ state at 37 kcal/mol above the 3 Σ g− ground state is vibrationally and quantitatively deactivated to the lower energy 1Δg state (22.5 kcal/mol) and does not have a su™cient lifetime to chemically react with organic substrates [10].
