Triplet ground state oxygen reactions

The most common sacrificial electron acceptor in the environment is molecular oxygen, whereas the main sacrificial donors are organic compounds. In consequence, the self-cleaning processes consist in oxidation of organic pollutants by molecular oxygen in its triplet ground state the reactions are driven by energy from solar radiation. In nature, many different photoinitiators or photosensitizers are reactive, but the most common environmental photosensitizers include hiunic substances (HS), whereas the best photoinitiators are transition metal complexes. 

Molecular oxygen is an important participant in reactions of triplet carbene because of its triplet ground state and its ubiquity as an impurity in reaction... 

The role of spin in the reaction is an especially interesting and important issue in carbene chemistry. Apparent violation of the spin selection rule, such as the reaction of ground state singlet carbene with triplet molecular oxygen in matrices as well as in solution, and the reaction of triplet ground-state carbenes with (singlet) CO, CO2, and N2 in matrices, are challenging issues for the near future. 

Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state (xAg), in which all electrons are paired. Unfortunately, this species, referred to as singlet oxygen, is generally too reactive and too short-lived for most situations (lb, lc). However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner (2). This latter type of activation is the subject of this chapter. 

As in 4-9, it is not the ground-state oxygen (the triplet), that reacts, but the excited singlet state,782 so the reaction is actually a Diels-Alder reaction (see 5-47) with singlet oxygen as dienophile 783... 

The reaction was reinvestigated by Cvetanovic (21) who found that within a small analytical uncertainty the exclusive primary step was reaction (11). Formation in this reaction of oxygen atoms in their triplet ground state (OIP) is required by the spin conservation rule and there is now ample chemical evidence that this is indeed so. A primary formar tion of an electronically excited N20 molecule, ruled out in the early work on spectroscopic grounds, is also incompatible with the lack of a pressure dependence of the rate of decomposition. 

Photolysis of matrix-isolated (trimethoxysilyl)diazomethane at X > 305 nm produced carbene 3e (equation 1), which was characterized by IR and UV-Vis spectroscopy23. Under these conditions, no other species besides the carbene could be detected spectroscopically. In an 02-doped (1%) argon matrix, the carbene rapidly reacted with oxygen to give carbonyl oxide 4 which was further photoisomerized to formylsilane 5. Again, the fast reaction of 3e with 302 points to a triplet ground state of the carbene. 

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