Molecular oxygen triplet ground state

Figure 1 Electronic configurations of molecular oxygen triplet ground state and the singlet first excited state. The figure shows the highest occupied molecular orbitals.

A principal appHcation for photomedicine is the photodynamic treatment of cancer. Photochemical and clinical aspects of this topic have been reviewed (10,11). Direct irradiation of tumors coupled with adininistration of a sensitizer is used to effect necrosis of the malignancy. In this process, an excited state sensitizer interacts with dissolved in vivo to effect conversion of the oxygen from its triplet ground state to an excited singlet state, which is highly cytotoxic. In principle, excited sensitizers in either the singlet or the triplet state can effect this conversion of molecular oxygen (8). In... 

Run a stability calculation on the true (triplet) ground state of molecular oxygen. What is the energy difference between the ground state and the singlet state ... 

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. 

As shown in Fig. 1, the electronic structure of molecular oxygen is quite unusual in that two of its twelve valence electrons are unpaired electrons of parallel spins in two degenerate 2prr antibonding orbitals. In this triplet ground state, oxygen therefore behaves as a biradical. In the singlet state, all electrons are paired. 

Fig. 3.13 Molecular orbital (MO) diagram of an oxygen molecule illustrating the electronic configuration of the triplet ground state ( Oj) and of the first excited singlet state (singlet...

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. 

The triplet ground state of molecular oxygen provides a considerable kinetic barrier to the auto-oxidation of normally diamagnetic organic molecules where reactions involving change of spin are generally very slow, and where products formed in triplet states are unstable. This barrier may be circumvented in three ways ... 

Figure 10.1 Orbital diagrams for the three configurations of molecular oxygen, O2. Shown from the left are The triplet ground state the singlet oxygen Ag excited state, and the second singlet oxygen Eg+ excited state. Adopted from Ref[l].

Figure 11.2. Molecular orbital diagrams for O2 and O2. The ground state normal O2 (dioxygen) = triplet ground state) has two unpaired ir-electrons (it is a biradical) and is paramagnetic. Two excited forms of singlet oxygen O2 ( A and E ) can be generated (often by light irradiation in the presence of a sensitizer). The first product in a one-electron reduction of oxygen is the superoxide ) anion. (Adapted from Sawyer, 1991.)...

---