The O2 molecule

When this overall sign is positive, the function F is gerade and its term symbol is appended with a "g" subscript (e.g., the level of the O2 molecule, which has 7iu" 7ig 2 occupancy is labeled 2ig) if the sign is negative, F is ungerade and the term symbol is so amended (e.g., the If level of the ItIu configuration of the... 

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

Where in this cycle is the essential influence of the catalyst Suppose we carry out the reaction in the gas phase without a catalyst. The reaction will proceed if we raise the temperature sufficiently for the O2 molecule to dissociate into two O atoms (radicals). Once these radicals are available, the reaction with CO to CO2 follows instantaneously. 

The setup used for crossed beam experiments is basically the same apparatus used in the H2O photodissociation studies but slightly modified. In the crossed beam study of the 0(1D) + H2 — OH + H reaction and the H + HD(D2) — H2(HD) + D reaction, two parallel molecular beams (H2 and O2) were generated with similar pulsed valves. The 0(1D) atom beam was produced by the 157 photodissociation of the O2 molecule through the Schumann-Runge band. The 0(1D) beam was then crossed at 90° with the... 

Excited states of the O2 molecules play a minor role. UVD occurs without the pigment being involved and is therefore of no further interest in this connection. 

Oxygen gas is normally present in electroless solutions unless steps are taken to reduce its concentration by inert gas purging, the concentration of dissolved O2 usually approaches 10 4 mol dm 3 at room temperature. In contrast to deliberately added solution stabilizers, the O2 molecule tends to adsorb weakly on catalytically active surfaces, where it may or may not undergo dissociation [125], Since at many metallic surfaces it is capable of being reduced via either a 2-electron or a 4-electron reaction route, dissolved O2 gas affects the mixed potential of the surface being coated in the electroless solution. Shown here is the case of a 4-electron reduction reaction in basic solution ... 

The O2 molecule is essential to all aerobic forms of life, but many anaerobic organisms (e.g. anaerobic bacteria such as Clostridia spp.) are killed after only brief exposures to molecular O2. However, it is well established that even aerobic organisms, including man and other animals, show signs of oxygen toxicity when exposed to O2 tensions above those normally found in air (i.e. >21% O2). Such toxicity does not normally occur because aerobic cells possess protective enzymes that prevent either the formation or the accumulation of oxygen metabolites. It is only when these protective systems be-... 

Figure 7.6 Ground-state electron configuration for the O2 molecule.

The triplet ground state (OkJ/mol) for the O2 molecule is represented by the term symbol Eg and has two unpaired electrons, one each in the rr or the K level. The first excited state (92kJ/mol above the ground state) is a singlet (electrons spin paired with both electrons in either the irf or the Kf level). The singlet state with paired spin electrons, one each in the rr and K levels, is the next excited level 155 kJ/mol above the ground state. Reduction of O2 by one electron yields the superoxide ion (O2) a radical anion. Reduction by two electrons yields the peroxide ion, (02 ). As noted in equation 7.1, the reduction potential for O2 in the presence of protons is thermodynamically favorable. Therefore reversible binding of O2 to a metal can only be achieved if competition with protons and further reduction to superoxide and peroxide are both controlled. ... 

Figures 3a and 3a depict the weak bond of an O2 molecule with the lattice. It is formed by an electron being drawn from an ion of the lattice to an O2 molecule. Owing to the greater electron aflSnity of the O2 molecule, the electron may be considered completely transferred from the lattice to the molecule as a result, a molecular ion 02 is formed and a localized hole appears in the lattice attached to the ion Oi, The entire system (the adsorbed O2 molecule + adsorption center) acquires a noticeable dipole moment with negative pole directed outward, but remains electrically neutral as a whole. The bond is effected without the participation of a free lattice electron. The transition to a strong acceptor bond entails the localization of an electron, or, what amounts to the same thing, the delocalization of a hole. Such a strong acceptor bond is depicted in Figs. 3b and 3b. ...

The following is review on the molecular and physical properties of this class of monooxygenases, which are also known as hydroxylases. A typical monooxygenase reaction is the hydroxylation of an alkane to an alcohol which involves a reduced cosubstrate that reduces a second atom within the O2 molecule to form water. Flavin-containing monooxygenases include lysine oxygenase and 4-hydroxybenzoate hydroxylase. Reduced pteri-dines are involved in the phenylalanine hydroxylase and tryptophan hydroxylase reactions. See also Cytochrome P-450... 

C. The average kinetic energy of the He molecules is greater than that of the O2 molecules. 

For each cluster size, the few lowest energy equilibrium isomers with molecular adsorption (MA) and two atoms adsorption (TAA) complexes are represented in Fig 8. Most of these structures are planar or near planar. For 4c, 5b, and 5d, only an O atom is not in the plane. Two atom adsorption leads to major structural changes in the gold cluster, particularly for n>5. For example, in the structures 5a, 6a, and 7a, a gold atom breaks the bonds to other Au atoms to form a highly stable linear O-Au-0 unit which bind each 0 atom to one of the remaining Au atoms. On the other hand, molecular adsorption induces only a modest relaxation in the host cluster, and the O2 molecule is attached on top of a Au atom preferably to the bridge position between two Au atoms. 

The reactivity of Au O clusters to oxygen as a function of n was measured by Salisbury and coworkers using time of flight mass spectroscopy. It was found that odd-n (even-n) clusters have negligible (significant) reactivity, which is explained by the fact that the O2 molecule, acting as a one-electron aceptor, binds to anionic gold clusters, which have an unpaired electron. 

In this subsection we investigate the adsorption of O2 molecules on neutral Aun clusters with 5cluster substrate, which is chosen among the equilibrium structures of pure An clusters obtained in a previous work . We do not consider initial configurations with two separated O atoms. Thus, O2 dissociative adsorption is obtained only when that process occurs without any barrier. 

In these experiments hydroxyl was obtained by photochemical decomposition of H202. Analysis of reaction products was also made. Hydroxyl concentrations were too low to be measured by the spectroscopic method, but were sufficient for detecting the reaction products such as acetone, which was apparently formed by the reaction of the iso-C3H7 radical with the O2 molecule. 

Fig. 7.1. The low-lying electronic states of the O2 molecule. The lowest vibrational levels of each electronic state are also indicated after [180].

It is well known that, besides the electronic ground state,. Y3Zg, of the O2 molecule, there are two low lying excited electronic states labeled... 

Fig. 1. Potential energy as a function of internuclear distance for states of the O2 molecule. Curve (a) after Wilkinson and Mulliken, (105). Curve (b) after Vander-slice et al. (86). Curve (c) after Carroll (21). Portions of this figure are taken from ref. (105) with the permission of The Journal of Chemical Physics.

Oxygen is the most abundant element in the earth s crust. Dioxygen (O2) can be prepared in the laboratory by electrolysis of water, by catalytic decomposition of hydrogen peroxide, or by thermal decomposition of KCIO3. Oxygen is manufactured by fractional distillation of liquefied air and is used in making steel. The O2 molecule is paramagnetic and has an... 

---