O2 chemistry

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

Tolman and co-workers brought about another breakthrough in the CU2/O2 chemistry in... 

Carrion H, Rogosky M, Nembhard HB, Joshi S. Characterization of the SilSpin etch-hack (hreakthrough) process for nanohthography with CHF3 and O2 chemistry. Int J Nanomanufact 2008. To appear. 

Tetramethyldioxetane is the prototype for all chemiluminescent processes. It will generally be the case that a dioxetane or similar structure will be formed. Thermal decomposition of this high energy structure then produces an excited state product. Details vary, but many of the basics of Figure 16.19 will be involved. Thus, species containing a strained 0-0 bond play a special role in chemiluminescent mechanisms. For that reason, we now discuss some aspects of O2 chemistry that are relevant to the formation of dioxetanes and related species. 

Popp and Stahl extended their studies of O2 chemistry at [PdL2] systems (described in Section 4.3 and also discussed in Chapter 13) to consider the... 

Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

The BWR water chemistry parameters are given in Table 4 (19). Originally, no additives were made to feedwater—condensate or the primary water. The radiolytic decomposition of the fluid produced varying concentrations of O2 in the reactor vessel, ranging from about 200 ppb O2 in the reactor recirculation water to about 20 ppm O2 in the steam. Stoichiometric amounts of hydrogen were also produced, ie, 2 mL for each mL of O2. Feedwater O2 was about 30 ppb, hence the radiolytic decomposition of the water was a primary factor in determining the behavior of materials in the primary system and feedwater systems. 

Compute the enthalpy change for the destruction of ozone by atomic chlorine by subtracting the dissociation energies of O2 and CIO from the dissociation energy for ozone. What model chemistry is required for accurate modeling of each phase of this process The experimental values are given below (in kcal-moT ) ... 

You ll need to run five calculations at each model chemistry oxygen atom, chlorine atom, O2, CIO and ozone (but don t forget that you can obtain lower-level energies from a higher-level calculation). Use the experimental geometries for the various molecules and the following scaled zero-point energy corrections ... 

The top table gives the predicted total energies for each molecule, and the bottom table lists the computed dissociation energies and AH. All three model chemistries do pretty well on all three phases of the process, with G2 and CBS-Q generally modeling it very accurately. The CBS-Q values are the most accurate, and they take only about half as long to compute as G2 theory. CBS-4 performs well for O2 and for the overall AH at substantially less cost about one sixth the cost of G2 and one third the cost of CBS-Q. ... 

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