Primary reactivity

Photochemical transformations of cyclic and short chain polysilane oligomers have been intensively investigated (39). Irradiation of these materials in the presence of trapping reagents, such as silanes or alcohols, has suggested that substituted silylenes and silyl radicals are primary reactive intermediates. The former have been... 

Fig. 3) show n orbitals to be the HOMO orbitals. Thus, the primary reactivity pattern should be the side-on reactivity. However, this reactivity is generally not allowed in the phosphido complexes with tris-amido ligand sets due to their kinetic stabilization by bulky substituents. Therefore, experimentally, only the less crowded and more flexible alkoxide complexes 18 and 21 show the anticipated side-on access to the triple bond. 

A fourth, often overlooked problem is most prominent in noble gas matrices which are notoriously poor heat sinks because only very low energy lattice phonons are available to accept molecular vibrational quanta. Hence, thermalization is very slow compared to solution, and the excess energy that may be imparted onto an incipient reactive intermediate in the process of its formation (e.g., from a precursor excited state) may therefore be dissipated in secondary chemical processes such as rearrangments or fragmentations, which may make it impossible to generate the primary reactive intermediate. Often, this problem can be alleviated by attaching alkyl groups that serve as internal heat sinks, but sometimes this is not acceptable for other reasons. 

Glutaraldehyde is a homobifunctional cross-linker containing an aldehyde residue at both ends of a 5 carbon chain. Its primary reactivity is toward amine groups, but the reaction may occur by more than one mechanism. As discussed in Chapter 4, Section... 

The primary reactive diluents are monoepoxy low-molecular-weight epoxy resins. These may be used at rather high concentration with little effect on cured properties. Often they are employed to make selective improvements on certain properties such as adhesion, thermal cycling resistance, and impact strength. Some lower-viscosity commercial epoxy resins are already reduced with these diluents. 

A varying and much more complex mechanistic situation exists in heterogeneous photocatalysis (Fig. 5-13). With respect to the transient oxygen species, comparable overall oxidation reactions are usually observed, but the set of primary reactive oxygen species is slightly different. It is commonly assumed, that superoxide radical anions and hydroxyl radicals are the primary species formed after photogeneration of the electron-hole pair of a semiconductor catalyst in the presence of water and air (Serpone, 1996). In the presence of ozone, ozonide radical anions or are formed by fast electron transfer reaction of superoxide radical anions with O3 molecules. The combination Ti02-03-UV/VIS is called photocatalytic ozonation (Kopf et al., 2000). For example, it was applied for the decomposition of tri-chloroethene in the gas phase (Shen and Kub, 2002). 

Well studied primary reactive species in radiation- or photo-initiated reactions of auxiliary oxidants in an aqueous phase are hydrated electrons (eaq), hydrogen atoms (H ) and hydroxyl radicals ( OH), the last being by far the most important ones in photo-initiated AOPs. The formation and reactivity of ejq and of H were described by Hart and Anbar (1970) and by Buxton et al. (1988). Hydrated electrons can be produced by VUV photolysis of water, by photolysis of aqueous solutions of [FelCNlq]" or of V with formation of [Fe(CN)5] and il2, respectively (cf. Buxton et al, 1988). 

There are two primary reactive modes for flame retardants either gas-phase or solid-state. Since a flame consists of a variety of gas-phase radicals and atoms (Eqs. 6 and 7), a gas-phase flame retardant is one that will scavenge the flame-propagating radicals such as OH and O. In particular, if the hydroxyl radical is suppressed, the exothermic formation of CO2 (Eq. 8) will be prevented thereby reducing the flame temperature. 

Fig. 7. fhe pfiotochianistry of reactive nitrogen oxide species. Double-lined arrows represent primary reactive pathways and single-lined arrows represent secondary reactive pathways. 

Most importantly, both in the thermal and the charge-transfer osmylation of anthracene, identical cycloadducts on the terminal ring are observed, which underscores the close relationship between the two reaction modes. Thus, a unifying electron-transfer mechanism is proposed for both thermal and photoactivated osmylation, which reveals the ion-radical pair [Ar+ , 0s04 ] as the common (primary) reactive intermediate [161]. 

TABLE 3. Yields of the primary reactive species in the radiolysis of cyclopropane (CP)... 

Dansette, P. M., Thang, D. C., el Amri, H., Mansuy, D. Evidence for thiophene-S-oxide as a primary reactive metabolite of thiophene in vivo formation of a dihydrothiophene sulfoxide mercapturic acid. Biochem. Biophys. Res. Common. 1992, 186, 1624-1630. 

The cleanness of this system with respect to (1) the generation of HO- radicals and (2) their primary reactivity with hydrocarbon substrates (KH, rather than solvent or ligand) has been demonstrated through the use of PhSeSePh as an efficient carbon radical trap. 

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