Chemical bond cleavage

The ionization and excitation may lead to chemical bond cleavage and production of highly reactive species, free radicals, ions and molecular fragments, which subsequently interact with each other and at last stable degradation products are created. This complex sequence of processes can deliberately be divided into two basic phases, the initial physical phase, in which the ion energy is dissipated to electrons and atoms, and the chemical one comprising interaction of the reaetive species and production of the final stable products. 

All three membranes are responsive to attack by halogens. Chemical interaction evidently proceeds by more than one reaction mechanism. A possible explanation involves halogen addition as evidenced by membrane tightening. A second process may result in chemical bond cleavage which ultimately causes membrane failure. Halogen attack on membrane U-1 is probably dominated by bond cleavage which is enhanced as pH decreases. 

Singlet excited molecules are usually relatively short-lived and, therefore, are not very likely to undergo bimolecular reactions. In many cases, however, chemical bond cleavage competes with physical monomolecular deactivation paths. For example, singlet excited carbonyl groups contained in a polyethylene chain can undergo the Norrish type I reaction, resulting in a free radical couple [see Eq. (1-17)]. 

A site-selective C-C bond formation is important for syntheses of valuable chemicals. Bond cleavage reactions, such as ester hydrolysis and amide hydrolysis, using cyclodextrin (CyD) have been extensively studied. However, there has been only a few reports on C-C bond formation using CyD catalyst [1]. 

Chemical bond cleavage in semi-rigid PUR foams can be caused by oxidative processes or by hydrolysis and re-cleavage of urethane and/or urea links and their resulting products [702]. 

Isocyanide has been widely utilized as a key reagent in organic synthesis. Besides, the insertion of isocyanides into M-C bonds is one of the powerful means for carbon chain construction [16-34]. Insertion of isocyanide into M-C bonds afforded / -iminoacyl-metal intermediates, such as / -iminoacyl-Zr complexes [19-27], which can be conveniently converted to one-carbon elongated products such as imines, aldehydes, or nitriles via various chemical bond cleavage including Zr-C, C=N, and N-R bonds (Scheme 2.8) [29-32]. / -Iminoacyl-Zr complexes... 

Under certain circumstances the properties of condensed systems can be understood in terms of characteristic points of the PES [37,38]. Transport in or on solids, chemical bond cleavage and formation, and reorganization processes in solid... 

Modem photochemistry (IR, UV or VIS) is induced by coherent or incoherent radiative excitation processes [4, 5, 6 and 7]. The first step within a photochemical process is of course a preparation step within our conceptual framework, in which time-dependent states are generated that possibly show IVR. In an ideal scenario, energy from a laser would be deposited in a spatially localized, large amplitude vibrational motion of the reacting molecular system, which would then possibly lead to the cleavage of selected chemical bonds. This is basically the central idea behind the concepts for a mode selective chemistry , introduced in the late 1970s [127], and has continuously received much attention [10, 117. 122. 128. 129. 130. 131. 132. 133. 134... 

The value of alkyl halides as starting materials for the preparation of a variety of organic functional groups has been stressed many times In our earlier discussions we noted that aryl halides are normally much less reactive than alkyl halides m reactions that involve carbon-halogen bond cleavage In the present chapter you will see that aryl halides can exhibit their own patterns of chemical reactivity and that these reac tions are novel useful and mechanistically interesting... 

One potential approach extends the idea of chemical amplification introduced in our preceding description of dry-film resists. In 1982, Ito and co-workers (37,38) recognized that if a photosensitizer producing an acidic product is photolyzed in a polymer matrix containing acid-labile groups, the acid will serve as a spatially localized catalyst for the formation or cleavage of chemical bonds. 

Hydrogenolysis is analogous to hydrolysis and ammonolysis, which involve the cleavage of a bond induced by the action of water and ammonia, respectively. Chemical bonds that are broken by hydrogenolysis reactions include carbon—carbon, carbon—oxygen, carbon—sulfur, and carbon—nitrogen bonds. An example of hydrogenolysis is the hydrodealkylation of toluene to form benzene and methane ... 

Chemically Controlled. These systems are classified together because of the hydrolysis or enzymatic cleavage of a chemical bond that allows dehvery of the dmg. There are two main types of systems, ie, pendent chain systems and bioerodible systems. 

In the pendent chain systems, the dmg is chemically bound to a polymer backbone and is released by hydrolytic or enzymatic cleavage of the chemical bond. The dmg may be attached directiy to the polymer or may be linked via a spacer group. The spacer group may be used to affect the rate of dmg release and the hydrophilicity of the system. These systems allow very high dmg loadings (over 80 wt %) (89) which decrease the cost of the polymeric materials used ia the systems. These systems have beea examiaed by many iavestigators (111,112). 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

The basic function of lysis processes is to split molecules to permit further treatment. Hydrolysis is a chemical reaction in which water reacts with another substance. In the reaction, the water molecule is ionized while the other compound is split into ionic groups. Photolysis, another lysis process, breaks chemical bonds by irradiating a chemical with ultraviolet light. Catalysis uses a catalyst to achieve bond cleavage. 

Technology Description Hydrolysis is the process of breaking a bond in a molecule (which is ordinarily not water-soluble) so that it will go into ionic solution with water. Hydrolysis can be achieved by the addition of chemicals (e.g., acid hydrolysis), by irradiation (e.g., photolysis) or by biological action (e.g., enzymatic bond cleavage). The cloven molecule can then be further treated by other means to reduce toxicity. 

Ceric ions react rapidly with 1,2-diols. There is evidence for chelation of cerium and these complexes are likely intermediates in radical generation10 106 The overall chemistry may be understood in terms of an intermediate alkoxy radical which undergoes p-scission to give a carbonyl compound and a hydroxyalkyl radical (Scheme 3.59). However, it is also possible that there is concerted electron transfer and bond-cleavage. There is little direct data on the chemical nature of the radical in termediates. 

Chemical reaction rate, see Rate of reaction Chemical reactions condensed phases, 42-46 enzymatic, see Enzymatic reactions gas phase, see Gas-phase reactions heterolytic bond cleavage, 46, 47, 51,... 

Excited states can also decay by means of chemical reaction via heterolytic bond cleavage, leading to ions, or by homolytic bond cleavage generating free radicals. 

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