Activation reactions

Kim S K, Guo J, Baskin J S and Zewail A H 1996 Femtosecond chemically activated reactions concept of nonstatistical activation at high thermal energies J. Phys. Chem. 100 9202-5... 

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

GVD Coatings. As in PVD, the stmcture of the deposited material depends on the temperature and supersaturation, roughly as pictured in Figure 8 (12). In the case of CVD, however, the effective supersaturation, ie, the local effective concentration in the gas phase of the materials to be deposited, relative to its equiUbrium concentration, depends not only on concentration, but on temperature. The reaction is thermally activated. Because the effective supersaturation for thermally activated reactions increases with temperature, the opposing tendencies can lead in some cases to a reversal of the sequence of crystalline forms Hsted in Figure 8, as temperature is increased (12). 

Because the integrity of the dihydrothiazine ring and its C-4 carboxyl substituent is crucial to useful antimicrobial activity, reactions involving this part of the cephalosporin molecule are usually undesirable. The possibilities for sulfur oxidation or alkylation, substitution at C-2 which is adjacent to both sulfur and a double bond, double bond isomerization and addition reactions, and the influence of a free carboxylic acid must all be considered in designing reactions to selectively modify other cephalosporin functionalities. 

The substitution of the lone proton on the benzhydryl carbon by a methyl group again affords compounds with antihistamine activity. Reaction of an appropriate acetophenone (21) with phenyl-magnesium bromide affords the desired tertiary alcohols (22). 

Substitution of an additional nitrogen atom onto the three-carbon side chain also serves to suppress tranquilizing activity at the expense of antispasmodic activity. Reaction of phenothia zine with epichlorohydrin by means of sodium hydride gives the epoxide 121. It should be noted that, even if initial attack in this reaction is on the epoxide, the alkoxide ion that would result from this nucleophilic addition can readily displace the adjacent chlorine to give the observed product. Opening of the oxirane with dimethylamine proceeds at the terminal position to afford the amino alcohol, 122. The amino alcohol is then converted to the halide (123). A displacement reaction with dimethylamine gives aminopromazine (124). ... 

Insertion of a longer spacer is compatible with antifungal activity. Reaction of epichlorohydrin with 4-chlorobenzyl-magnesium chloride leads to substituted phenylbutane Dis-... 

A substituted thiazole ring attached to a reduced imidazole moiety is present in a compound that displays anti hypertensive activity. Reaction of thiourea 61 with methyl iodide to... 

Because most thermoset composites cure by a thermally activated reaction, a complicated heat transfer process occurs during solidification, the result of an exothermic cross linking reaction in the resin. The complications of thermoset resin curing are compounded by the competing mechanisms of... 

However, recently, a theoretical paper has been published, which provides interesting arguments for a conventional silylmetal hydride rather than for a 3c2e M(H)Si bond [132,133]. For the great implications of these compounds for Si —H bond activation reactions consult, e.g., on the work of Crabtree [129]. 

It appeared to be a logical consequence to transfer this synthetic principle to more suitable metals like ruthenium and introduce bulky, kinetically stabilizing ligands at the metal. An interesting example for this approach is the complex 78. The latter is synthesized from Cp RuCl(PR3)2 with ClMgCH2SiMe2H through 77 by a thermal Si — H activation reaction. 

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

Thru 1967, emphasis was given to the use of neutrons as the bombarding source of radiation. Almost all possible neutron reactions were considered including moderation of fast neutrons by hydrogen in the expl, thermal capture reactions, elastic and inelastic scattering of neutrons and neutron activation reactions. These neutron reactions are listed as follows ... 

Neutron activation reactions have also been considered for mine detection. Here a radioactive element is produced in the mine which in the process of decay, emits nuclear radiation, either alpha or beta particles or yrays or two of these or all three in combination. For buried mines the penetrating 7iays are of most in-... 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

Interestingly, the final inactive by-products of these phosphorus-activated reactions, triphenylphosphine oxide or diphenylphosphate, can be converted back to starting TPPC12 or DPCP by reaction with COCl2 and SOCl2, respectively. 

In a plasma-activated reaction, the substrate temperature can be considerably lower than in thermal CVD (see Ch. 5, Sec. 9). This allows the coating of thermally sensitive materials. The... 

The value of is the difference in partial molal volume between the transition state and the initial state, but it can be approximated by the molar volume. Increasing pressure decreases the value of AV and if A V is negative the reaction rate is accelerated. This equation is not strictly obeyed above lOkbar. If the transition state of a reaction involves bond formation, concentration of charge, or ionization, a negative volume of activation often results. Cleavage of a bond, dispersal of charge, neutralization of the transition state and diffusion control lead to a positive volume of activation. Reactions for which rate enhancement is expected at high pressure include ... 

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