Long-lived active intermediates

It was shown (Ovanesyan et al., 2000) that iron complexes formed during the thermal treatment of FeZSM-5 zeolite perform single-turnover cycles of methane oxidation to methanol at ambient conditions when nitrous oxide is used as a source of oxygen. The long-living active intermediate is capable of transferring an accepted O atom into a C-H bond of methane to produce methanol at 100% selectivity. On the basis of joint Mossbauer and catalytic data, the structure and composition of iron active centers are suggested. 

Preparation of Novel Photosensitive Pofyimide Systems via Long-Lived Active Intermediates... 

Figure 5. Photoreaction of a diazo group in PI(DZDA/DEDPM) to generate a carbene, a long-lived active intermediate (LLAI), which insert into C-H bond.

Design of the reactions via long-lived active intermediate was found to be important for developing photosensitive polyimide tystems, and this concept is especially effective for the reactions in solid state 11), because the solid-state reactions are controlled by the molecular motions 12). In addition, change in the electronic state in polymer solid was found to affect the efficient of their photoreactions. Charge-transfer structure is one of the characteristic nature of aromatic polyimides, which is affected by the change in their physical properties, and which in turn controls their photoreactivities. 

The rate (if any) at vduch chain carriers are formed, their lifetime and the relative abundance of various active forms, from polarised ester molecules to free carbenium ions, depends mostly on how these four variables are aii usted. All types of limiting situations are possible very fast or extremely dow initiation reactions relativdy long-lived active s cies or very ephemeral ones and predominance of free ions or almost exclusive formation of ester molecules. Usually, however, initiation processes di lay a behaviour which is intermediate between extreme cases. A few examples will illustrate the effect of each variable. The basicity of the monomer can alter the course of initiation in a given system. 

In electron transfer between metal ions, a metal-ion catalyst normally reacts by nonassociative activation, in which the species do not form long-lived binuclear intermediates. The catalytic process often can be rationalized by reactivity patterns e.g., the Cu catalyses of the oxidation "of V(III) by Fe(III). This catalysis by Cu occurs by outer-sphere mechanisms as in ... 

However, the density of the medium affects the course of radiolysis in another well-known, and very important way. Long-living reactive intermediates such as free atoms and radicals remain localized for an appreciable period of time in the close neighborhood of the parent activations because of their limited mobility in condensed phase. Hitherto neglected space and time correlations between the individual elementary acts thus begin to influence considerably the subsequent processes. This fact has to be taken into account in the theory of primary processes. 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

The highly fractionated nature of the and Th series nuclides is illustrated by the measured activities in some representative waters in Figure 1. The highest activities are typically observed for Rn, reflecting the lack of reactivity of this noble gas. Groundwater Rn activities are controlled only by rapid in situ decay (Table 1) and supply from host rocks, without the complications of removal by adsorption or precipitation. The actinide U, which is soluble in oxidizing waters, is present in intermediate activities that are moderated by removal onto aquifer rocks. The long-lived... 

This equation is particularly useful to derive apparent estuarine water mass ages (Fig. 6) because the term /em is removed. Using ( Ra/ " Ra) isotope ratios in this manner is based on the assumption that the initial ( Ra/ " Ra) activity ratio must remain constant. This conclusion is reasonable as the long-lived parent isotopes ( Pa and Th) have relatively constant activity ratios in sediments, and the intermediate Th isotopes ( Th and Th) are scavenged efficiently in the near-shore water column. The utility of Ra as... 

Little has investigated monoactivated and doubly activated alkenes tethered to butenolide with respect to electroreductive cyclization [202]. The geminally activated systems 227 undergo cyclization to diastereomeric products 228 and 229 in an 1 1 mixture, whereas both the a,j8-unsaturated monoester and a,/ -unsaturated mononitrile fail to cyclize. Only saturation of the C-C double bond of butenolide is observed. The author explains these results by distinct reactivity and lifetime of the intermediate radical anions. The radical anions derived from the monoactivated olefins are less delocalized than those of 227 and therefore should be shorter lived and more reactive. In this case preferential saturation occurs. The radical anions derived from the doubly activated alkene 227 are comparatively long-lived and less basic and thus capable of attacking the C-C double bond of the butenolide moiety. A decrease in saturation, accompanied by a marked increase in cyclization, is observed. 

The overall TOF of this reaction was unexpectedly low and may be due to deactivation by long lived intermediates blocking reactive sites on the bimetallic particles. We are currently attempting to determine the cause of this loss of activity. 

The mechanism of the cleavage of the pyruvate in Eq. 15-37 is not obvious. Thiamin diphosphate is not involved, and free C02 is not formed. The first identified intermediate is an acetyl-enzyme containing a thioester linkage to a cysteine side chain. This is cleaved by reaction with CoA-SH to give the final product. A clue came when it was found by Knappe and coworkers that the active enzyme, which is rapidly inactivated by oxygen, contains a long-lived free radical.326 Under anaerobic conditions cells convert the inactive form E to the active form Ea by an enzymatic reaction with S-adenosylmethionine and reduced flavodoxin Fd(red) as shown in Eq. 15-38.327-329 A deactivase reverses the process.330... 

Low-level waste is waste with only low concentrations of intermediate-level, short-lived waste or intermediate-level, long-lived waste as defined by IAEA (1981). Low-level waste would be suitable for disposal in a municipal/industrial landfill that met current EPA standards and would include relatively low-activity Class-A waste, as defined in NRC s 10 CFR Part 61 (NRC, 1982a). 

Within the low-level and intermediate-level waste classes, a further distinction was made between short- and long-lived waste, as well as alpha-bearing waste (IAEA, 1981). Short-lived waste referred to waste that would decay to low activity levels during the time period of perhaps a few centuries when administrative control over the waste can be expected to last, and long-lived waste referred to waste that would not decay to low levels during an administrative control period. Alpha-bearing waste referred to waste that contains one or more alpha-emitting radionuclides in amounts above acceptable limits established by national authorities. 

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