Intrinsic rate, substrate oxidants

H2 flow rate, and total time), but also on the Intrinsic nature of the oxide (a, y, Fe203, Fe304) etc., as well as on the nature of the substrate. The Mossbauer spectra for the Fe/carbon systems in general showed a six-line spectra with a central doublet arising from the Fe (reduced) clusters and a S.P. component (Fig. 2A). 

High etch rates and selectivity can be achieved by judicious selection of feed gases to a plasma reactor. The atomic and radical species formed by electron impact dissociation depend largely on feed gas composition, and the intrinsic etch rates measured in the absence of a plasma (i.e., downstream etching) provide a useful indicator of chemical selectivity in the presence of a plasma. For example, the ratio of (100) silicon (34) to thermal oxide (Si02) (37) etching by F atoms is 41 1 at room temperature. As etch rates generally follow an Arrhenius type dependence on substrate temperature. 

The rates of asymmetric sulfoxidation of thioanisole in nearly anhydrous (99.7%) isopropyl alcohol and methanol catalyzed by horseradish peroxidase (HRP) were determined to be tens to hundreds of times faster than in water under otherwise identical conditions (Dai, 2000). Similar effects were observed with other hemo-proteins. This dramatic activation is due to a much higher substrate solubility in organic solvents than in water and occurs even though the intrinsic reactivity of HRP in isopropyl alcohol and in methanol is hundreds of times lower than in water. In addition, the rates of spontaneous oxidation of the model prochiral substrate thioanisole in several organic solvents was observed to be some 100- to 1000-fold slower than in water. This renders peroxidase-catalyzed asymmetric sulf-oxidations synthetically attractive. 

The electrochemistry and surface chemistry of such UPD species has been the subject of several previous reviews [6, 7, 99, 100) and many original papers Ref 99 reviews, in thorough detail, electrocatalysis induced or modified by UPD metal adatoms which really change the intrinsic catalytic nature of the substrate metal surfaces. It is surprising, however, that very little work has been done until recently (cf Refs. 75, 101-106) on the adsorbed species that are the kinetically involved intermediates in overall Faradaic reactions proceeding continuously at appreciable net rates (or equivalent current densities), for example, in the reactions of H2, O2, and CI2 evolution and other processes such as O2 reduction (more work, relatively, has been done on that reaction) or H2 oxidation proceeding at appreciable overpotentials. Such intermediates are conveniently referred to as OPD species. 

The situation Is different for a substrate module however, which will employ a weatherable plastic-film front cover. Because all plastic films are permeable to oxygen and water vapor (the only difference Is permeation rate), the pottant is exposed to oxygen and water vapor, and also to UV If the plastic film is non-UV screening. Because Isolation of the pottant from oxygen and water vapor is not practically possible in this design option. It becomes a requirement that the pottant be intrinsically resistant to hydrolysis and thermal oxidation, but sensitivity to UV is... 

Incubation of SNAC 32 yielded no observable acylation of KSl, suggesting that the amide substrate should be derived from an a-amino acid. Equally, no acylation was detected with SNAC 33 indicating that the distal carbonyl is not a factor in substrate viability. Previous work by Claderone et al. revealed that the reduction of the 2-keto-4-methylpentanoyl intermediate to the 2-hydroxy-analogue is performed by the KR domain of module 3 (Scheme 4.1) [6]. This unorthodox reduction step is believed to occur post-KSl elongation, therefore suggesting that the oxidation level of SNAC 27 is correct, and therefore a substrate mimic for KSl. To probe this issue further, a 2-hydroxy-4-methyl-pentanoyl SNAC (34) was synthesised. Upon incubation of SNAC 34 with KSl, a slight reduction in acylation rate was observed ( 20 %) compared to SNAC 27. The reduction in acylation rate may be due to unfavourable loss of planarity in the substrate, or decreased intrinsic reactivity of the thioester (Fig. 4.4). 

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