Carbon-substrate interactions

This example shows that a nonoxidic support can give rise to interesting properties in active particles. The catalytic performance is not simply correlated to size distributions. The experiments presented in this section reveal how few indisputable facts are yet known concerning the metal-support interaction for carbon substrates. Interactions of the type (metal d-states)-(carbon sp )-(carbon sp2) mediated via amorphous accommodation particles (Fig. 32) of intermediate layers should be considered in the prevailing picture of a yet unproved epitaxy between transition metals and graphite (001) surfaces. 

The main reason for the existence of the incubation period frequently observed in diamond nucleation is the chemical interactions of the gas phase widi the substrate surface. Lux and Haubnerl l classified substrate materials into three major groups in terms of carbon-substrate interactions, as listed in Table 1. 

Urchaga P, Weissmann M, Baranton S, Girardeau T, Coutanceau C (2009) Improvement of the platinum nanoparticles-carbon substrate interaction by insertion of a thiophenol molecular bridge. Langmuir 25 6543-6550... 

Graphite was tised as substrate for the deposition of carbon vapor. Prior to the tube and cone studies, this substrate was studied by us carefully by STM because it may exhibit anomalotis behavior w ith unusual periodic surface structures[9,10]. In particular, the cluster-substrate interaction w as investigated IJ. At low submonolayer coverages, small clusters and islands are observed. These tend to have linear struc-tures[12j. Much higher coverages are required for the synthesis of nanotubes and nanocones. In addition, the carbon vapor has to be very hot, typically >3000°C. We note that the production of nanotubes by arc discharge occurs also at an intense heat (of the plasma in the arc) of >3000°C. 

A wide range of carbonaceous materials can be modified with a stable DNA adsorbed layer. The multi-site attachment of DNA on carbon surfaces seems to be strongly dependent on hydrophobic interactions between DNA bases and carbon substrates such as GC and GC(ox)> HOPG, CNTs and GECs. Al-... 

With the support of quantum mechanics this proteolysis study has readily shown that fluorinated amino acid side chains are able to direct enzyme substrate interactions, which can have an influence on proteolytic stability. Depending on the absolute stereochemistry and on the position within the sequence, aTfm amino acids can considerably stabilize peptides against proteolysis. The unique electrostatic properties of carbon-bound fluorine, however, may also induce a contrary effect. The conformational restrictions of C -dialkylation seem to be partly dimin-ishable by the electrostatic consequences of fluorination. With this knowledge. 

There is an ongoing controversy about whether there is any stabilization of the transition state for nucleophilic substitution at tertiary aliphatic carbon from interaction with nucleophilic solvent." ° This controversy has developed with the increasing sophistication of experiments to characterize solvent effects on the rate constants for solvolysis reactions. Grunwald and Winstein determined rate constants for solvolysis of tert-butyl chloride in a wide variety of solvents and used these data to define the solvent ionizing parameter T (Eq. 3). They next found that rate constants for solvolysis of primary and secondary aliphatic carbon show a smaller sensitivity (m) to changes in Y than those for the parent solvolysis reaction of tert-butyl chloride (for which m = 1 by definition). A second term was added ( N) to account for the effect of changes in solvent nucleophilicity on obsd that result from transition state stabilization by a nucleophilic interaction between solvent and substrate. It was first assumed that there is no significant stabilization of the transition state for solvolysis of tert-butyl chloride from such a nucleophilic interaction. However, a close examination of extensive rate data revealed, in some cases, a correlation between rate constants for solvolysis of fert-butyl derivatives and solvent nucleophicity. " ... 

Note how the key interaction boosting the catalytic effect is the protonation of the carbonyl group on the TG. Such catalyst-substrate interaction increases the electrophilicity of the adjacent carbonyl carbon atom, making it more susceptible to nucleophilic attack. Compare this to the base-catalyzed mechanism where the base catalyst takes on a more direct route to activate the reaction, creating first an alkoxide ion that directly acts as a strong nucleophile (Figure 4). Ultimately, it is this crucial difference, i.e., the formation of a more electrophilic species (acid catalysis) v.s. that of a stronger nucleophile (base catalysis), that is responsible for the differences in catalytic activity. 

This reaction rearranges the carbonyl and hydroxyl groups on carbons 1 and 2. However, more than 80% of the enzymatic rate acceleration has been traced to enzyme-substrate interactions involving the phosphate group on carbon 3 of the substrate. This was determined by a careful comparison of the enzyme-catalyzed reactions with glyceraldehyde 3-phosphate and with glyceraldehyde (no phosphate group at position 3) as substrate. 

The importance of the interaction of organic compounds with calcium carbonate surfaces has long been recognized. It has been demonstrated that aragonite precipitation is inhibited by uncharacterized dissolved organic matter (Chave and Suess, 1967), and that humic and fulvic acids, and certain aromatic carboxylic acids, inhibit seeded aragonite precipitation from seawater (Berner et al., 1978). The selective adsorption of amino acids on carbonate substrates has received considerable attention. A preferential adsorption of aspartic acid has been shown from humic and fulvic acids and proteinaceous matter (Carter and Mitterer, 1978 Carter, 1978 Mitterer, 1971). 

The predominance of van der Waals interactions at solid metal/A1203 interfaces is also shown by the fact that whatever the orientation of monocrystalline AI2O3 surface, Cu particles are orientated with (111) faces parallel to the A1203 surface (Soper et al. 1996). This orientation maximises the number of metal atoms per unit area in nearest-neighbour interactions with A1203. A similar behaviour was found for non-reactive fee metals on carbon substrates i.e., for systems with predominant van der Waals interfacial interactions (Section 8.1). 

Ragsdale, S. W., Ljungdahl, L. G., and DerVartanian, D. V., 1982, EPR evidence for nickel substrate interaction in carbon monoxide dehydrogenase from Clostridium thermoaceticum, Biochem. Biophys. Res. Comrmin. 108 6589663. 

An electron transfer mechanism has been proposed to account for the formation of carbon dioxide on irradiation of alkyl pyruvates the yield of carbon dioxide is enhanced by the presence of electron acceptors such as methyl viologen. The dye-sensitized photo-oxidation of a-oxo-carboxylic acids and esters also leads to the production of carbon dioxide. An initial dye-substrate interaction rather than singlet oxygen appears to be responsible for this fragmentation. 

BINAPHOS L3 and its derivative L4, which has not been tested experimentally so far, possess two and one chiral axes, respectively, whereas JOSIPHOS L5 and its constitutional isomer L6 combine planar chirality with a carbon stereocenter. In the case of L3, the relative configuration of the two chiral axes decides the total stereoselectivity. Synergistic/antagonistic backbone-substrate interactions are the main reason for the good/bad performance of L3-(/f,S)/L3-(/f,R) and their enantiomers L3-(S,R)/L3-(S,5). This is reminiscent of the matched/ mismatched concept [38]. Furthermore, one chiral axis, as in L4, seems to be insufficient for a high ee. 

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