Carbon hydride adsorbates

Experimental evidence for the influence of adsorbates on surface structures during growth have been reported for GaAs(OOl) by Creighton and Baucom [133, 149). After TMGa deposition under UHV conditions, a CHj-stabilized Ga-rich (1 x 2) reconstruction and a CH2-stabilized Ga-rich (1 x 4) reconstruction were found. Creighton et al. also reported a CHj-stabilized As-rich (1 x 2) reconstruction. Comparative RAS measurements carried out under UHV and MOVPE growth conditions showed that the CHj-stabilized (1 x 2) reconstruction also appears in the latter case [149]. 

These microscopic surface modifications, however, have strong influence on the macroscopic surface morphology. While under gaUium-rich surface conditions the 

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Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

A complete separation of a carbonium ion from the hydride ion is very probably not necessary. It has been shown [73] by MO calculations that any attack by a charged species on an atom bonded to a carbon atom causes activation of the bonds from a /3-carbon atom to the substituents. In this way, the splitting of the Cp—Cy bond can be induced by adsorption of the alkane on a strongly acidic site. The preferential cracking of a saturated hydrocarbon chain in /3-positions to the position where a carbonium ion might be formed was observed early and named the /3-rule by Thomas [2], The question remains open as to which type of acidic centre is able to activate an alkane molecule. The fact that an aluminosilicate catalyst is poisoned for the cracking of alkanes by irreversibly adsorbed ammonia suggests a Lewis site [240], viz. 

Reductive cleavage of epoxides,1 Epoxides are usually reduced by hydride reagents, particularly LiAlH4, by attack at the less-substituted carbon to provide the more-substituted alcohol. However, reductions with Zn(BH4)2 adsorbed on Si02 in THF at 25° can show the opposite regioselectivity. Thus the epoxide of a 1-alkene is... 

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