Activation energies, toxicant

The COMPACT (computer-optimized molecular parametric analysis of chemical toxicity) procedure, developed by Lewis and co-workers [92], uses a form of discriminant analysis based on two descriptors, namely, molecular planarity and electronic activation energy (the difference between the energies of the highest occupied and lowest unoccupied molecular orbitals), which predict the potential of a compound to act as a substrate for one of the cytochromes P450. Lewis et al. [93] found 64% correct predictions for 100 compounds tested by the NTP for mutagenicity. 

They also studied decomposition rates at several different temperatures. These data were fitted into the Arrhenius equation with an excellent correlation (half-life in minutes was plotted against 1000/T in °K.). The slope of the lines is E/2.303R, where E is the activation energy. Stabilizers were shown to raise this activation energy. This is not surprising in that a stabilizer actually physically separates the toxicant from the catalytic site. 

Many studies on the direct reaction of methyl chloride with silicon-copper contact mass and other metal promoters added to the silicon-copper contact mass have focused on the reaction mechanisms.7,8 The reaction rate and the selectivity for dimethyldichlorosilane in this direct synthesis are influenced by metal additives, known as promoters, in low concentration. Aluminum, antimony, arsenic, bismuth, mercury, phosphorus, phosphine compounds34 and their metal complexes,35,36 Zinc,37 39 tin38-40 etc. are known to have beneficial effects as promoters for dimethyldichlorosilane formation.7,8 Promoters are not themselves good catalysts for the direct reaction at temperatures < 350 °C,6,8 but require the presence of copper to be effective. When zinc metal or zinc compounds (0.03-0.75 wt%) were added to silicon-copper contact mass, the reaction rate was potentiated and the selectivity of dimethyldichlorosilane was enhanced further.34 These materials are described as structural promoters because they alter the surface enrichment of silicon, increase the electron density of the surface of the catalyst modify the crystal structure of the copper-silicon solid phase, and affect the absorption of methyl chloride on the catalyst surface and the activation energy for the formation of dimethyldichlorosilane.38,39 Cadmium is also a structural promoter for this reaction, but cadmium presents serious toxicity problems in industrial use on a large scale.41,42 Other metals such as arsenic, mercury, etc. are also restricted because of such toxicity problems. In the direct reaction of methyl chloride, tin in... 

The a/AP parameter, where AE is the difference in HOMO and LUMO energies, is an orbital energy-weighted polarizability term. The last equation implies that the acute toxicity of nitriles is a function of molecular size/polarity and electronic activation energy. As the dipole moment of the compounds did not correlate with the activity for this series, it was suggested that the enzyme-substrate interaction might be of secondary importance. 

A number of alternative precursor compounds have been investigated. The low cost and ready availability of aluminum 2-ethylhexanoate (7) has prompted its use as a less toxic precursor [107], Of particular note is that the activation energy for deposition using Al(2-ethylhexanoate)3, (28.1 kimol" ) is nearly identical to that of Al(acac)j under similar conditions. However, carbon contamination is lower, and the deposition may be performed in air. 

The incentive to utilize for the first time tt-tt complexation for selectively removing overdosed and toxic lipophilic aromatic compounds from blood originated from the work of Dust on binding dopamine derivatives to trinitrobenzene [47]. In that and other subsequent publications [42,48-50], spectroscopic methods are described for quantitative determination of complexation as well as how to calculate binding constants and activation energies. 

Similar studies by Fukuto et al. have correlated chemical reactivity, anti-ChE activity, and toxicity to insects of a series of oximes of substituted acetophenones and benzalde-hydes with the free-energy parameters, F, ° R, ir, and Excellent correlation was obtained between these... 

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