Substitution different sites

Acylation of 1,4-dimethoxynaphthalene with acetic anhydride (1.2 equiv) and aluminum chloride (2.2 equiv) in ethylene dichloride (60°C, 3 h) gives two products, 6-acetyl-l,4-dimethoxynaphthalene (30%) and l-hydroxy-2-acetyl-4-methoxynaphtha-lene (50%). Suggest a rationalization for the formation of these two products and, in particular, for the differing site of substitution in the two products. 

The following scheme gives examples of typical compounds and of the distribution (substitution) of the atoms in the different sites ... 

The term has also been applied to Hammett equations (See Selectivity Factor) and for other substituted reactants undergoing parallel reactions at different sites with the same rate law. 

In the absence of dynamic and static disorder, all partially filled band systems would exhibit coherent transport over long distances. With static and dynamic disorder, the modulation of the simple molecular orbital or band structure by nuclear effects entirely dominates transport. This is clear both in the Kubo linear response formulation of conductivity and in the Marcus-Hush-Jortner formulation of ET rates. The DNA systems are remarkable for the different kinds of disorder they exhibit in addition to the ordinary static and dynamic disorder expected in any soft material, DNA has the covalent disorder arising from the choice of A, T, G, or C at each substitution base site along the backbone. Additionally, DNA has the characteristic orientational and metric (helicoidal) disorder parameters arising from the fundamental motif of electron motion along the r-stack. 

The importance of maintaining the active site water network in CA II for efficient proton transfer was investigated by substituting different amino acids of varying size at position 65 and measuring the rate constants for proton transfer in the variant carbonic anhydrases... 

Hetero site reactivity The simplest difference between a cluster and a mononuclear complex is that the cluster can do two or more things where a mononuclear complex can do one. Simple as it is, this difference has hardly ever been verified other than in multiple ligand substitutions. One verification is hetero site reactivity, i.e., different modes of reaction at different sites on one and the same cluster. Two examples of this appear to exist. Different phosphine ligands substitute CO on different metal atoms in H2FeRu3(CO),3 depending on their size and basicity (210), and Ru2Co2 (CO), 3 reacts with H2 at the ruthenium atoms (cf. Section IV, A) and... 

In its recommendations, IAEA emphasizes that waste classification, even if it focuses on waste disposal, does not provide an adequate substitute for site-specific safety assessments of particular disposal systems to ensure the acceptability of waste disposal. IAEA also recognizes the role of national authorities in implementing waste classification systems and ensuring the safety of waste disposal, and that different countries may choose to classify waste in different ways depending on their particular situations. However, IAEA believes that, if for no other reason than to facilitate communication, it would be desirable to achieve some level of uniformity of waste classification systems in different countries. IAEA recommends that it is particularly important to obtain an international consensus on the boundary for determining unconditionally exempt material that may be transferred from one country to another, especially for purposes of recycle/reuse. 

Transition-state selectivity is sometimes difficult to distinguish from product shape selectivity. A recent study by Kim et al. (8) shows that the high para-selectivity for the alkylation of ethylbenzene with ethanol in metallosilicates (MeZSM-5) is not due to product selectivity alone. They conclude that the primary product of the alkylation on ZSM-5 type metallosilicates is p-diethylbenzene which isomerizes further inside the cavity of ZSM-5 to other isomers. As the acid sites of zeolites becomes weaker (achieved by substituting different metals into the framework of the zeolite), the isomerization of the primarily produced p-isomer is suppressed. Although Kim et al. attribute this suppression of the isomerization activity to restricted transition-state selectivity, it is more likely that this suppression is due to the decrease in acid strength. 

The structural chemistry in these compounds is very rich, with a wide range of substitutions into the different sites occurring, particularly for the plane and chain copper sites in YBa2(Cui. vM.v)307.52-55 Some elements (e.g. Fe, Co, A1... 

Suzuki and co-workers <68CPB750> have correlated chemical shift values of protons attached to the carbon at different sites in thiazolo[5,4-J]pyrimidine with the ease of substitution of halogen in 2,5,7-trichlorothiazolo[5,4-J]pyrimidine. The order of reactivity of chlorine was thus concluded to be C-2 > C-5 > C-7 which is the order of chemical shift protons at C-2 resonate at lower field than those at C-5, and C-7 protons resonate at highest field. It seems, however, that the reactivity at C-5 and C-2 is very similar as compound (175) reacts with hydrazine hydrate to yield a mixture of the isomers (176) and (177) (Equation (15)) <79IJC307>. 

Third, the relative intensities of the Si(nAl) signals in realuminated samples are strikingly different from those in the as-prepared zeolites with the same framework composition, which means that the distribution of Si and A1 in the treated zeolites is different. This is a consequence of the different site selectivities discussed above, but also of the fact that both the original Si(OAl) sites and the Si(OAl) sites created during ultrastabilization are available for A1 substitution. 

However, recently it has proved possible to positively identify tryptophan radicals in cytochromec peroxidase[147] and tyrosine radicals in ribonucleotide reductase, prostaglandin H synthase and photosystem II of chloroplasts [148], This has been achieved by a combination of the techniques discussed already, but with the powerful, additional non-invasive tool of isotopic substitution. As deuterons (5=1) give different splitting than protons (S = 1/2), substituting different labelled amino-acid residues into the enzyme should reveal the nature of the radical-containing residue. This is easily achieved in an auxotrophic mutant that requires this amino acid to be supplied in the medium. The specific residue can then be identified by site-directed mutagenesis of the evolutionary conserved amino-acid residues [108,149-151]. 

With alloys and substitutional solid solutions, it is possible that a mixture of atoms (of similar size, valence, etc.) may reside at a general or special position and all its equivalent coordinates. The fraction of atoms of one type residing at that position is given by the site occupancy, or site occupation factor. The sum of the site occupation factors for that site must equal unity. The distribution of two or more types of atoms over a single site is completely random. Where two atoms are distributed over all the equivalent coordinates of different sites with similar local coordination environments (but not identical site symmetry), electronic, or other, effects can result in partial site preferences. That is, there can be a nonstatistical distribution over the two sites. 

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