Base effect

Kurnikov I V and Beratan D N 1996 Ab initio based effective Hamiltonians for long-range electron transfer Hartree-Fock analysis J. Chem. Phys. 105 9561-73... 

The augmentation of the work of adhesion by acid-base effects has, as a consequence, an effect on wettability properties. Identifying /sl in Eq. 26 as W and substituting it into Eq. 3 and Eq. 14, respectively leads to ... 

The literature also contains other less quantitative but direct indications of the importance acid-base effects in contact adhesion [35,78,103]. 

Methylation of nitrogen at the 2 position also proves to be consistent with diuretic activity. Condensation of 160 with urea affords the heterocycle, 193. Treatment of this compound with methyl iodide and base effects alkylation on the more acidic ring nitrogen (194). Basic hydrolysis then gives the N-methylated aminosulfonamide (195). Condensation of this with chloroacetalde-... 

Acid-base effect—coupling agents alter acidity of substrate surface. 

A sound development/design manufacturing base Effectiveness... 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

H202 in TS-1. On the basis of the observed solvent and acid/base effects on the kinetics and yield in alkene epoxidation in various alcohols, an end-on (7) group with a simultaneously coordinated alcohol group was envisioned as the reactive intermediate. 

OCOH)(PH3)2 and a dihydrogen molecule [44], The introduction of a NH3 molecule in the system converts this single-step process in a three-step process, with three transition states. The energy change occurs smoothly and the highest barrier is only 2.1 kcal/mol (MP2//MP2) which is much smaller than the barrier in the absence of NH3 (11.4 kcal/mol, Table 4). This means that the four-center metathesis occurs with nearly no barrier in the presence of base. However, the base effects are likely overestimated in this calculations. The experimental triethylamine system cannot do all the interactions ammonia does, and polar or protic solvents should weaken the interaction between the base and the complex. 

In the case of protonated pyrroles, the p Ta value lies in the range between 4 and —4, whereas the pATa value of acetonitrile is about —10. Therefore, the oligomerization of pyrrole in pure acetonitrile may already stop at the level of a-intermediates of hi- or more likely of tetrapyrrole. Acetonitrile is a weaker base than the a-intermediates. Consequently, a stronger base must be used to initiate the elimination of protons. Water fulfills this condition. Pyrrole can be polymerized in acetonitrile in the presence of 1% water [6, 37]. A similar effect results from the application of a sterically hindered base such as 2,6-di-tert-butylpyridine [38]. However, the concentration should be kept low because, at high concentrations proton, abstraction from the monomeric radical cation may occur, thus forming a neutral radical [28d]. The base effect can be also observed in the case of thiophenes. 

It should be recognized that already Togni and co-workers had observed a similar base effect in the context of the intramolecular hydroamination of 2-... 

Zhang (1993) proposed the modified effective binary approach (also called activity-based effective binary approach). In this approach, the diffusive flux of a component is related to its activity gradient and all other components are treated as one combined component. The diffusive flux for any component i is expressed as (by analogy with Equation 3-61)... 

To quantify the diffusion profiles is a difficult multicomponent problem. The activity-based effective binary diffusion approach (i.e. modified effective binary approach) has been adopted to roughly treat the problem. In this approach. 

Figure 3-24 Calculated diffusion-couple profiles for trace element diffusion and isotopic diffusion in the presence of major element concentration gradients using the approximate approach of activity-based effective binary treatment. The vertical dot-dashed line indicates the interface. The solid curve is the Nd trace element diffusion profile (concentration indicated on the left-hand y-axis), which is nonmonotonic with a pair of maximum and minimum, indicating uphill diffusion. The dashed curve is the Nd isotopic fraction profile. Note that the midisotopic fraction is not at the interface.

For inter diffusion between same-valence ions (ionic exchange) in an aqueous solution, or a melt, or a solid solution such as olivine (Fe +, Mg +)2Si04, an equation similar to Equation 3-135c has been derived from the Nemst-Planck equations first by Helfferich and Plesset (1958) and then with refinement by Barter et al. (1963) with the assumption that (i) the matrix (or solvent) concentration does not vary and (ii) cross-coefficient Lab (phenomenological coefficient in Equation 3-96a) is negligible, which is similar to the activity-based effective binary diffusion treatment. The equation takes the following form ... 

Charge Transfer Between Native DNA Bases. Effects of Water Surroundings... 

Lewis bases effect dramatic changes in microstructure, initiation rates, propagation rates, and monomer reactivity ratios for alkyllithium—initiated polymerizations of vinyl monomers (1-6). Some insight into the molecular basis for these observations has been provided by a variety of NMR, colligative property, and light-scattering measurements of simple and polymeric organolithium compounds in hydrocarbon and basic solvents... 

The enzymatic activity versus pH is thus generally a bell-shaped curve as in Fig. 6. An autocatalytic effect may appear when the reaction products have an acid-base effect (often the case). A simple example is the glucose oxidase reaction, shown in Fig. 7. Notably, the rate versus product (H+) curve indicates an autocatalytic effect on the alkaline branch, that is, for pH>pH (see Fig. 6). Systems presenting analogous properties have been studied by R. Caplan et al.27 and we also learn more about them from D. Thomas in this volume. 

The influence of deuterium substitution on the acid base effects in hydrogen bonds has been considered [3], The treatment given here has been designed to enable a more quantitative consideration of these effects to be made and to interpret the known contraction phenomena. 

Acid-base effects—coupling agents were altering the acidity of a glass surface. The amine group of the aminopropyl silane probably bonded to glass. 

Although acid-base effects can be important in adhesion our studies indicate that bonding to metals through silane coupling agents is not by an acid-base mechanism, but probably through Si-O-M oxane bonds. As with glass, the hydrolysis and formation of oxane bonds are true equilibria, but the individual equilibrium constants are not known. 

The work by Mills et al. (32) includes an early example of catalytic titration behavior. Figure 10 taken from their study shows that cumene cracking at 425°C drops sharply as nitrogen bases are chemisorbed in increasing amounts on silica-alumina catalyst. Base effectiveness decreases in the order quinaldine > quinoline > pyrrole > piperidine > decylamine > aniline. 

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