Metastable product

Theoretically, the mechanism for ethoxylated alcohol sulfation is similar to primary alcohol sulfation, involving the rapid formation of a metastable product. The stoichiometry of this almost instantaneous and highly exothermic initial reaction corresponds again to more than one molecule of S03 per molecule of feedstock (Table 4). The desired ethoxylate acid sulfate product formed is... 

On the other hand, the large activation energy for the formation of sulfate from 8g and water makes it possible to prepare polysulfides as well as other reduced sulfur compounds as metastable products in aqueous solution at ambient conditions. 

Also included in Table IV are the metastable product yields for comparison to the ion beam and IR activation results. From these data it appears that the processes involving elimination of hydrogen and methane involve a competitive dissociation from a common intermediate as shown in Figure 16. However, a common intermediate may not be involved in the elimination of ethylene and propylene (the latter product appears to be formed in a faster process), and Scheme HI is overly simplistic. 

Cornish, T.J. Cotter, R.J. A Curved Field Reflection TOF Mass Spectrometer for the Simultaneous Focusing of Metastable Product Ions. Rapid Commun. Mass Spectrom. 1994, 8. 781-785. 

Depending on the data available, Eqs (6.17)-(6.23) reproduce experimental pressure effects with considerable accuracy in many cases. In particular, Eq. (6.18) can be used to confirm entropy data derived using more conventional techniques and can also provide data for metastable allotropes. Ti again provides a leading example, as pressure experiments revealed that the u -phase, previously only detected as a metastable product on quenching certain Ti alloys, could be stabilised under pressure (Fig. 6.14). Extrapolation of the P/w transus line yields the metastable allotropic transformation temperature at which the / -phase would transform to w in the absence of the a-phase, while die slope of the transus lines can be used to extract a value for the relevant entropy via Eq. (6.18). 

A very similar situation has been observed in Similar but less well studied indications for metastable products have been obtained for the DHP s formed from the m-substituted stilbenes Some observations about time-dependent changes in the absorption spectmm of 1 could well be due to such processes ... 

Let us now consider bathorhodopsin, which appears to be the first metastable product of the photochemical reaction. Resonance Raman measurements indicate that the retinal in bathorhodopsin has isomerized to the all-trans form but is still held as a protonated Schiff s base. 

Table 12.4 summarizes the voltammetric oxidation potentials and peak currents for l,4-(MeO)2Ph and other alkoxy-substituted benzenes, phenols, and benzyl alcohols. Only the 1,4-(MeO)2PhX members of the series exhibit an initial irreversible anodic cyclic voltammogram via the sequence of Eq. (12.37). These plus the l,2-(MeO)2Ph isomer yield a metastable product from the second oxidation [species A, Eq. (12.37)] that undergoes a reversible reduction. Thus, the two-electron oxidation of dimethoxy benzenes yields the corresponding quinone. 

Materials chemistry contains all the elements of modem chemistry. These include synthesis, structure, dynamics, and properties. In synthesis, one employs all possible methods and conditions from high-temperature and high-pressure techniques to mild solution methods (chimie douce or soft chemistry).3 Chemical methods generally tend to be more delicate, often yielding novel, metastable products. Kinetic rather than thermodynamic control of reactions favors the formation of such structures. Supramolecular organization provides new ways of designing materials. 

Catalysis relies on changes in the kinetics of chemical reactions. Thermodynamics acts as an arrow to show the way to the most stable products, but kinetics defines the relative rates of the many competitive pathways available for the reactants, and can therefore be used to make metastable products from catalytic processes in a fast and selective way. Indeed, cafalysis work by opening alternative mechanistic routes with lower activation energy barriers than those of the noncatalyzed reactions. As an example, Figure 1 illustrates how the use of metal catalysts facilitates the dissociation of molecular oxygen, and with that the oxidation of carbon monoxide. Thanks to the availability of new pathways, catalyzed reactions can be carried out at much faster rates and at lower temperatures than noncatalyzed reactions. Note, however, that a catalyst can shorten the time needed to achieve thermodynamic equihbrium, but caimot shift the position of that equihbrium, and therefore cannot catalyze a thermodynamicaUy unfavorable reaction. ... 

Assuming a constant surface area, dissolution at a solution-solid interface (Case I) results in linear kinetics in which the rate of mass transfer is constant with time (equation 1). Analytical solutions to the diffusion equation result in parabolic rates of mass transfer (, 16) (equation 2). This result is obtained whether the boundary conditions are defined so diffusion occurs across a progressively thickening, leached layer within the silicate phase (Case II), or across a growing precipitate layer forming on the silicate surface (Case III). Another case of linear kinetics (equation 1) may occur when the rate of formation of a metastable product or leached layer at the fresh silicate surface becomes equal to the rate at which this layer is destroyed at the aqueous... 

Yet another possible mechanism that can cause an induction period is reversible coupling (or other reaction with itself) of the most plentiful free radical to yield a product of low stability. Initially, none of that product is present. In the early stages of the reaction, its formation can be the dominant and highly effective termination, keeping the reaction at a low rate. With time, however, the metastable product builds up and approaches equilibrium with the free radicals from which it is formed. Picture the product as a reservoir into which the reaction drains free radicals until it is filled to capacity. By default, another and slower termination mechanism then takes over, and the reaction speeds up accordingly. 

Fig. to. Chemical state of carbon in the solar nebula (Hayatsu et al., 1980b). SoUd lines give the temperatures at which 50% of the CO should have reacted according to the equilibrium shown (the 0.1 % lines would be typically 100-200° higher). In each field, principal stable products are shown in roman type, and metastable products, in italics those that do not form for kinetic reasons are encl( ed in brackets. 

Fig. 5, 0°, structure on the left) which in FET results in a metastable product radical cation, and and one derived from a twisted (deformed) state where the electron density is shifted more to the hetero-atom (Fig. 5, 90°, right hand side) which in the prompt ionization should result in an extremely unstable and dissociative radical cation with high charge density at the hetero-atom. 

Similar to the porosils, the dense, thermodynamically stable Si02 modification a-quartz is also prepared under hydrothermal conditions. However, in the industrial process for the production of quartz, the temperatures are rather high (around 400°C). In this process, NaOH is added as a mineralizer to the aqueous solution to promote dissolution of the silica precursor. The reaction mixtures for the preparation of porosils and other zeotype materials also generally contain a mineralizer, but the reaction conditions are much milder. Synthesis temperatures are below 200°C, typically between 140 and 180°C. Some zeolites can even be prepared from aqueous solutions under reflux at normal pressure. These mild synthesis conditions provide the kinetic control necessary to form metastable products [5-9]. 

Figure 1. Photochemical reaction process of 6-hydroxy-1, 3, 3 -trimethylspiro[2H-1 -benzopyran-2,2 -indoline] (6-OH-BIPS 7). The reaction proceeds along the diabatic surface (dashed line) i.e., direct dissociation of the Cipjro-0 bond takes place. B is one of the several possible merocyanine isomers, and X refers to the initial metastable product formed soon after the C,pjro—O bond cleavage. (Reprinted from Ref. 10 with permission of the American Chemical Society.)...

The red metastable product, produced from P4 and KOH or KOEt, does not appear to be an intermediate in these, tertiary phosphine oxide syntheses, since when separately prepared, gave only minor amounts of phosphine oxide. This result indicates that the olefinic compounds must attack an earlier intermediate. The initial step was thought to involve nucleophilic attack of hydroxide ion on tetrahedral white phosphorus to give a phosphide ion that subsequently underwent a Michael-Addition to the electrophilic unsaturated compounds present (Rauhut, Bemheimer and Semsel). 

Silicate mineral dissolution is usually incongruent, with precipitation of relatively amorphous metastable products that may crystallize with time to form minerals such as gibbsite, kaolinite, illite, and montmorillonite (Helgeson et al. 1984). The incongruency means that the net release rates of individual components from a silicate mineral into the water may not be equal (cf. White and Claassen 1979 Helgeson et al. 1984). 

Neither AG° nor A0 depends upon the mechanism of the reaction. But even if AG° is strongly negative, the yield of thermodynamically favored products may be negligible if the reaction proceeds too slowly on the human timescale or if the slowness of a critical step in the reaction sequence relative to some alternative steers the reaction to other (metastable) products. Thus, as Taube1 emphasized, we need to understand what makes some ligand substitution reactions fast and others slow. The mechanism of reaction is a simplified hypothetical model, an approximation to reality that purports to trace the progress of the system from reactants to products, and is significant only insofar as it helps us understand the kinetics and stereochemistry of the reaction (rather than vice versa as some workers tend to believe). 

Il in, Turutina, and co-workers (Institute of Physical Chemistry, the Ukrainian S.S.R. Academy of Sciences, Kiev) (113-115) investigated the cation processes for obtaining crystalline porous silicas. The nature of the cation and the composition of the systems M20-Si02-H20 (where M is Li+, Na+, or K+) affect the rate of crystallization, the structure, and the adsorption properties of silica sorbents of a new class of microporous hydrated polysilicates (Siolit). These polysilicates are intermediate metastable products of the transformation of amorphous silica into a dense crystalline modification. The ion-exchange adsorption of alkali and alkaline earth metals by these polysilicates under acidic conditions increases with an increase in the crystallographic radius and the basicity of the cations under alkaline conditions, the selectivity has a reverse order. The polysilicates exhibit preferential sorption of alkali cations in the presence of which the hydrothermal synthesis of silica was carried out. This phenomenon is known as the memory effect. 

The red metastable product obtained from potassium hydroxide and phosphorus does not appear to be an intermediate in this tertiary phosphine oxide synthesis. A solution of the red compound prepared separately in etlianol was not decolorized by acrylamide, and only minor amounts of the tertiary phosphine oxide were obtained. This result... 

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