Thermodynamic selectivity between

Marchand and coworkers102 reported a difference in site selectivity between the thermodynamically and kinetically controlled Diels-Alder reactions of cyclopentadiene with 2,3-dicyano-p-benzoquinone (126) (equation 37). Under kinetic conditions, the more reactive double bond of 126 reacted with cyclopentadiene affording 127, whereas the less substituted double bond reacted under thermodynamic conditions affording 128. Both reactions proceeded with complete endo selectivity. These findings were in agreement with ab initio HF/3-21G calculations. 

Furthermore, the organic functionalization studies have indicated that multiple reaction products can form even for simple systems. Kinetic and thermodynamic influences must be considered in any analysis of the product distribution. Moreover, the studies have revealed differences in the dominance of kinetic vs. thermodynamic control between the silicon and germanium surfaces. The dissimilarity primarily stems from the fact that adsorbate bonds are usually weaker on Ge than on Si. This difference in energetics leads to observable differences in the degree of selectivity that can be achieved on the two surfaces. Another important motif is the significance of interdimer bonding in the products. Many molecules, even as small as ethylene, have been observed to form products that bridge across two dimers. Consequently, each analysis of adsorption products should include consideration of interdimer as well as intradimer species. 

The systems selected for evaluation are the PDMS-C02 system studied by Gerhardt et al. (1997, 1998) and PS-gas systems studied by Kwag et al. (1999). Properties for these systems are listed in Table 11.1. The variation in physical properties between these systems provides a very broad basis for evaluating the rheological properties of polymer-gas systems. The PDMS C02 system exhibits a favorable thermodynamic affinity between the polymer and dissolved gas, and provides the opportunity to evaluate the rheology of melts with very high dissolved gas content (up to 21 wt %). Carbon dioxide is much less soluble in polystyrene than in PDMS, so the PS-C02... 

Similarly, a difference between the kinetic and thermodynamic selectivities was observed by Taguchi and coworkers while generating zirconaaziridines by... 

Because of the different degree of steric congestion in the interstices between the NO, Cp, and PPhs ligands (NO/PPhs most congested > Cp/PPhs > NO/Cp least congested), the chiral (Re)+ fragment is able to discriminate between both faces of prochiral alkenes and aldehydes with high kinetic and thermodynamic selectivities. For example, the thermodynamic... 

Retention of Rohrschneider-McReynolds standards of selected chiral alcohols and ketones was measured to determine the thermodynamic selectivity parameters of stationary phases containing (- -)-61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in poly(dimethylsiloxane) . Separation of selected racemic alcohols and ketones was achieved and the determined values of thermodynamic enantioselectivity were correlated with the molecular structure of the solutes studied. The decrease of the ionic radius of lanthanides induces greater increase of complexation efficiency for the alcohols than for the ketone coordination complexes. The selectivity of the studied stationary phases follows a common trend which is rationalized in terms of opposing electronic and steric effects of the Lewis acid-base interactions between the selected alcohols, ketones and lanthanide chelates. The retention of over fifty solutes on five stationary phases containing 61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in polydimethylsiloxane were later measured ". The initial motivation for this work was to explore the utility of a solvation parameter model proposed and developed by Abraham and coworkers for complexing stationary phases containing metal coordination centers. Linear solvation... 

However, Eqs. 3 and 5 are different equations even though they are based on the same definition of the preferential binding parameter and have the same theoretical basis the Kirkwood-Buff theory of solutions. To make a selection between Eqs. 3 and 5 a simple limiting case, the ideal ternary mixture, will be examined using the traditional thermodynamics, and the results will be compared to those provided by Eqs. 3 and 5. 

According to Fig. 2-4, these parameters may also be obtained from the chromatogram. The selectivity is determined by the properties of the stationary phase. In case of HPLC, the selectivity is additionally affected by the mobile phase composition. If a = 1, then there are no thermodynamic differences between two sample components at the given chromatographic conditions therefore, no separation is possible. At equilibrium, which is established in reasonably close approximation in chromatography, the selectivity a is a thermodynamic quantity, which at constant temperature only depends on the specific properties of the sample components to be separated and on the properties of the mobile and stationary phase being used. 

The relative adsorption between components i and j of a gas mixture is expressed in terms of the selectivity of adsorption (Sij = niyjjnjy. Component i is more selective than component j if Sij > 1. The thermodynamic selectivity decreases with increasing T for any given values of Ui. For adsorption on a homogenous adsorbent at constant T, S j can be constant, increase, or decrease with adsorbate loading depending on the size differences between the molecules of components... 

Most practical gas separations using activated carbons are based on thermodynamic selectivity created by van der Walls , pole-pole, and pole-induced pole interactions between the gas molecules and the surface molecules at the walls of the carbon pores [19]. Examples of separation by kinetic selectivity are sporadic. True cases of molecular sieving by activated carbons are rare. 

Marchand and coworkers reported a difference in site selectivity between the thermodynamically and kinetically controlled Diels-Alder reactions of cyclopentadiene with... 

However, correlations have been suggested which allow prediction or rationalization of regioselectivity with a modest degree of confidence. With some significant exceptions as discussed below, the difference between the kinetic and thermodynamic selectivity has not been determined or is small. 

In rubbery polymers, such relationships can be obtained in a rather straightforward way, since true thermodynamic equilibrium is reached locally immediately. In such cases, one simply has to choose the proper equilibrium thermodynamic constitutive equation to represent the penetrant chemical potential in the polymeric phase, selecting between the activity coefficient approacht or equation-of-state (EoS) method ", using the most appropriate expression for the case under consideration. On the other hand, the case of glassy polymers is quite different insofar as the matrix is under non-equilibrium conditions and the usual thermodynamic results do not hold. For this case, a suitable non-equilibrium thermodynamic treatment must be used. 

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