Entropic control

A strictly entropically controlled tendency for statistical ligand distribution was discussed 150) for ligand exchange when the sum of the Sb—X and Sb—Y bond energies remains constant. Calculations show that due to the electronic interaction in the entire molecule an energetic tendency also exists to form Lewis acids with mixed ligand spheres ... 

We also note that the values obtained for the prefactor Q are the same for room-temperature and low-temperature glass also, in both glasses, the values of are about two orders of magnitude lower than in the case of bare protoheme. This latter difference was interpreted by Ye et al. (2007) as entropic control in the protein. This idea may be rationalized upon glancing at Equation (1.1), which predicts that, if not used explicitly in data analysis, a possible entropic contribution to the change in Gibbs free energy would be collected in the prefactor Q. 

In the opposite case (i.e., if both isomers would have identical formation molar enthalpies), the equilibrium constant and populations would be determined only by the difference in their entropies (see Table 1) and, accordingly. A = exp(-tAAS /R)=l/3 (see Eq. 7). Therefore, under such circumstances the equilibrium constant of this entropically controlled process would remain unchanged with the temperature and the nature of the solvent. 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

B. A. Heymann and H. Grubmiiller (2001) Molecular dynamics force probe simulations of antibody/antigen unbinding Entropic control and nonadditivity of unbinding forces. Biophys. J. 81, pp. 1295-1313... 

Such a mechanism based on entropic control would circumvent the Cordes dilemma and would account for the mechanistic importance of the 2 -ribo hydroxyl. In the absence of the properly configured 2 -hydroxyl, electrostatic interactions with basic groups (carboxylates ) in the active site would be redirected to the nicotinamide ring. Such interactions would then serve both to promote... 

Blixt, J. Detellier. C. Conformational dynamics of calixarenes. Kinetics of conforinational interconversion in 5.11,17,23-tetra-p-tgrt-butyl-25,26,27,28-tetramethoxy-calix[4]arene under entropic control. J. Am. Chem. Soc. 1994, 116. 11957-11960. 

Harnessing entropically controlled phenomena, such as molecular recognition and self-assembly, can also result in energy savings. After all, it is because of intermolecular interactions that nature is able to produce such highly complicated and organized structures as biomacromolecules at ambient temperature and in aqueous media. 

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