Molecular entropy effects

The solvophobic model of Hquid-phase nonideaHty takes into account solute—solvent interactions on the molecular level. In this view, all dissolved molecules expose microsurface area to the surrounding solvent and are acted on by the so-called solvophobic forces (41). These forces, which involve both enthalpy and entropy effects, are described generally by a branch of solution thermodynamics known as solvophobic theory. This general solution interaction approach takes into account the effect of the solvent on partitioning by considering two hypothetical steps. Eirst, cavities in the solvent must be created to contain the partitioned species. Second, the partitioned species is placed in the cavities, where interactions can occur with the surrounding solvent. The idea of solvophobic forces has been used to estimate such diverse physical properties as absorbabiHty, Henry s constant, and aqueous solubiHty (41—44). A principal drawback is calculational complexity and difficulty of finding values for the model input parameters. 

Figures 13.2,13.3,13.4, 13.5,13.6,13.11 and 13.12 are reproduced with permission from the Royal Society of Chemistry from Chemical Society Reviews, 31, Krishna, R., Smit, B., and Calero, S., Entropy effects during sorption of alkanes in zeolites, p. 185-194, Copyright The Royal Society of Chemistry (2002) http //www.rsc.org/Puhlishing/Joumals/ CS/article.asp doi=bl01267n Figures 13.7 and 13.8 are reprinted with permission from the Journal of Physical Chemistry B, 101 (8), Eder, E., Lercher, J.A., On the role of the pore size and tortuosity for sorption of alkanes in molecular sieves, p. 1273-1278,...

The basic unit of elastin structure is tropoelastin, which has a molecular weight of about 72,000 and contains 800-850 amino acid residues. It has been proposed that tropoelastin units are present in the random coil conformation and are extensively cross-linked. This makes such a network kinetically free free to stretch and to recoil. It is the entropy effects that permit the stretched elastin... 

Marked differences between observed and predicted results occur, however, when the entropy effects are too great to be ignored. In the derivation of Eqs. 4 and 5, only dispersion forces between molecular components were taken into... 

Exact determination of entropy effects in enzymatic reactions is not an easy task even nowadays when sophisticated Monte Carlo and molecular dynamics methods are available for calculations (Warshel, 1991 Aqvist and Warshel, 1993). One way to examine the importance of entropy is to analyse the configuration space available to the system in its ground and transition states, both in the enzyme and in solution. The entropic contribution to the catalytic effect, relative to the uncatalysed solution can be expressed as... 

An extended Flory and Huggins equation is added in the UNIQUAC equation to account for the entropy effects of molecular size differences. The UNIQUAC is made up of two parts. 

The higher diffusivities of p-xylene compared to those of benzene in silicahte-1 can be ascribed to the combination of enthalpy and entropy effects [60-62]. The sorbed benzene molecules have to lose their rotational freedom around their hexagonal axis, Ce, when they move from an intersection to a channel segment, i.e., there is a large decrease in entropy in this jump step. For p-xylene, the molecules are, however, orientated with their long molecular axis along either channel direction when they are located at an intersection. The diffusion jump step involves, therefore, only a very small... 

Fig. 12 Reactant and intermediate structures for investigation of a stepwise mechanism where the C-C bond is cleaved prior to protonation of C6. In the QM optimization of the structures, the atoms marked with asterisks were kept frozen, from the structures obtained by molecular dynamics simulations. The relative energy reported does not include thermal or entropy effects...

Summarizing, CoOi fN f oxynitride formation appears to be a kinetically controlled phenomenon [371], in accordance with the reported experimental observation of its thermal sensitivity [366]. The phases decompose into rock-salt CoO, metallic Co, and molecular nitrogen at higher temperatures. The broad phase width is also in accordance with theoretical calculations. Further entropy effects will additionally favor, but only very weakly, the formation of ternary phases due to substitutional entropy (O/N). In addition, there is no indication of anionic ordering in the calculations, and a random distribution of N and O is clearly preferred. 

Tbe cbelate effect is important in bioebemistry and molecular biology. Tbe additional tbermodynamic stabilization provided by entropy effects helps stabilize biolo cal metal-cbelate complexes, such as porphyrins, and can allow changes in the oxidation state of the metal ion while retaining the structural integrity of the complex. 

Overview Over the last few decades, quanmm chemistry has evolved into a predictive tool for the calculation of energy differences. Of equal importance as a driving force for chemical reactions is the entropy that has received less attention from the computational community. The predictive value of computed rate constants depends not only on the accuracy of the electronic structure calculations but also on the correct description of the molecular entropy, including anharmonic effects in the vibrational modes, which is subject of Section 7.3.1.3. 

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