In solvents, effects

Most of the theoretical arguments support the statement of Evans and Polanyi (20), who first argued that AG represents the situation at 0°K better than does AH. The main reason is seen in solvent effects. Dewar expressed this meaning in the most radical manner, saying that determination of AH and AS in solution is simply a waste of time (21). Laidler offered similar ideas and stressed that a theory for AG can be more easily developed than for AH (13, 225). The approach of Hammett is still more general and not restricted to solvent effects (226). According to Hammett, a reaction that is more complex than it appears to the observer and consists of two parallel independent processes will affect the value of AH more than will AG. 

C. J. Cramer and D. G. Truhlar, Continuum solvation models, in Solvent Effects and... 

These configurational changes will affect the total pair correlation in the model compounds. In addition, in the real molecules we have also a large effect due to changes in proton-proton distances (as well as differences in solvent effects and intramolecular hydrogen bonding). 

Gonzalez-Lafont, A., Lluch, J. M. and Bertran, J. (1996) In Solvent Effects and Chemical Reactivity, Tapia, O. and Bertran, J., Eds. Kluwer Academic Pubhshers, Boston, pp. 125-178. 

Reichardt, C., in "Solvent Effects in Organic Chemistry (H. F. Ebel, ed.), Monogr. Mod. Chem. Ser., Verlag-Chemie, Weinheim, 1979 (for a wider discussion of the problem). 

In the last few years more information on excited state pA-values has accumulated and the present review contains extensive reference tables of the experimental results in the literature available up to August 1974. We have confined our attention throughout to Br nsted acids and bases, though work on Lewis acid (donor-acceptor) systems continues (Weller, 1961 Birks, 1970 Ottolenghi, 1973) and may prove directly relevant to a deeper understanding of prototropic reactions, which must often be preceded by the formation of hydrogen-bonded complexes. Such interactions also play a role in solvent effects upon the absorption frequencies of acid and base molecules. 

In solvent effect studies, a fundamental quantity is the solvent shift, 8, on the energy, defined as the difference between the energy gap values calculated in solution and in vacuo ... 

The Swain Equation (34) provides very accurate predictive power for a limited range of solvents and processes based on a statistical analysis of a five-parameter equation. The origins of the Swain parameters are not explicit, although the acity (A) and basity (B) coefficients are related to electrophilic and nucleophilic processes respectively. Many fundamental processes have been implicated in solvent effects and the equation of Koppel and Palm (Equation 37) incorporates the major factors thought to be involved. 

Our Monte Carlo (MC) simulation uses the Metropolis sampling technique and periodic boundary conditions with image method in a cubic box(21). The NVT ensemble is favored when our interest is in solvent effects as in this paper. A total of 344 molecules are included in the simulation with one solute molecule and 343 solvent molecules. The volume of the cube is determined by the density of the solvent and in all cases used here the temperature is T = 298K. The molecules are rigid in the equilibrium structure and the intermolecular interaction is the Lennard-Jones potential plus the Coulombic term... 

It is interesting to mention that the statistical inefficiency is obtained only after performing the simulation and therefore it is of little practical use until the simulation is halted. However, for our applications in solvent effects it shows that within a given accuracy determined only by the total number of MC steps there is a great saving of the total calculation if only uncorrelated structures are used in the subsequent quantum mechanical calculations. For the case of the acetone-water calculations only 40 uncorrelated structures are used for the ensemble average of the calculated quantum mechanical results, instead of the total number of 32000. 

C. Cramer and D. Truhlar, in Solvent Effects and Chemical Reactivity, Ed. O. Tapia and J. Bertran, Kluwer, Dordrecht, 1995. 

Ultraviolet spectrophotometry was also used to monitor valence-bond isomerizations of various 2//-pyrans (see Section V.C.l) in solvent-effect investigations (85IZV1075 87IZV821 88KGS1325) and to demonstrate the site of methoxide-ion attachment to pyrylium cations [86JOC4385 89JCS(P2)1393]. 

Mechanism of Nonoxidative Thermal Dehydrochlorination. This subject is still very controversial, with various workers being in favor of radical, ionic, or molecular (concerted) paths. Recent evidence for a radical mechanism has been provided by studies of decomposition energetics (52), the degradation behavior of PVC-polystyrene (53) or PVC-polypropylene (54) mixtures, and the effects of radical traps (54). Evidence for an ionic mechanism comes from solvent effects (55) and studies of the solution decomposition behavior of a model allylic chloride (56). Theoretical considerations (57,58) also suggest that an ionic (El) path is not unreasonable. Other model compound decompositions have been interpreted in terms of a concerted process (59), but differences in solvent effects led the authors to conclude that PVC degrades via a different route (59). 

Because of the large number of lines in a spectrum, overlapping of lines can lead to difficulties though these have largely been removed by the availability of computer simulation facilities. Furthermore, if one is interested in solvent effects then one obviously chooses a solute whose spectrum is capable of rapid analysis. Having obtained the isotropic hyperfine splitting constants it is necessary to relate them to the electronic structure of the radical. 

Mathematically, limiting value when b < , for example, lim = Vz-In Fig. 4 we have plotted common solvents. It appears that for (b > 10, v (solvent effects between materials like EtBr( a = 9.50) and DMSO (e = 48.9). This is contrary to practically any experimental evidence, which indicates, for instance, a sizable difference in solvent efficiency even between DMF ( = 36.7) and DMSO. This important shortcoming is shared by all the preceding theories and the inclusion of the solute polarizability and multipoles does not improve the picture. This fact has been recognized by... 

A similar pattern is seen in solvent effects on pmr shifts of chloroform (19a) as reported by Lichter and Roberts (143). The shifts in nine solvents for which j8 values are known (2,3 6,7,18,24,25,26,50) follow the regression equation,... 

Victor Akpe, Hjalmar Brismar, Tebello Nyokong, Osadebe, P. O. Photophysical and photochemical parameters of octakis (benzylthio)phthalocyaninato zinc, aluminum and tin Red shift index concept in solvent effect on the ground state absorption of zinc phtha-locyanine derivatives. 

Absorbance vs. wavelength Dissolved O2 in solvents Effects of gradient work, 7,18 General, 3-9 Normal-phase solvents... 

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