Linear solvent energy relationships,

LORG (localized orbital-local origin) technique for removing dependence on the coordinate system when computing NMR chemical shifts LSDA (local spin-density approximation) approximation used in more approximate DFT methods for open-shell systems LSER (linear solvent energy relationships) method for computing solvation energy... 

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

Kamlet-Taft Linear Solvation Energy Relationships. Most recent works on LSERs are based on a powerfiil predictive model, known as the Kamlet-Taft model (257), which has provided a framework for numerous studies into specific molecular thermodynamic properties of solvent—solute systems. This model is based on an equation having three conceptually expHcit terms (258). 

Another method for studying solvent effects is the extrathermodynamic approach that we described in Chapter 7 for the study of structure-reactivity relationships. For example, we might seek a correlation between og(,kA/l ) for a reaction A carried out in a series of solvents and log(/ R/A R) for a reference or model reaction carried out in the same series of solvents. A linear plot of og(k/iJk ) against log(/ R/ linear free energy relationship (LFER). Such plots have in fact been made. As with structure-reactivity relationships, these solvent-reactivity relationships can be useful to us, but they have limitations. 

The second aspect is more fundamental. It is related to the very nature of chemistry (quantum chemistry is physics). Chemistry deals with fuzzy objects, like solvent or substituent effects, that are of paramount importance in tautomerism. These effects can be modeled using LFER (Linear Free Energy Relationships), like the famous Hammett and Taft equations, with considerable success. Quantum calculations apply to individual molecules and perturbations remain relatively difficult to consider (an exception is general solvation using an Onsager-type approach). However, preliminary attempts have been made to treat families of compounds in a variational way [81AQ(C)105]. 

The effect of solvent has been treated quantitatively (for SnI mechanisms, in which the solvent pulls off the leaving group) by a linear free energy relationship "... 

One of the things our discussion of linear free energy relationship has not yet made any endeavour to take into account is the role played in reactions by the solvent. This despite the fact that the very great majority of organic reactions do take place in solution, with the solvent often playing a crucial role. 

Kamlet MJ, Dickinson C, Taft RW (1981) Linear solvation energy relationship. Solvent effects on some fluorescent probes. Chem Phys Lett 77 69-72... 

Kamlet, M. J., M. E. Jones, J.-L. M. Abboud, and R. W. Taft. 1979. Linear Solvation Energy Relationships. Part 2. Correlations of Electronic Spectral Data for Aniline Indicators with Solvent tt and 3 Values. J. Chem. Soc., Perkins Trans. 2, 342. 

M. H. Abraham, New solute descriptors for linear free energy relationships and quantitative structure-activity relationships, in Quantitative Treatments of Solute/Solvent Interactions, P. Politzer and J. S. Murray, eds., Elsevier, Amsterdam (1994) pp. 83-134. 

The data for influence of solvents on oxidation propanthiole by chlorine dioxide are satisfactorily generalized by means of five parameters equation according to principles of Linear Free Energies Relationships (LFER). An essential role plays the density of media cohesion energy, that bears out radical process nature. 

Kamlet M. J., Dickinson C. and Taft R. W. (1981) Linear Solvation Energy Relationships. Solvent Effects on Some Fluorescent Probes, Chem. Phys. Lett. 77, 69-72. 

A. Wang and P.W. Carr, Comparative study of the linear solvation energy relationship, linear solvent strength theory, and typical conditions model for retention prediction in reversed-phase liquid chromatography. J. Chromatogr.A 965 (2002) 3-23. 

Certain expressions describing a solvent acidity function, where S is a base that is protonated by an aqueous mineral acid solution. The equations describe a linear free-energy relationship between log([SH+]/[S]) + Ho and Ho + log[H ], where Ho is Hammett s acidity function and where Ho + log[H+] represents the activity function log(7s7H+/ysH ) for the nitroaniline reference bases to build Ho. Thus, log([SH+]/[S]) log[H+] = ( 1)... 

A linear solvation energy relationship (LSER) has been developed to predict the water-supercritical CO2 partition coefficients for a published collection of data. The independent variables in the model are empirically determined descriptors of the solute and solvent molecules. The LSER approach provides an average absolute relative deviation of 22% in the prediction of the water-supercritical CO2 partition coefficients for the six solutes considered. Results suggest that other types of equilibrium processes in supercritical fluids may be modeled using a LSER approach (Lagalante and Bruno, 1998). 

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