Solvation organic compounds, energy calculations

Fig. 6. Calculated (y-axis) and experimental (x-axis) molecular properties. Top Left free energy of solvation in kcal/mol for 291 small organic molecules. Top Right boiling points (in Kelvin) of 298 small organic molecules. Bottom Left the log concentration ratio between the blood and brain for 75 compounds. Bottom Right the solubility in water of 1438 small organic compounds (units are log concentration ratios).

In a polar solvent, heterolytic cleavage leading to proton abstraction is usually facilitated because of the favorable solvation energy of the proton, and cation radicals are ordinarily much more acidic than the corresponding neutral compounds. Table 1-5 combines acidity constants of organic compounds (AH) and their cation radicals (AH+ ) calculated for their solutions in dimethylsulfoxide (DMSO, a very polar solvent) at 25°C. 

The calculated individual contributions to the total aqueous solvation free energies of 30 organic compounds are given in Table 1. The electrostatic (SCRF) contributions were calculated using semiempirical AMI (Austin Model 1 [60,61]) method. The dispersion energies were calculated using INDO/1 parameterization [62] and AMI optimized molecular geometries in solution. A comparison of different columns in Table 1 with the experimental solvation... 

AMI SCRF Calculated Electrostatic Solvation Energies, Eei, INDO/1 Calculated Dispersion Energies, Edisp, SPT Cavity Formation Free Energies, AGcav, and Experimental Solvation Free Energies, AG(exp) (kcal/mol), [63] of 30 Organic Compounds in Aqueous Solution. 

The data presented above demonstrate that the total solvation free energy of organic compounds in aqueous solution can be calculated with some confidence using a minimum number of parameters (the dielectric constant and the solute cavity size contibution for the solvent) and provided that appropriate quantum chemical and statistical physical models are used for the description of the reaction field and dispersion interactions, and the cavity formation in solution. 

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Revelli, A.-L. Mutelet, F. Jaubert, J.-N. (2010a). Prediction of partition coefficients of organic compounds in ionic liquids Use of a linear solvation energy relationship with parameters calculated through a group contribution method. Ind. Eng. Chem. Res., 8,49, 3883-3892. 

Whether AH for a projected reaction is based on bond-energy data, tabulated thermochemical data, or MO computations, there remain some fundamental problems which prevent reaching a final conclusion about a reaction s feasibility. In the first place, most reactions of interest occur in solution, and the enthalpy, entropy, and fiee energy associated with any reaction depend strongly on the solvent medium. There is only a limited amount of tabulated thermochemical data that are directly suitable for treatment of reactions in organic solvents. Thermodynamic data usually pertain to the pure compound. MO calculations usually refer to the isolated (gas phase) molecule. Estimates of solvation effects must be made in order to apply either experimental or computational data to reactions occurring in solution. 

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