Solvation Gibbs Free Energy Calculations

When we began our work on pK, calculations, using carboxylic acids and phenols as test cases, we tried to use the then accepted value of -261.4 kcal/mol. Like previous researchers, it was not possible to calculate accurate absolute pK values [47-50], Karplus and co-workers reported that the continuum dielectric methods were not accurate enough to yield accurate absolute solvation Gibbs free energies [49]. By 2001, new solvation methods had been developed, and relative pK values were easily... 

The absolute solvation Gibbs free energy of a proton can also be calculated using high-level gas phase calculations with a supermolecule-continuum approach, involving a self-consistent reaction field model. 

Within the framework of the same dielectric continuum model for the solvent, the Gibbs free energy of solvation of an ion of radius and charge may be estimated by calculating the electrostatic work done when hypothetically charging a sphere at constant radius from q = 0 q = This yields the Bom equation [13]... 

In principle, Gibbs free energies of transfer for trihalides can be obtained from solubilities in water and in nonaqueous or mixed aqueous solutions. However, there are two major obstacles here. The first is the prevalence of hydrates and solvates. This may complicate the calculation of AGtr(LnX3) values, for application of the standard formula connecting AGt, with solubilities requires that the composition of the solid phase be the same in equilibrium with the two solvent media in question. The other major hurdle is that solubilities of the trichlorides, tribromides, and triiodides in water are so high that knowledge of activity coefficients, which indeed are known to be far from unity 4b), is essential (201). These can, indeed, be measured, but such measurements require much time, care, and patience. 

These differences stimulated our interest and prompted us to study the effects of solvation on (R,R )-tartaric acid amides. Encouraged by the widespread usage ofAMSOL [42] (vide infra) we decided to use it to calculate Gibbs free energies of hydratation. We performed the calculations with the use of the solvation model SM5.4 [43-46] and hamiltonian PM3 [47] for all structures, optimized at the RHF/6-31G level [20],... 

FIGLfRE 2. Calculated Gibbs free-energy values of unsolvated and solvated mixed complexes between a chiral lithium amido ether and methyllithium... 

IR spectra, total energy, Gibbs free energy, and the highest n and c electronic states calculated for tautomers of 5-substituted imidazoles by ab initio (MP2, RHT/6-311++G ) show that such substituents as N02, NH2, CN, etc. stabilize the N3-H tautomer more [1042], Therefore the conclusions derived from these data are related to molecules in the gas state and cannot be extended to the imidazoles in the solvated state. 

The Henry s law constants (77) for triazine-derived herbicides have been calculated using quantum-chemical solvation models, SM2, SMS, PCM-DFT, and CPCM-DFT, and their performances have been discussed. The results showed considerable differences in performance among the different levels of theory. The differences were discussed in terms of the different contributions, electrostatic and nonelectrostatic, to Gibbs free energy of solvation <2003JCI1226>. 

The largest source of error in pK calculations is the change in Gibbs free energy of solvation calculation for the reaction, which is based on the type of solvation model used and the specific level of theory [8.11.15.39.55]. The... 

Along with deciding whether to use implicit or implicit-explicit solvent models, a specific level of theory and basis set must be used for the calculation of the change in Gibbs free energy of solvation. Similar to the gas phase Gibbs free energy, there are a variety of methods and it can be difficult to determine what combination is the most accurate. 

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