Differential solvation free energies

The 2-substituted system has proven especially attractive to modelers because the experimental equilibrium constants are known both in the gas phase and in many different solutions. As a result, the focus of the modeling study can be on the straightforward calculation of the differential solvation free energy of the two tautomers, without any requirement to first accurately calculate the relative tautomeric free energies in the gas phase. However, in 1992 Les et al. [290] suggested that prior experimental data [240,266,288], primarily in the form of ultraviolet spectra in the gas phase and in low-temperature matrices, had been misinterpreted and that the reported equilibrium constants referred to homomeric dimers of tautomers (i.e., (42)2 (43)2). Parchment et al. [291] contested this... 

Calculations on the differential solvation free energies of the two relevant tautomers are presented in the following table for several different models implemented at a number of levels of theory. The following discussion will focus on comparing specific calculations in the table. 

Young et al. [195] have provided a calculation in which they compared expanding the multipole series up to /= 6 in a spherical cavity of 3.8 A. These results may be compared directly to those of Wong et al. [297] at the identical level of theon asis set in order to assess the effect of including higher moments. In each case, the differential solvation free energy increases by about 40%. This illustrates nicely the relationship between cavity radius and model... 

Only the force-field energy term associated with interactions between the biotin and avidin fragments remains. This is added to the differential solvation free energies and differential thermal terms to determine the full binding free energy. 

There are many assumptions typically made in these calculations. These include assuming the rate of complex formation is the same for R as for S analyte and that only the relative stabilities of the complexes are important complete neglect of mobile phase additives, ions or solvent, although we know that diastereomers have differential solvation free energies and experimentally we can sometimes find reversal in retention orders depending on solvent elimination or truncation of the... 

This issue has been considered in some detail [16-18], where, when dealing with aqueous phase electrode potentials, on the other hand, it is emphasized [18] that differential solvation free energy terms can be very large, and certainly very dependent upon net charge. 

Considering that, roughly speaking, the electrostatic component of the solvation free energy varies as the cube of the molecular dipole moment, it becomes obvious that the corrective term (13.1) should be taken into account in the determination of differential solvation properties of very polar solutes. In the computation of transfer free energies across an interface, it has been suggested that equation (13.1) be expressed as a function of the number density of one of the two media, so that the correction is zero in solvent 1 and zl,l lsl lll in solvent 2 [115]. 

It can be anticipated that the computation of A//soi and AAsoi is more delicate than the prediction of AGsoi, which benefits from the enthalpy-entropy compensation. Accordingly, the suitability of the QM-SCRF models to predict the enthalpic and entropic components of the free energy of solvation is a challenging issue, which could serve to refine current solvation continuum models. This contribution reports the results obtained in the framework of the MST solvation model [15] to estimate the enthalpy (and entropy) of hydration for a set of neutral compounds. To this end, we will first describe the formalism used to determine the MST solvation free energy and its enthalpic component. Then, solvation free energies and enthalpies for a series of typical neutral solutes will be presented and analyzed in light of the available experimental data. Finally, collected data will be used to discuss the differential trends of the solvation in water. 

The usual way to proceed consists in directly differentiating the expression of the solvation free energy introducing the same approximation used in the previous section (i.e. the HF/DFT approach with MOs expanded as a linear combination of atomic orbitals (LCAO)), eq.(1.20) leads to the... 

Potential 14.21 signifies the mean field approximation, which follows essentially from the use of the solvation free energy in the form of Equation 14.15. The analytical first derivative with respect to the nuclear coordinates R is obtained by differentiation of the free energy... 

Differential free energies of solvation (kcal/mol) for 2-pyridone 42 and 2-hydroxypyridinc 43 for different dielectric constants and solvent models."... 

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