Solvation absolute free energy

In Eqs. (25) and (26), the summations are over the incremental steps in going from X to Y in the gas phase or in solution. The Hj are the intermediate Hamiltonians (or force fields in a classical treatment). Thus, Hi=0,gas = Hx,gas, Hi=Njgas = HY,gas, etc. It is of course desirable that the molecules X and Y be structurally similar, so that the perturbation of X that produces Y be small. Another option is to let Y be composed of noninteracting dummy) atoms,75 so that its free energy of solvation is zero. Then Eq. (24) gives the absolute free energy of solvation of X ... 

Table 11.3 Absolute free energies of solvation (kcal mol ) and chloroform/water partition coefficients (logio units) for nucleic acid bases at the SM5.4/AM1 Icvcf ...

Continuum solvation models have been applied to many chemical processes in the liquid phase. Determining absolute free energies of activation is important because it allows one to predict the time scale on which a chemical process can take place. In addition,... 

Hydrophobic Effect and Absolute Free Energies of Aqueous Solvation. 

The SMx aqueous solvation models, of which the most successful are called AM1-SM2,27 AMl-SMla, and PM3-SM3, °- adopt this quantum statistical approach, which takes account of the ENP and CDS terms on a consistent footing. The NDDO models employed are specified as the first element (AMI or PM3) of each identifier. It is worth emphasizing that the SMx models specifically calculate the absolute free energy of solvation—a quantity not easily obtained with other approaches. We have reviewed the development and performance of the models elsewhere.203 We anticipate our further observations later in this chapter by noting that the mean unsigned error in predicted free energies of solvation is about 0.6-0.9 kcal/mol for the SMx models for a data set of 150 neutral solutes that spans a wide variety of functionalities. A number of examples are provided later in this chapter. 

C. J. Cramer and D. G. Truhlar, Science, 256, 213 (1992). An SCF Solvation Model for the Hydrophobic Effect and Absolute Free Energies of Aqueous Solvation. 

J. Gao,/. Phys. Chem., 96, 537 (1992). Absolute Free Energy of Solvation from Monte Carlo... 

Poisson-Boltzmann model The PB approximation is one of the more elaborate continuum solvation models that take into account not only the charge density of the solute but also the mobile charge density within the surrounding continuum. Continuum models of solvation are in general capable of calculating absolute free energies of solvation. 

An important quantity required for analysis of ion-solvent interactions and structural properties is the absolute free energy (or enthalpy and entropy) of solvation. Most methods of obtaining these quantities involve some extra-thermodynamic assumption such as the extrapolation of solvation energies versus some function of crystal radii (see sect. 2.11.4). The method based on measurements of volta potential differences avoids the controversy involving the significance of these radii. This method has been used by Frumkin, Klein and Lange, Randles and Parsons et... 

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. 

Mean Unsigned Errors in Absolute Aqueous Solvation Gibbs Free Energies of Ions and Ion-Water Clusters, with a Single Water Molecule, for Various Continuum Solvent Models... 

In this procedure, the electronic terms of the QM/MM interaction Hamiltonian for A and B are first annihilated, leaving only the Lennard-Jones terms, which are gradually mutated from structure A into structure B. The latter mutation consists of empirical potentials, while standard methods are employed for the conversion.Absolute free energies of solvation can also be determined following a procedure developed by Cieplak and Koll-man, in which A is made to vanish in solution ... 

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