Solvation equation

H-bonding is an important, but not the sole, interatomic interaction. Thus, total energy is usually calculated as the sum of steric, electrostatic, H-bonding and other components of interatomic interactions. A similar situation holds with QSAR studies of any property (activity) where H-bond parameters are used in combination with other descriptors. For example, five molecular descriptors are applied in the solvation equation of Kamlet-Taft-Abraham excess of molecular refraction (Rj), which models dispersion force interactions arising from the polarizability of n- and n-electrons the solute polarity/polarizability (ir ) due to solute-solvent interactions between bond dipoles and induced dipoles overall or summation H-bond acidity (2a ) overall or summation H-bond basicity (2(3 ) and McGowan volume (VJ [53] ... 

Statishcal criteria of Eq. (24) are too good the standard deviation, which was created on the basis of different measurements by various authors, is much less than even the experimental error of determinahon. This could be due to mutual intercorrelation of descriptors leading to over-ophmistic statistics [18]. Another reason may be the lack of diversity in the training set. The applicahon of the solvation equation to data extracted from the MEDchem97 database gave much more modest results n = 8844, = 0.83, root mean square error = 0.674, F = 8416... 

As in isocratic mode, the estimate of log P is indirect and based on the construction of a linear retention model between a retention property characteristic of the solute (logkw) and a training set with known logP ci values. To assess the most performing procedures, the three hydrophobicity indexes (( )o, CHI and logkw) were compared on the basis of the solvation equation [41]. These parameters were significantly inter-related with each other, but not identical. Each parameter was related to log P with values between 0.76 and 0.88 for the 55 tested compounds fitting quality associated with the compound nature. 

Du, C. M., Valko, K., Bevan, C., Reynolds, D., Abraham, M. H. Rapid gradient RP-HPLC method for lipophilicity determination a solvation equation based comparison with isocratic methods. Anal. Chem. 1998, 70, 4228-4234. 

Abraham, M. H., Chadha, H. S., Applications of a solvation equation to drug transport properties, in Lipophilicity in Drug Action and Toxicology. Pliska, V., Testa, B.,... 

The retention depends on the nature of both the stationary phase and the organic modifier in the mobile phase. Therefore CHI values obtained using different systems show different sensitivities towards solute characteristics. This has been studied systematically and used for the quantitative calculation of solute molecular descriptors (H-bond donor capacity, H-bond acceptor capacity and dipolarity/polarizability) for application in a general solvation equation [21]. 

Vs,mm and Vs are negative, this means that each term except e is promoting solvation. Equation (10) is one of the few GIPF relationships that include Vmin, the overall most negative value of the electrostatic potential. The correlation coefficient for Eq. (10) is R = 0.988 the average absolute deviation from experiment is 0.27 kcal/mole for AGsoivation values varying over 9.59 kcal/mole. 

One of the most comprehensive hydrogen-bonding scales is due to Abraham and his coworkers50, who have derived the general solvation equation 651... 

Abraham et al. [2] published a number of papers in which they analyzed Young s data set using MLR and gave a general solvation equation in which various solvent-solute interactions were described by solute descriptors and equation coefficients (Eq. 16)... 

The free energy of activation in the continuum theory is based on the prequantal Born continuum solvation equation " for ions in solution,... 

The Born solvation equation is based on the difference in the energy needed to charge a sphere of radius r,- in a solvent of dielectric constant e, and in vacuum having a dielectric constant of unity. Thae are basic flaws in the concept of the Born solvation equation (5) on which the continuum theory of ET reactions is based. First, Bom Eq. (5) does not take into account the interaction of ions with a water solvent that has a dielectric constant of approximately 80 at room temperature. Hence, the Born solvation energy will have negligible contribution from solvents with high dielectric constants. Consequently, for solvents of high dielectric constant, Eq. (5) can be written as... 

These points indicate that the continuum theory expression of the free energy of activation, which is based on the Born solvation equation, has no relevance to the process of activation of ions in solution. The activation of ions in solution should involve the interaction energy with the solvent molecules, which depends on the structure of the ions, the solvent, and their orientation, and not on the Born charging energy in solvents of high dielectric constant (e.g., water). Consequently, the continuum theory of activation, which depends on the Born equation,fails to correlate (see Fig. 1) with experimental results. Inverse correlations were also found between the experimental values of the rate constant for an ET reaction in solvents having different dielectric constants with those computed from the continuum theory expression. Continuum theory also fails to explain the well-known Tafel linearity of current density at a metal electrode. ... 

Several computational methods require the user to input experimental parameters in order to make predictions. Examples include Yalkowsky s general solvation equation [18], which requires melting points, and Abraham s method [19], which requires five experimentally derived parameters. These methods are not useful for cases before the compounds are synthesized (e.g., in library design or virtual screening), and so they will not be considered further here. 

What is the Born solvation energy of Fe3+ in water having a dielectric constant of 78, and what is its value when it is in ethanol, which has dielectric constant of 36 (b) What is the self-energy of Fe3+ in a vacuum (c) Is there any meaningful difference between the Born solvation energy and the self-energy of an Fe3+ ion The radius of this ion is 0.64 A and the diameter of water is 2.76 A. (c) Where do you think there is a possible flaw in Born s solvation equation (Khan)... 

This is because the definition of Eq. 4.21 seems not to apply in this case. Thus, for methane, with log Pcydohex. = 1-33 and VM = 30.3 cm3, the log BB value calculated by Eq. 4.20 is 1.62 log units but the real value is only 0.04 log units [26], Therefore, the authors attempted to evaluate systematically those factors influencing the distribution of solutes between blood and brain, starting with their general solvation equation ... 

Equation (3.42), the general solvation equation, can also be used to predict the partition coefficient ... 

Description of various lipophilicity scales hy molecular descriptors (solvation equations)... 

A linear free-energy relationship has been suggested 113 by Abraham to describe various partition processes of molecules. Eq. (12.14) shows the so-called linear solvation equation ... 

THE RELATIVE COEFFICIENTS OF THE SOLVATION EQUATIONS OBTAINED FOR SEVERAL DISTRIBUTION SYSTEMS... 

The power of the solvation equation approach is that it can characterise various partition systems and lipophilicity scales by five molecular descriptors (dispersivity, size, polarisability-dipolarity, H-bond acidity and basicity). All of the five descriptors should be included into the linear regression equations by definition. When one or two coefficients are not significantly different from zero, it means that that particular property does not play a significant role in the partition. 

Similar to the blood-brain barrier distribution there is a large number of biochemical and toxicological proces.ses that involve aqueous solutes interacting with a given system. In principle, the general solvation equation. Eiq. (12.14), could be applied to any such... 

A well-designed compound set that has a wide variety of known descriptors (preferably without inter-correlation) can be used to set up the solvation equation in a particular partition system. The coefficients obtained for the equation will characterise the system. 

THE NORMALISED REGRESSION COEFFICIENTS OF THE SOLVATION EQUATIONS SET UP FOR THE GRADIENT RETENTION DATA (CHI) OBTAINED ON SEVERAL STATIONARY PHASES... 

In conclusion, the solvation equation could be applied to describe several liquid chromatographic partition systems in terms of their sensitivity towards molecular properties. The standard error for estimating the retention data was low and the multiple correlation coefficients of the solvation equations were high. The parameters of the solvation equations help us to understand and describe the different selectivity of the stationary phases, and also to understand the retention of the compounds in a given system based on its molecular properties. 

It is somewhat surprising that the solvation equation worked on the estimation of solubility, although it was originally set up to describe liquid-liquid partition. There... 

THE NORMALISED REGRESSION COEFFICIENTS OF THE GENERAL SOLVATION EQUATION OBTAINED FOR THE INVESTIGATED HPLC SYSTEMS... 

THE CX3EFFICIENTS OF THE GENERAL SOLVATION EQUATION, EQ. (12.14). OBTAINED FOR SELECTED DIVERSE CHROMATOGRAPHIC STATIONARY PHASES... 

Fig. 12.19) that seemed to have the widest range of coefficients for the molecular descriptors in the solvation equation. Table 12.10 shows the solvation equations obtained and their normalised coefficients. The equations were obtained from the chromatographic hydrophobicity index data (CHI) of the compounds listed in Table 12.7 by using a 2.5 min linear gradient of the oi anic solvent from 0 to 1009F with 2 ml/min flow rate. The columns used were 4.6 x 50 mm short columns. The application of this fast gradient method makes it possible to obtain CHI values in a given HPLC system in 5 min. 

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