Hydrogen bonding and acidity

Where FCl is the solute gas-liquid partition coefficient, r is the tendency of the solvent to interact through k- and n-electron pairs (Lewis basicity), s the contribution from dipole-dipole and dipole-induced dipole interactions (in molecular solvents), a is the hydrogen bond basicity of the solvent, b is its hydrogen bond acidity and I is how well the solvent will separate members of a homologous series, with contributions from solvent cavity formation and dispersion interactions. 

The difference in the hydrogen bond acidities and basicities was far more marked. The a value is largely determined by the availability of hydrogen bond donor sites on the cation. Values range from 0.8-0.9 for the monoalkylammonium salts, and are slightly lower (0.3-0.8) for the imidazolium salts. In the absence of a... 

Overall, we have shown that family-independent correlations can be obtained for solute hydrogen bond acidity and basicity, as quantitated by a 1 and log KHB. These are well represented at the HF/6-31G level by an electrostatic potential term alone, V or V, . , respectively (Hagelin et al. 1995). 

Attempts to evaluate the effective solute hydrogen bond acidity and basicity40 produce quantitative scales of solute hydrogen-bonding, which may be of interest for application also to biochemical processes41. The indicator pair 4-nitroanilinc/4-nitro-/V,/V-dimethylaminoaniline are used to state the equivalence42 of solute and solvent scales of hydrogen bond basicity. 

The accurate determination of gas-phase basicities and gas-phase acidities opened the way to analyses of the effect of solvation on proton acidities, and on hydrogen-bond acidities and basicities, as well as on substituents effects. 

Abraham s solute descriptors to yield a predictive regression equation. Further, the solute dipolarity/polarizability, hydrogen bond acidity, and hydrogen bond basicity were found to favor blood and solute size favor brain. 

Our detailed knowledge of the mechanism of proton transfer from intramolecularly hydrogen-bonded acids and our understanding of the factors that affect the strength of intramolecular hydrogen bonds have now reached a stage when correlations of the proton-transfer behaviour with the nature of the hydrogen bond will soon be possible. 

One very important use of E and C numbers is the calculation of heats of interaction for systems which have not been examined experimentally. From our knowledge of the standard deviations of the parameters and their correlation coefficients, we have calculated the expected standard deviations for calculated heats for all possible combinations of all but a few of the acids and bases listed in Tables 3 and 4. For the hydrogen bonding acids and sulfur dioxide, these predicted standard deviations nearly all lie between 0.1 and 0.3 kcal/mole. For other systems with much larger heats, the errors are somewhat worse than this averaging around 0.7 kcal mole-i. 

Zhao and coworkers [53] also constructed a linear model using the Abraham descriptors. The MLR model possesses good correlation and predictability for external data sets. In this equation, E is an excess molar refraction (cm3/mol/ 10.0) and S the dipolarity/polarizability, A and B are the hydrogen bond acidity and basicity, respectively, and V is the McGowan characteristic volume (cm3/ mol/100). The large coefficients of A and B indicate too polar molecules having poor absorption. 

Here E is the solute excess molar refractivity, S is the solute dipolarity/ polarizability A and B are the overall or summation hydrogen-bond acidity and basicity, respectively and V is the McGowan characteristic volume lower-case letters stand for respective coefficients which are characteristic of the solvent, c is the constant. By help of sfafisfical methods like the principal component analysis and nonlinear mapping, the authors determined the mathematical distance (i.e., measure of dissimilarify) from an IL fo seven conventional solvents immiscible with water. It appears that the closest to the IL conventional solvent is 1-octanol. Even more close to IL is an aqueous biphasic system based on PEG-200 and ammonium sulfate (and even closer are ethylene glycol and trifluoroethanol, as calculated for hypofhefical water-solvenf sysfems involving fhese solvenfs). 

During the last 15 years, Abraham and his co-workers have established a set of five descriptors for the general description of logarithmic partition coefficients by linear regression. Their so-called linear free energy relationship (LFER) descriptors E, S, A, B, and V are effective parameters for the polarizability, polarity, hydrogen-bond acidity and basicity, and volume of the solute molecules, respectively [113-116]. They are mainly derived from experimental refraction and partition coefficients of the solutes. 

At the other end of the two-component spectrum are tectons that possess mutually complementary shapes or functionality e.g. hydrogen bond acid and base), which crystallise together preferentially... 

Fig. 7 Hydrogen bond acidity and vertical pK. Vertical pA a values show a reasonable correlation with Abraham s hydrogen bond acidity parameter, while the equilibrium pKa values show much more scatter. ( ) Points used in the least squares fit, ( ) vertical pATa values for carboxylic acids not used in the least squares fit, and ([>) equilibrium pK.A values not used in the least squares fit.

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