H-bond basicity

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] ... 

As the solute descriptors (E, S, A, B and V) represent the solute influence on various solute-solvent phase interachons, the regression coefficients e, s, a, h and V correspond to the complementary effect of the solvent phases on these interactions. As an example, consider the product aA in Eq. (4). Since A is the H-bond acidity of the solute, a is the H-bond basicity of the system. In other words, the intermolecular forces discussed in Sections 12.1.1.2 and 12.1.1.3 are present in all Abraham s log P factorization equations, with the exception of those interactions involving ions. This is the reason why Abraham s equahons are valid for neutral species only. 

In this equation E (R2) is the excess molar refraction, S (tt ) is the solute dipolarity-polarizabiUty, A (2a ) and B(2 3 ) are the solute H-bond acidity and basicity, respectively, and Vis the McGowan characteristic volume (in cm mol /100). The solute size, V, (molecule favors octanol) together with solute H-bond basicity, B, (favors water) are the dominating parameters of this equation. The use of Bo(2P ) resulted in equation... 

Valko et al. [37] developed a fast-gradient RP-HPLC method for the determination of a chromatographic hydrophobicity index (CHI). An octadecylsilane (ODS) column and 50 mM aqueous ammonium acetate (pH 7.4) mobile phase with acetonitrile as an organic modifier (0-100%) were used. The system calibration and quality control were performed periodically by measuring retention for 10 standards unionized at pH 7.4. The CHI could then be used as an independent measure of hydrophobicity. In addition, its correlation with linear free-energy parameters explained some molecular descriptors, including H-bond basicity/ acidity and dipolarity/polarizability. It is noted [27] that there are significant differences between CHI values and octanol-water log D values. 

Product of overall H-bond acidity and basicity (Absolv) Molecular polarizability in A3 (HYBOT) [23] also Alpha Overall H-bond basicity (Absolv) also as Sp 1 [8]... 

FIGURE 9.7 Variation of Kamlet-Taft solvatochromic parameters for methanol/C02 mixtures as a function of added CO2 at 298 K and 17.2 MPa (A)n (dipolarity/polarizability, ( ) (H-bond acidity), and (T)/8 (H-bond basicity). (Adapted from Y. Cui, S. V. Olesik, Anal. Chem., 63 1812(1991).)... 

Barbour and Petersen (1974) observed strong H-bonding basicity of asphaltenes with phenol, having enthalpy of interactions between 6 and 8 kcal/mole. Phenol-base interactions for ir aromatic bases are typically as low as 0.5 kcal/mole but can reach 9 kcal/mole for nitrogen- or sulfur-containing molecules (Barton et al., 1972 Osawa et al., 1967 Whetsel, 1968). 

Using a solver technique (we have used the solver add-in in Microsoft Excel 6.0 ), one can calculate the 3 molecular descriptors (H-bond acidity, H-bond basicity and polarisability-dipolarity). Plass et al. 1122] published the molecular descriptors of tripeptide derivatives based on the above-described method. Although reasonably sensible data were obtained, the method has not yet been validated on a large number of... 

Using the measured CHI data of the model compounds we can set up a correlation equation to express the H-bond acidity (a), H-bond basicity (f) and the polarisability-dipolarity (n) parameters by the CHI values obtained on a particular stationary phase with a particular mobile phase. The plot of the database descriptors as a function of the estimated ones based on the measured CHI values can be seen in Figs. 12.20-12.22. The best equations for the estimation of the descriptors are also shown in the figures. It can be seen that the CHI values obtained on Luna C-18 columns with acetonitrile and trifluoroethanol gradients are used for the fi and the ir calculations. The CHI values... 

P is the effective H-bond basicity, a summation of the basicity from all H-bonds... 

More recently, Laurence et al. reported the development of the pA Hx H-bond basicity scale based on a set of 1338 experimental values related to 1164 HBAs [32], This approach uses the H-bonding free energies determined in CCl for a large number of chemically diverse H-bond acceptor molecules using 4-fluorophenol as the reference donor. The p bhx meaning similar to the log AT H-bond... 

Laurence and coworkers (Lanrence et al., 2014) emphasize the distinction between basicity parameters of bnik solvents and those derived from measurements on substances in solntion in inert solvents. In the same paper, they discnss some difficulties in the assignment of basicity parameters based on solvatochromism, such as the Taft-Kamlet p, to amphiprotic solvents, and propose another measure of H-bond basicity, p, based on the difference between the NMR chemical shifts of 4-fluorophenol and... 

Several biocatalytic reactions have been investigated in ILs or IL-based systems. From these studies, a significant amount of information has been accumulated regarding the physicochemical properties of ILs (viscosity, polarity, hydrophobicity, nucleophUicity, H-bond basicity, and kosmotropicity/chaotropicity) that affect the activity and selectivity of enzymes. However, one should keep in mind that physical and chemical characteristics of ILs can be affected by several factors, for instance, by the presence of impurities (halide ions, acids, residual solvents, etc.) [12], which can further affect the enzyme activity and ultimately the performance of biocatalytic transformations [13-15]. 

In contrast with the above findings in water, condensations in chloroform exhibit a considerable specificity of the base used as a catalyst, as shown by two model reactions ethyl nitroacetate with norbornene and benzoylnitromethane with styrene [65]. A screening of several organic bases (tertiary amines and heteroaromatic A-bases with one or two basic sites) applied to these reactions indicates that the results are not related to the base strength. In fact, the most effective bases are those with the highest H-bonding basicity [74], particularly those having two basic sites, like DABCO, NMI, etc. [75-77]. 

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