Hydrogen bond acidic experimental

The first experimental data for a reaction involving proton transfer from a hydrogen-bonded acid to a series of bases which were chosen to give ApK-values each side of ApK=0 are given in Fig. 15 (Hibbert and Awwal, 1976, 1978 Hibbert, 1981). The results were obtained for proton transfer from 4-(3-nitrophenylazo)salicylate ion to a series of tertiary aliphatic amines in aqueous solution, as in (64) with R = 3-nitrophenylazo. Kinetic measurements were made using the temperature-jump technique with spectrophoto-metric detection to follow reactions with half-lives down to 5 x 10"6s. The reciprocal relaxation time (t ), which is the time constant of the exponential... 

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. 

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. 

HF/6-31G computed Vs ax 3iid experimental hydrogen bond acidities... 

The master retention equation of the solvation parameter model relating the above processes to experimentally quantifiable contributions from all possible intermolecular interactions was presented in section 1.4.3. The system constants in the model (see Eq. 1.7 or 1.7a) convey all information of the ability of the stationary phase to participate in solute-solvent intermolecular interactions. The r constant refers to the ability of the stationary phase to interact with solute n- or jr-electron pairs. The s constant establishes the ability of the stationary phase to take part in dipole-type interactions. The a constant is a measure of stationary phase hydrogen-bond basicity and the b constant stationary phase hydrogen-bond acidity. The / constant incorporates contributions from stationary phase cavity formation and solute-solvent dispersion interactions. The system constants for some common packed column stationary phases are summarized in Table 2.6 [68,81,103,104,113]. Further values for non-ionic stationary phases [114,115], liquid organic salts [68,116], cyclodextrins [117], and lanthanide chelates dissolved in a poly(dimethylsiloxane) [118] are summarized elsewhere. 

Figure 3.17 (Top) Crystal structure of maleic acid, 137 displaying both inter- and intramolecular hydrogen bonding. (Bottom) Experimental and spectral fitting MAS spectra of maleic acid at 14.1 T.

The three parameters that are central to the method are tt, a, and The tv parameter provides a comprehensive measure of the ability of a solvent to stabilize a solute molecule based on the dielectric effects. It is a quantitative index of solvent dipolarity and polarizability. The acidity a for a solvent is a measure of its strength as a hydrogen-bond donor HBD, its ability to donate a proton in a soIvent-to-solute hydrogen bond. The estimation of a is based on the experimental determination of n and t(30). The t(30) scale, developed by Reichardt et al., indicates a solvent strength by combining polarity and HBD acidity, which itself serves as a useful solvent parameter for physicochemical correlations in a wide range of solvents. ° The basicity parameter denotes the solvent s hydrogen-bond acidity HBA or an index of the solvent s ability to accept a proton in a... 

Alternating values of E and of In A were found for n = 0 to 4 and increasing values above n = 5. This observation was correlated with the hydrogen-bonding ability of these acids E+ is proportional to the concentration of associated carboxy groups19. Their experimental observations are, to some extent, in agreement with the results from Mat-suzaki and Mitani268. ... 

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