Hydrogen Bonding and Charge-Transfer Complexing

Essential to a discussion of hydrogen bonding is tire concept of electronegativity. According to valence-bond theory, the atoms which combine to form a molecule share electrons. If tire bonded atoms are identical (e.g., the Cl atoms in a CI2 molecule), tire bonding electrons 

Statistical Mechanics for Thermophysical Property Calculations, sec. 11.5 and app. 11, PTR Prentice Hall, Englewood Cliffs, NJ, 1994. 

The highly electronegative elements F, O, and N serve as atomic hydrogen-acceptor sites. Hence HF, HOH, HOOH, ROH, RCOOH, H NH, R2NH and NCH are hydrogen acceptors. But so are aldehydes (ROCH), ketones (ROCR), ethers (ROR), esters (ROCOR), and tertiary amines (R3N), species which have no active hydrogens. 

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Theoretical Predictions of the Infrared Spectra of Hydrogen-Bonded and Charge Transfer Complexes Ammonia and Boron Trifluoride Dimers. ... 

This equation shows that even for uncharged molecules with no net dipole moment may be significant owing to the quadrupole term. A detailed treatment of the theory has been presented by Abraham and Bretschneider (1974). The reaction-field model has been tested for a number of conformational equilibria, and usually gives excellent results, but is limited to solutions in which no specific interaction exists between solute and solvent, such as hydrogen bonding and charge-transfer complex formation. Thus water and alcohols are excluded, and aromatic solvents such as benzene and toluene also often show anomalous behaviour. Solvent mixtures can in principle be treated by the theory but such a treatment is usually avoided. 

The substituents in a benzene derivative may affect the rate of electrophilic attack at the ortho position by steric interaction and secondary bonding (e.g., hydrogen bonding or charge-transfer complexing) as well as by electrical influence. Therefore ortho substitution is less satisfactory. (See Section 2.2, p. 61 and Figure 2.2.)... 

Ward and Upchurch (340) found an inverse relationship between the solubilities of 18 substituted anilines and their adsorption by nylon (R2 = 0.77) and cellulose triacetate (R = 0.80). It was suggested that steric and electronic effects of the molecules were also responsible for the adsorption differences of the compounds. Because of the low solubilities of the substituted anilines, adsorption by organic matter probably occurs at lipophilic sites through dipole-dipole interactions—e.g., hydrogen bonding or charge-transfer complexes. Low adsorption at clay surfaces probably occurs because the hydrophobic molecules do not readily associate with hydrated surfaces. Adsorption of the substituted anilines by dry clays might occur on the soil surface for instance, but in the... 

The interaction that occurs between the solvent and the transition state is sometimes described in terms of specific and nonspecific, depending on the nature of the interaction. Specific interaction refers to hydrogen bonding or charge transfer complexation. Nonspecific interaction is the result of general attraction due to van der Waals forces. Some of the correlations that have been devised are restricted to only nonspecific solvation of the transition state by the solvent. 

Solubility of some substances in the supercritical fluids, among different parameters, mostly depends on the vapor pressure, the substance polarity, and substance molar mass. Compounds of smaller molar mass and a higher vapor pressure at supercritical condition are more soluble in SFs, compared to other with lower vapor pressure and higher molar mass. An enhancement factor (dimensionless parameter) is defined as the ratio of solubility of substances in the SF (solvent) compared to its solubility in the ideal gas. Usually, this parameter has the common value between 10" and 10 [3]. Different mechanisms reported in the literature have been used for explaining the enhancement of solute solubility in supercritical fluids. They included the hydrogen bonding, the charge transfer complex formation, dipole-dipole noninduced and induced interactions, and solute-solvent with and without cosolvent interactions. 

Cooperation of hydrogen-bond and charge-transfer interactions in molecular complexes in the solid state 13BCJ183. 

We have referred to the influence of hydrogen bonding in one-component systems and mentioned the two-component system benzoic acid-pyridine (108). A variety of acid-base systems in addition to the latter are known to give 1 1 complexes. Pfeiffer gives in his book (88) a wealth of information from the older literature on such complexes, as well as on two-component organic-inorganic systems and charge-transfer complexes. 

Results obtained by Brillouin scattering range from the determination of the elastic and photoelastic constants of materials to the analysis of material transformations phase transitions, polymerization, glass transitions, pho-toinduced transformations, etc. (It is out of the scope of this presentation to present all.) We will limit our discussion to some examples selected in the field of supramolecular products defined as complexes consisting of two or more chemical entities associated through van der Waals interactions, hydrogen bonds, or charge-transfer mechanisms. " ... 

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