Acid-base interactions measurement Lewis acidic properties

A study of the interaction of Lewis acids and bases (or electron acceptors and donors) in surface dynamics has led to new insight into interactions with various solid surfaces [26,64,99,100,104,110,130-142], as well as interactions at interfaces between two different substances. It is noted that the acid-base interactions of Lewis, including the orientational properties of charge transfer forces of Mulliken [143], occur between specific (or polar) groups in substances. These interactions are quite dependent on the Stockmayer degree of polarity, <5, [126] as measured by dipole moment in Eq. (58). Furthermore, it can be found that a concept of acids attract bases may be substituted... 

Because of the rather localized negative charge at the phenoHc oxygen atom , the standard dye (44) is capable of specific HBD/HBA and Lewis acid/base interactions. Therefore, in addition to the nonspecific dye/solvent interactions, the betaine dye (44) predominately measures the specific HBD and Lewis acidity of organic solvents. On the other hand, the positive charge of the pyridinium moiety of (44) is delocalized. Therefore, the solvent Lewis basicity will not be registered by the probe molecule (44). If this solvent property is relevant for the system under study, other empirical measures of Lewis basicity should be used cf. Section 7.7. 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

Inverse gas chromatography involves the sorption of a known probe molecule (adsorbate, vapour) and an unknown adsorbent stationary phase (solid sample). IGC may be experimentally configured for finite or infinite dilution concentrations of the adsorbate. The latter method is excellent for the determination of thermodynamic properties such as surface energies and Lewis acid-base parameters. Measurements in this range are extremely sensitive due to the low concentration regime where the highest energy sites of the surface interact with the probe molecules. 

Among the cosolvents studied, methanol and acetone have received the greatest interest (36-38). Methanol may act as either a Lewis acid or a Lewis base while acetone is a weaker Lewis base and very slightly acidic ( ). The dipole moment of acetone is 2.88 Debeye compared to 1.7 Debeye for methanol. Based on these properties, Walsh, et al., (40), interpret the data of Van Alsten (37) and Schmitt (38) and present liquid phase IR measurements which show Lewis acid-base interactions in the systems methanol/acridine and acetone/benzoic acid. Supercritical solubility data of Dobbs, et al., ( ), exhibit trends which indicate the importance of acid-base interactions. Van Alsten and Schmitt present data which show that acid-base interactions are a secondary cosolvent effect superimposed on a primary effect determined by cosolvent concentration. 

Empirically measured parameters are additional solvent properties, which have been developed through the efforts of physical chemists and physical organic chemists in somewhat different, but to some extent related, directions. They have been based largely on the Lewis acid base concept, which was defined by G. N. Lewis. The concept originally involved the theory of chemical bonding which stated that a chemical bond must involve a shared electron pair. Thus, an atom in a molecule or ion which had an incomplete octet in the early theory, or a vacant orbital in quantum mechanical terms, would act as an electron pair acceptor (an acid) from an atom in a molecule or ion which had a complete octet or a lone pair of electrons (a base). Further developments have included the concepts of partial electron transfer and a continuum of bonding from the purely electrostatic bonds of ion-ion interactions to the purely covalent bonds of atoms and molecules. The development of the concept has been extensively described (see Ref. 11 for details). 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

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