Acid-base concepts ionization potential

The second method considers the surface acidity to result from the electron acceptor characta- of the oxide surface (29). This is related to the Lewis acid-base concept where, for an ionic oxide, the acid entity is the cation with the base being the oxygen anions. For an oxide M ,Oy, the surface acidity has been shown to be related to the ionization potential (IP) of the metal M according to... 

Yokoyama and coworkers investigated the discolouration of titanylphthalocyanine 60, and related porphyrins by silyl radicals derived from methylphenylpolysilane by molecular orbital methods60. Calculated values of AN, the index derived from the hard and soft acids and bases concept was determined from HF/3-21G calculated ionization potentials, electron affinities, and HOMO and LUMO orbital energies of the systems of interest. These studies predict that PhMe2Si should donate an electron readily to 60 to form 60 (equation 18). This prediction is of direct relevance to observations that irradiation of titanylphthalocyanine-coated methylphenylpolysilane films leads to discolouration of the film60. 

The most immediate reason for Equation (2.12), however, was that it agreed with the chemical observations that led to the concept of hard and soft acids and bases. For example, consider Ca and Fe. The third ionization potential, A, and the second, I2, would be / and A, respectively (Table 2.1). Accordingly, Fe + is much softer than Ca , as expected. Also Ca is more EN than Fe , meaning that it is much less likely to find Ca + than Fe . Similar results are found or almost all the metal ions. 

Internal and External Phases. When dyeing hydrated fibers, for example, hydrophUic fibers in aqueous dyebaths, two distinct solvent phases exist, the external and the internal. The external solvent phase consists of the mobile molecules that are in the external dyebath so far away from the fiber that they are not influenced by it. The internal phase comprises the water that is within the fiber infrastmcture in a bound or static state and is an integral part of the internal stmcture in terms of defining the physical chemistry and thermodynamics of the system. Thus dye molecules have different chemical potentials when in the internal solvent phase than when in the external phase. Further, the effects of hydrogen ions (H" ) or hydroxyl ions (OH ) have a different impact. In the external phase acids or bases are completely dissociated and give an external or dyebath pH. In the internal phase these ions can interact with the fiber polymer chain and cause ionization of functional groups. This results in the pH of the internal phase being different from the external phase and the theoretical concept of internal pH (6). 

Denmark has also made significant contributions to the evolution of this chemistry by introducing the concept of Lewis base activation of Lewis acids. Its validity and synthetic potential were demonstrated first by achieving the highly enantiose-lective allylation and propargylation of aldehydes with 2 as an optimal catalyst as exemplified in Scheme 7.4 [5]. The key to this asymmetric catalysis is the generation of a chirally modified and activated silicon Lewis acid by the 2-promoted ionization of the weakly Lewis acidic SiCLj. 

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