Lewis basicity definition

The surface basicity of a solid catalyst can be defined in a way analogous to that applied to conventional bases. Thus, a surface Lewis base site is one that is able to donate an electron pair to an adsorbed molecule. If we take the definition of surface basicity in a more general way, it could be said that the active surface corresponds to sites with relatively high local electron densities. This general definition will include not only Lewis basicity but also single electron donor sites. We emphasize that the literature of heterogeneous catalysis often reports that both single-electron and electron-pair donor sites exist on basic catalysts. 

This definition is completely independent from water as the reaction medium and is more general than the previous ones. In terms of Lewis acidity, the Br0nsted-type acid HA is the resnlt of the interaction of the Lewis-type acid species H+ with the base A . According to the definitions given, Lewis-type acids (typically, but not only, coordinatively unsatnrated cations) do not correspond to Br0nsted-type acids (typically species with acidic hydroxyl groups). On the contrary, Lewis basic species are also Br0nsted bases. 

Each of the three definitions expands our concept of acids and bases. Arrhenius basic definition is adequate for understanding many of the properties of acids and bases. It is important to recognize, though, that acids and bases are not fixed labels that can be applied to a substance. Bronsted-Lowry and Lewis showed that acid-base characteristics are dependent on the reactions that take place between substances. A... 

Physical organic chemists have tended to examine parameters based on shifts in the absorption peaks in the spectra of various dyes or indicator molecules. The a and P scales of Taft and Kamlet, the ET(30) scale of Dimroth and Reichardt, the 7t scale of Taft and co-workers and the Z value of Kosower are all examples of this type of parameter. The definitions and measurement means for these parameters, as well as important references, are shown in Table 5. An alternative definition of the Dimroth-Reichardt parameter is the dimensionless, ETN, which is now preferred by some organic chemists (for a discussion see Ref. 15). The Z value is important in that it led to the scale of Dimroth and Reichardt, which overcomes many of the limitations of the earlier scale. Several workers have shown that relationships exist, with good correlation coefficients, between similar parameters. Thus, DN is linearly related to p, both parameters being designed to measure the donor properties (or Lewis basicity) of solvent molecules. Also, Lr(30) is related to a as well as to AN all three parameters purport to measure the electron acceptor properties (or Lewis acidity) of solvent molecules. It has been found that different solvent types have different coefficients in linear relationships between n and the dipole moment. The Taft and Dimroth-Reichardt parameters, in particular, have been found to correlate with free energies and... 

Though the strong aurophilicity of gold which leads to cluster formation may not be described by Lewis basicity in the classical definition, the formation of clusters containing direct Au-Au interactions falls within the scope of the relativistic discussion in this chapter161a. 

As we will learn later in this section, these processes are practically very important. The sites of highest catalytic reactivity are often the edged corner positions. The concentration of such sites is enhanced by Freund s adaption processes. The effective charges on the broken-bond surfaces are such that they induce Lewis acid- or Lewis basic-type reactivity features. The local charge excesses on an ionic surface, considered to exist as a series of point charges, can be estimated using Pauling s valency definitions 1 1. 

Lewis Basicity and Affinity Measurement Definitions and Context... 

Two definitions of acids and bases are used nowadays, the Bronsted definition and the Lewis definition. This book deals with the quantitative behaviour of Lewis bases. However, since Lewis bases are also Bronsted bases, this chapter begins with a short presentation of the Bronsted definition and of the quantitative behaviour of Bronsted bases [1]. The Lewis definition and the many ways for its quantification will then be studied. This introductory chapter is intended to help in the understanding and use of the tables in Chapters 2-6, which contain quantitative data on Lewis basicity and affinity, and not to discuss the Lewis acid/base concept in depth. This subject has been excellently treated in a book [2] and a review [3] by Jensen, and books and chapters by Mulliken and Person [4], Gur yanova et al. [5], Drago [6], Finston and Rychtman [7] and Weinhold and Landis [8], to quote just a few. 

A Lewis basicity scale can be constructed from this set of equilibrium constants. The basicity scale may be expressed as K, or as pAi or as AG. In order that the definition and... 

Lewis Basicity and Affinity Measurement Definitions and Context 43 Table 1.15 Lewis basicity of halide ions in aqueous solution. ... 

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