Acid-base interactions/pairing

X = number of moles of acid-base interaction pairs per unit area of interface... 

Acid-base interactions in the most general Lewis sense occur whenever an electron pair from one of the participants is shared in the formation of a complex, or an adduct . They include hydrogen bonding as one type of such a bond. The bond may vary from an ionic interaction in one extreme to a covalent bond in the other. Acid-base interactions and their importance in interfacial phenomena have been reviewed extensively elsewhere [35,78] and will be described only briefly here. 

Click Chemistry Interactive for the self-study module acid-base conjugate pairs. 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

Steric interactions between bulky substituents such as t-Bu, leading to larger C-E-C bond angles, obviously affect the Lewis basicity caused by the increased -character of the electron lone pair. However, the strength of the Lewis acid-base interaction within an adduct as expressed by its dissociation enthalpy does not necessarily reflect the Lewis acidity and basicity of the pure fragments, because steric (repulsive) interactions between the substituents bound to both central elements may play a contradictory role. In particular, adducts containing small group 13/15 elements are very sensitive to such interactions as was shown for amine-borane and -alane adducts... 

In the same year, Lewis proposed an alternative definition According to Lewis, an acid is ary species that can accept a pair of electrons because of the presence of an incomplete electronic grouping. Hence, a Lewis base is any species that possesses a nonbonding electron pair that can the donated to form a coordination or dative bond. The Lewis acid-base interaction is given by... 

Basic Mechanisms of Adhesion Acid-Base Interactions. The understanding of polymer adhesion has been greatly advanced in recent years by the recognition of the central role of acid-base interactions. The concept of an acid was broadened by G. N. Lewis to include those atoms, molecules, or ions in which at least one atom has a vacant orbital into which a pair of electrons can be accepted. Similarly, a base is regarded as an entity which possesses a pair of electrons which are not already Involved in a covalent bond. The products of acid-base interactions have been called coordination compounds, adducts, acid-base complexes, and other such names. The concept that... 

The participation of the germanium dimers in nucleophilic/electrophilic or Lewis acid/base reactions has been the subject of several investigations on the Ge(100)-2x1 surface [16,49,255,288,294,313-318]. As for the case of silicon, adsorption of amines has provided an excellent system for probing such reactions. Amines contain nitrogen lone pair electrons that can interact with the electrophilic down atom of a tilted Ge dimer to form a dative bond via a Lewis acid/base interaction (illustrated for trimethylamine at the Si(100)-2 x 1 surface in Ligure 5.17). In the dative bond, the lone pair electrons on nitrogen donate charge to the Ge down atom [49]. 

The hypothesized delocalization of lone pair electrons in the above silicon compounds is supported by the lowered basicity of the silyl compounds as compared to the corresponding carbon compounds. This reduced basicity is contrary to that expected on the basis of electronegativity effects operating through the a system since silicon is less electronegative than carbon. It is consistent with an internal Lewis acid-base interaction between the nitrogen and oxygen lone pairs and empty acceptor d orbitals on the silicon. Experimentally this reduced basicity is shown by the absence of disiioxane adducts with BF3 and BO ... 

The molecules or ions that surround the central metal ion in a coordination compound are called ligands, and the atoms that are attached directly to the metal are called ligand donor atoms. In cisplatin, for example, the ligands are NH3 and Cl-, and the ligand donor atoms are N and Cl. The formation of a coordination compound is a Lewis acid-base interaction (Section 15.16) in which the ligands act as Lewis bases (electron-pair donors) and the central metal ion behaves as a Lewis acid (an electron-pair acceptor). 

Lewis has defined acids and bases in a general way as electron acceptors and donors, respectively (21). Accordingly, a compound or element capable of accepting electrons (electron seeking) is termed a Lewis acid. Conversely a compound or element capable of giving (or sharing electrons) is a Lewis base. In other words, those elements which are deficient in electrons --that is, have unfilled electron shells -- will seek out those elements carrying extra electrons (lone pairs). This is the basis for the Lewis concept of "acid -base interaction. Thus ... 

The structure of the DNA molecule is basically determined by nucleic acid base interactions. Although the three-dimensional double helix structure of DNA is influenced by various contributions, the hydrogen bonding in DNA base pairs is of particuar importance. Because it is difficult to obtain gas phase experimental data for isolated base pair characterisation (only a limited number of experimental studies are available [21]) quantum chemical calculations can represent a useful tool to obtain reference data on the structure, properties and interactions of nucleic acid pairs. Theoretical studies can help us to understand the properties of nucleic acids and they are fundamental for verification... 

Metal ions can be considered as Lewis acids, with the formation of complexes rationalized in terms of Lewis acid-base interactions. This interaction can be visualized in terms of the ability of a metal atom or ion M to accept a pair of electrons (and thus act as a Lewis acid) from a ligand X, which is an electron donating base with an accessible lone pair of electrons, i.e.,... 

Dienes and heterodienes differ in the regioselectivity of their cycloadditions. In case of dienes they are controlled by substituent effects, in case of heterodienes by the strongest primary Lewis acid-base interaction [silylene to lone-pair of heteroatom with O > N (Scheme 3)]. 

Thus, the bonding in metal compounds and complexes has traditionally been viewed as ionic, with a positive metal center interacting with negative ions, such as HO , 0 , Cl , AcO , and coordinate donor, Lewis acid-base interactions with a positive metal center interacting with negative ions and electron-pair Lewis bases, such as iNHs, iPPhs, HOH. Examples of ionic versus covalent bonding illustrate the tradition H+CH versus H-Cl (ls-3p), C +(C1 )4 versus C(-C1)4 [2sp -(3p)4], Fe +(C1 )3 versus Fe(-Cl)3 [3d sp2-(3p)3], H+-OH versus H-OH (ls-2p), C +(Q2-)2 versus 0=C=0 [2sp -(2p )2], and Fe +(0 ) versus Fe=0 [3d sp-(2p )]. Such ionic formulations for these molecules in an inert matrix are not consistent with their physical... 

This orbital can act as an electron-pair acceptor (for example, from the HOMO of NH3) in Lewis acid-base interactions. 

The preferential interaction between [SiOAl] and Na+ at one hand and between [SiOB]" and TPA+ at the other can be understood on the basis of the hard and soft acid-base interaction. It is well known that hard acids accompany better hard bases and soft acids link preferentially to soft bases. As Na+ is a harder acid than TPA+, [SiOAl] is also a harder base than [SiOB]. The preferential interactions lead then to the TPA+-[SiOB] pairs, as it was demonstrated previously [22]. 

In copyrolytic reactions of the aminosilylenes with unsaturated ketones or imines (heterodienes) we mainly obtained isomeric mixtures. The chemo- and regioselectivity of main- and byproducts can be explained with multistep-cycloadditions. We assume a primary Lewis acid-base interaction between the lone electron pair of the heteroatom (oxygen or nitrogen) and the electron gap at silylene, which is followed by a [2+l]-cycloaddition and a radical ring-opening ring-closure reaction. 

Since IGC is able to generate adsorption isotherms and to evaluate acid/base interactions for specified adsorbate-adsorbent pairs, it follows that the technique is able to develop a detailed picture of surface properties for non-volatile stationary phases. This is illustrated, again for carbon fibers, by Vukov and Gray (48). They combine IGC information at essentially zero coverage of the injected probes with finite concentration data to obtain heat of adsorption values ranging from zero to multi-layer coverage. Their meticulous study shows the effects of thermal pretreatment on fiber surface characteristics, and underscores the convenience and power of IGC to generate information otherwise far more difficult to obtain. 

A further illustration of IGC as a source of data for acid/base characterization of polymers and of solid constituents of complex polymer systems, is given by Osmont and Schreiber (49), who rate the inherent acid/base interaction potentials of glass fiber surfaces and of polymers by a comparative index, based on the Drago acid/base concepts (SO). The interaction index is conveniently measured by IGC and is shown to differentiate clearly among untreated and variously silane-modified glass fiber surfaces. Conventional methods are used to determine adsorption isotherms for fiber-polymer pairs, and the IGC data ate used to demonstrate the relationship between acid/base interactions and the quantity of polymer retained at fiber surfaces. 

As the dye possesses a large dipole moment it is well suited to monitor permanent dipole/dipole as well as permanent dipole/induced dipole interactions with the solvent. The extended n-electron system of Reichardt s dye is prone to dispersion forces. In addition, the phenolate group represents an electron pair donor centre and can get involved in hydrogen bonding as well as in Lewis acid-base interactions. 

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