Acid-base chemistry polarizability

The general or universal effects in intermolecular interactions are determined by the electronic polarizability of solvent (refraction index n0) and the molecular polarity (which results from the reorientation of solvent dipoles in solution) described by dielectric constant z. These parameters describe collective effects in solvate s shell. In contrast, specific interactions are produced by one or few neighboring molecules, and are determined by the specific chemical properties of both the solute and the solvent. Specific effects can be due to hydrogen bonding, preferential solvation, acid-base chemistry, or charge transfer interactions. 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

To rationalize observations such as these, Pearson presented the concept of hard and soft acids and bases (HSABs), designating polarizable acids and bases as soft and nonpolarizable acids and bases as hard. Much of the hard-soft distinction depends on polarizability, the degree to which a molecule or ion is distorted by interaction with other molecules or ions. Electrons in polarizable molecules can be attracted or repelled by charges on other molecules, forming slightly polar species that can then interact with the other molecules. The HSAB concept is a nseful gnide to explain acid-base chemistry and other chemical phenomena. Pearson stated, Hard acids prefer to bind to hard bases, and... 

One goal of this chapter was to develop tools for comparing the thermodynamics of one reaction to another, where acid-base chemistry was our setting. In that regard, this chapter allowed us to analyze again many of the chemical effects that chemists routinely call upon to explain trends—namely, induction, resonance, aromaticity, solvation, hybridization, polarizability, and electrostatics. Although none of these effects were new, this chapter showed for the first time how their interplay affects the thermodynamics of reactions. Really, the only common effect that was not focused upon in this chapter is that of sterics, because a proton is so small that steric effects are minimal. 

Popular qualitative chemical concepts such as electronegativity [1] and hardness [2] have been widely used in understanding various aspects of chemical reactivity. A rigorous theoretical basis for these concepts has been provided by density functional theory (DFT). These reactivity indices are better appreciated in terms of the associated electronic structure principles such as electronegativity equalization principle (EEP), hard-soft acid-base principle, maximum hardness principle, minimum polarizability principle (MPP), etc. Local reactivity descriptors such as density, Fukui function, local softness, etc., have been used successfully in the studies of site selectivity in a molecule. Local variants of the structure principles have also been proposed. The importance of these structure principles in the study of different facets of medicinal chemistry has been highlighted. Because chemical reactions are actually dynamic processes, time-dependent profiles of these reactivity descriptors and the dynamic counterparts of the structure principles have been made use of in order to follow a chemical reaction from start to finish. 

We have already alluded to one of the most useful and pervasive principles in aU of chemistry, that being the hard-soft interaction principle (HSIP). This principle relates to many areas, but it is most directly applicable to interactions in which there is electron pair donation and acceptance (Lewis acid-base interactions). The terms hard and soft relate essentially to the polarizability of the interacting species. For example, 1 has a large size, so its electron cloud is much more distortable than that of F . Likewise, Hg2+ is a large metal ion having a low charge, while Be " " is a very small ion. The result is that Hg is considered to be a soft Lewis acid while Be is considered to be a hard Lewis acid. As a result of these characteristics, Hg + interacts preferentially with 1 rather than F , while Be " " interacts preferentially with F . The hard-soft interaction principle indicates that species of similar electronic character (hard or soft) interact best. It does not say that hard Lewis acids will not interact with soft Lewis bases, but the interaction is more favorable when the acid and base are similar in hard-soft character. 

Measurements of gas phase dissociation constants have shown that the Lewis basicity towards hard reference acids such as BMc3, BF3 or BCI3 falls down Group V in the order MejN > Me P > MejAs > Me3Sb ( MejBi). Towards the softer acid BH3, however, the order is Me3 > MejN > MejxAs. Orders of basicity are strongly influenced by the nature of the acceptor. This is especially evident from the chemistry of transition elements, which often form stable complexes with phosphines and arsines for which there are no amine analogues (Chapter 5). Phosphines and arsines are, in the classification of Pearson, typical soft bases which form their most stable complexes with soft acids such as polarizable transition metal and heavy post transition metal acceptors. 

R. Cammi, B. Mennucci, Structure and properties of molecular solutes in electronic excited states A polarizable continuum model approach based on the time-dependent density functional theory, in Radiation Induced Molecular Phenomena in Nucleic Acids A Comprehensive Theoretical and Experimental Analysis, ed. by M.K. Shukla, J. Leszczynski. Series Challenges and Advances in Computational Chemistry and Physics, vol 5 (Springer, Netherlands 2008)... 

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