Polarizability and acidity

Nile Red was recently introduced as a solvatochromic dye for studying supercritical fluids (10). Although not ideal, Nile Red does dissolve in both nonpolar and polar fluids and does not lose its color in the presence of acids, like some previously used dyes. Major criticisms of Nile Red include the fact that it measures several different aspects of "polarity" simultaneously (polarizability and acidity (15)) yet it is insensitive to bases (10). However, in chromatography other single dimension polarity scales, like P, are routinely used. Measurements with Nile Red and other dyes indicate that the solvent strength of binary supercritical fluids is often a non-linear function of composition (10-14). For example, small... 

PVP is a nonionic water-soluble polymer that interacts with water-soluble dyes to form water-soluble complexes with less fabric substantivity than the free dye. Additionally, PVP inhibits soil redeposition and is particularly effective with synthetic fibers and synthetic cotton blends. The polymer comprises hydrophilic, dipolar imido groups in conjunction with hydrophobic, apolar methylene and methine groups. The combination of dipolar and amphiphilic character make PVP soluble in water and organic solvents such as alcohols and partially halogenated alkanes, and will complex a variety of polarizable and acidic compounds. PVP is particularly effective with blue dyes and not as effective with acid red dyes. 

The Si chemical shifts of silatranes depend on whether it is in the crystal or in solution, on the medium and the temperature " - . The greatest Si shielding is observed in powder silatrane pattems " . Dissolution of silatranes and decrease in the polarity, polarizability and acidity of the solvent, as well as heating of a solute result in a downfield shift of their Si signal . It indicates once more that the Si- —N bond is vay deformable and sensitive to the inllnence of the physical environment. The weakCT the Si- -N bond in a silatrane crystal, the higher the solvent and temperature effects on the Si chemical shift. That is why the latter effects were proposed to be nsed for a fast estimation of the related strength of the Si- —N bond within a series of these componnds . Despite its simplicity, this method proved to be rather reliable and after some modifications it was successfully applied to numerous (N—Si)— and (O—Si)chelate... 

Schade A, Behme N, Spange S (2014) Dipolarity versus polarizability and acidity versus basicity of ionic liquids as a function of their molecular structures. Chem Eur J 20 2232-2243... 

Soft Acids. The acceptor atoms are large, have low positive charge, and contain unshared pairs of electrons (p or d) in their valence shells. They have high polarizability and low electronegativity. 

Hard acids (HA) have low polarizability and small dimensions, higher oxidation numbers and the hardness increases with increasing oxidation number they bind bases primarily through ionic bonds. Typical examples are H+, Na+, Hg2+, Ca2+, Sn2+, V02+, V022+, (CH3)2Sn2+, A1(CH3)3,I7+, I5+, Cl7+, co2, R3C+, so3. 

MolSurf parameters [33] are descriptors derived from quantum mechanical calculations. These descriptors are computed at a surface of constant electron density, with which a very fine description of the properties of a molecule at the Van der Waals surface can be obtained. They describe various electrostatic properties such as hydrogen-bonding strengths and polarizability, as well as Lewis base and acid strengths. MolSurf parameters are computed using the following protocol. 

Earlier it was described how PH3 is a much weaker base than NH3. That is certainly true when the interaction of these molecules with H+ is considered. However, if the electron pair acceptor is Pt2+, the situation is quite different. In this case, the Pt2+ ion is large and has a low charge, so it is considered to be a soft (polarizable) Lewis acid. Interaction between Pt2+ and PH3 provides a more stable bond that when NH3 bonds to Pt2+. In other words, the soft electron acceptor, Pt2+, bonds better to the softer electron donor, PH3, than it does to NH3. The hard-soft interaction principle does not say that soft Lewis acids will not interact with hard Lewis bases. In fact, they will interact, but this is not the most favored type of interaction. 

A diradical is likely to be more polarizable and a stronger base than the corresponding singlet state. If so, reaction with an acid should shift the equilibrium enough for the diradical to be detectable in otherwise diamagnetic substances. This seems to be the case for the substances shown below, in which paramagnetic resonance absorption indicates the presence of the diradical conjugate acid to the extent... 

The experimental evidence shows that the carbon of terminal carbonyl groups is positively polarized (or polarizable) and, contrary to the behaviour of free carbon monoxide, is easily attacked by strong nucleophiles (OH-, OR-) a behaviour which is general in the chemistry of metal carbonyls. Moreover, the negative polarisation (or polarizability) of oxygen atoms of carbonyl groups, particularly bridging carbonyls, is illustrated by the facile formation of adducts with Lewis acids as shown inEq. (16) 7. ... 

According to the hard and soft acids and bases (HSAB) principle, developed by Pearson in 1963232,233, Lewis acids and Lewis bases are divided into two groups hard and soft. Pearson correlated the hardness of acids and bases with their polarizability, whereby soft acids and bases are large and easily polarizable, and vice versa. A selected list of Lewis acids ordered according to their hardness in aqueous solution is presented in Table 18. The HSAB principle predicts strong association of like partners. Hard acid-soft base complexes mainly result from electrostatic interactions, while soft acid-soft base complexes are dominated by covalent interactions. 

The most important properties of an organic pollutant which determine its mode of interaction with SPHS/SP0M are the chemical character of the molecule, shape and configuration, acidity (plCa) or basicity (pKb), water solubility, polarity, molecular size, polarizability, and charge distribution. 

The resulting estimated average polarizabilities for the hydroxylamines, oximes and hydroxamic acids that we have discussed are given in Table 8. (It should be noted that Miller s approach does not distinguish between oxime isomers.) As anticipated from the correlation between average polarizability and volume (equation 10), the a in Table 8 increase with molecular size. 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

The HSAB (hard and soft acids and base) principle is that hard acids prefer to interact with a hard base, and soft acids with soft bases. Hard bases are not polarizable, and inclnde those with 0-donor atoms. Soft bases are more polarizable, and inclnde S-donor bases. Solvent hardness/softness can be assessed by comparing the Gibbs free energy of transfer of a soft cation like Ag from hard water to the solvent with the Gibbs free energy of transfer of similarly sized hard cations like Na and K. Table 3.9 shows some solvents listed in increasing softness. ... 

NH3) are those derived from small atoms with high electronegativity and generally of low polarizability. Soft acids (e.g., Ag Hg I2, 1,3,5-trinitrobenzene, tetracya-noethene) and soft bases e.g., H, r, RC RS, RSH, R2S, alkenes, CeHg) are usually derived from large atoms with low electronegativity and are usually polarizable. 

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