Lewis base strengths

Norinder et al. [34] Polarity, Lewis base strength, and hydrogen-bonding... 

Lewis base strengths of, 11 253 handling of, 11 250-284 Siliranes, from silylenes and alkenes, 29 12,... 

In THF or diglyme, 9-fluorenyllithium, which is dissociated extensively into ion pairs, catalysed through a strong anionic propagation to produce syndiotactic polymethylmethacrylate. The effectiveness of ethers to help dissociate the ion pairs was found to be related to the Lewis base strength of the ethers. 

We have recently studied systematically the Lewis basicity of zeolites using pyrrole as a probe molecule. Pyrrole has H-donor properties and adsorbs on basic zeolites forming NH—O bridges. The NH stretching vibration was shown by Barthomeuf [51-53] and others [54] to be a measure of the Lewis base strength of 0 in zeolites. 

Since anions are more adaptable in their bonding than cations, it is convenient to distinguish between the normal Lewis base strength, calculated by assuming that oxygen is 4-coordinate, and the maximum Lewis base strength, which equals the valence of the strongest bond that the undistorted anion can form. This is usually... 

Fig. 10.8. Derivation of normal Lewis base strengths for oxyanions. The numbers are bond valences. The O atoms are assumed to form four bonds and the bond valence sum at O is -2. The base strength of PO is 0.25 v.u., of SO4 is 0.17 v.u., and of CIO4 is 0.08 v.u.

Lewis base strengths correlate well with other measures of basicity such as the p. a [7]. They are given for a variety of anions in Table 10.2 and References [7, 38]. 

The particular virtue of these definitions is that the Lewis acid and the Lewis base strengths are both estimates of the valence of the bond that links the cation with the anion. The most effective bonds will therefore occur between a cation whose Lewis acid strength (Sg) is close to the Lewis base strength (Sb) of the anion. This is known as the Valence Matching Principle. Compounds between badly matched ions, i.e. Be (Sa = 0.5 v.u.) and ClO j" (Sb = 0.08 v.u.), are difficult if not impossible to form as both the cation and the anion will be forced into unusual coordination. If they... 

In the Bond Valence Model, atoms are characterized by their atomic valence (oxidation state) and by their Lewis acid or Lewis base strength, the latter two quantities correlating with electronegativity and respectively. The acid and base strengths are both estimates of the valence of the bonds formed by the atoms. Hence, for a... 

Section 6.4.2 discussed that the donor-acceptor transition in I2—Lewis base complexes is modified depending on the extent of the donor interaction with the I2 LUMO. The blue shift (to higher energy) in the d-n/ 9o- transition upon I2 complexation has also been correlated to Lewis base strength. As shown in Figure 6.10, this transition increases in energy as the base strength increases. Table 6.11 lists blue shifts induced by selected bases, used to assess Lewis basicity. 

Additional substitution increases the effect, with the following resulting order of Lewis base strength in the gas phase ... 

An additional point emerges from this highCT syndiotactic contexts are obtained when the Lewis-base strength of the solvait increases, and this factor probably accounts for the efficiency of the ether in this system. Whrai the catalyst is 9-lluQrraiyl hthium, the reaction of methyl methacrylate at 195 K in tolume leads to isotactic polymer, whereas a change of solvent to tetrahydrofuran results in a syndiotactic product... 

Maciel et al [Ma 66] used the chemical shift of Li to study solutions of lithium salts in eleven organic solvents. Symons [Sy 67] used the chemical shift of Na to investigate solutions of sodium iodide in fourteen donor solvents. The data show a correlation with the Lewis base strength of solvent. Sodium resonance measurements in acetonitrile solution clearly point to a 7r-interaction between the dissolved salt and the C=N bond of the solvent [B168]. Sodium NMR has been used to study the solvation of the sodium ion in solutions of sodium methoxide in methanol and in mixtures of methanol and dimethyl sulphoxide [Ba 81]. 

Dimethyl sulphoxide is an aprotic solvent, in which the solvation of the halides is negligibly small. Hence, the sequence of the stabilities of the halide complexes in dimethyl sulphoxide is governed by the magnitude of the affinity between the central ion and the halide ligand. Accordingly, the complex stabilities follow the sequence of the Lewis base strengths of the ligands Cl" >Br" >1". 

Finally, accurate and robust models for corrosion inhibition efficiencies have been found for multiple sets of corrosion inhibition data. This finding is rather remarkable as corrosion inhibition depends on a number of molecular properties such as hydrophobicify (LogP) and Lewis Base strength as well as the solubiUfy of the inhibitor. This corroborates the concept that similarity based on electron density descriptors may in fact capture more than one aspect of the phenomenon being studied. 

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