Lewis bases, thermodynamic affinities

By far the most important body of data on the thermodynamics of this reaction has been provided by the group of Gal and Maria at the University of Nice-Sophia Antipolis. For technical reasons, these authors chose to study reaction 3.5 between gaseous BF3 and the Lewis base in a dilute solution of dichloromethane at 298 K. The affinity scale can be defined as the negative enthalpy of reaction 3.6 ... 

The existence of numerous theoretical studies on the Lewis affinity of BF3 and on the nature of the dative bond in BF3 complexes, generally performed at high theoretical levels, illustrates that the choice of BF3 as a reference Lewis acid for constructing an affinity scale of Lewis bases is well founded, not only from the experimental but also from the computational point of view. Indeed, the electronic structure of BF3 is very simple, since this trigonal planar molecule contains only four first-row atoms. Calculations on the thermodynamics of BF3 complexes can significantly improve our knowledge of Lewis affinity, as illustrated below. 

A number of scales proposed in the literature for measuring the hydrogen-bond basicity (affinity) of Lewis bases are summarized in Table 4.1. In this book, we will select four of them. The first is a thermodynamic scale of hydrogen-bond basicity built from the equilibrium constant of the reaction 4.2 of 4-fluorophenol with a series of Lewis bases in CCI4 at 25 °C ... 

Studies of the X—Y stretching vibration in complexes of XY with different Lewis bases reveal a characteristic decrease in frequency as the strength of the base increases [35, 36]. Hence spectroscopic scales of halogen-bond basicity can be built [37] in the manner described in Chapter 4 for the O—H stretching vibration in hydrogen-bonded complexes. Spectroscopic scales based on the shifts of the v(I—I) band of diiodine at 211 cm , the u(I—Cl) band of iodine monochloride at 376 cm and the v(I—CN) band of iodine cyanide at 485 cm will be presented and compared with thermodynamic basicity and/or affinity scales. 

Ionization rate constants in aqueous acetonitrile were obtained for trityl chlorides, bromides, and acetates covering 21 units in the pA1r+ of the trityl cation. This study observed solvolyses with and without common ion return, solvolyses where the trityl cation could be observed to form and then decay, solvolyses where water addition occurs before complete formation of the cation, and at the other extreme, solvolyses that yield persistent carbocations. Mayr and coworkers showed how electrophilicity and nucleophilicity parameters rationalize reactivity patterns and resolve mechanistic controversies in organocatalytic cyclizations. The thermodynamic affinities of a large number of Lewis bases were computed for addition to the methyl, diphenylmethyl, and triphenyl-methyl cations. ... 

The heats of adduct formation with many Lewis acids have been measured experimentally in the gas phase using the technique of ion cyclotron resonance [10]. The absolute proton affinity (PA) of a gaseous base molecule has a precise thermodynamic definition in relation to the negative of the enthalpy variation for a hypothetieal reaction of attachment of an isolated proton to a molecule M in the gas phase. 

Besides direct measurement of the thermodynamic quantities K and AH discussed above, spectroscopic estimates of basicity (affinity) have been proposed. Their main attraction is the ease with which they can be carried out. Moreover, many systems that do not possess the physical properties needed for a thermodynamic study can be characterized by spectroscopic parameters. Spectroscopic scales of basicity (affinity) are based on the change of a spectrochemical property (NMR, UV-Vis, IR, etc.) of the Lewis acid upon complexation... 

Acidic/Basic Lewis acidity/basicity determines the solvent s ability to donate or accept a pair of electrons to form a coordinate bond with solute and/or between solvent molecules. A scale for this acid/base property was proposed by Gutman (DN and AN donor and acceptor number, respectively) based on calorimetric determination. The complete proton transfer reaction with formation of protonated ions is determined by proton affinity, gas phase acidity, acid or base dissociation constants. Both concepts differ in terms of net chemical reaction. Acidity functions are not unique properties of the solvent system alone, but depend on the solute (or family of closely related solutes) with respect to which the thermodynamic tendency is measured. ... 

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