Solvents HBA basicity

Solution calorimetry of the molecular probes pyrrole, A-methylpyrrole, benzene, and toluene in 35 solvents has been used by Catalan el al. to determine a solvent HBA basicity scale, ranging from the gas phase to HMPT [31a]. Analogously, a solvent HBD acidity scale was derived calorimetrically using A-methylimidazole and A-methylpyrrole as probe molecules in 3 6 solvents, ranging from the gas phase to 2,2,2-trifluoroethanol [31b]. 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

The most important scales for solvent HBA basicity are reported here. 

It was of particular interest that the ordering of the —b values in the solvatochromic equations (i.e., the spectral sensitivities to solvent HBA basicities)... 

It is of interest that, as might be expected, the b/s ratios in the solvatochromic equations reflect smaller relative effects of solvent HBA basicity as the hydrocarbon moiety R in R—C=CH grows larger. 

Kamlet MJ, Taft RW (1976) The solvatochromic comparison method. 1. The /6-scale of solvent hydrogen-bond acceptor (HBA) basicities. J Am Chem Soc 98 377-383. 

The LSER approach relates a bulk property, P, to molecular parameters thought to account for cavity formation, dipole moment/polarizability, and hydrogen-bonding effects at the molecular level. The cavity term models the energy needed to provide a solute molecule-sized cavity in the solvent. The dipole moment/polarizability terms model dipole and induced dipole interactions between solute and solvent these can be viewed as related to dispersion interactions. The hydrogen-bonding terms model HBA basicity and EIBD acidity interactions. 

Because of the rather localized negative charge at the phenoHc oxygen atom , the standard dye (44) is capable of specific HBD/HBA and Lewis acid/base interactions. Therefore, in addition to the nonspecific dye/solvent interactions, the betaine dye (44) predominately measures the specific HBD and Lewis acidity of organic solvents. On the other hand, the positive charge of the pyridinium moiety of (44) is delocalized. Therefore, the solvent Lewis basicity will not be registered by the probe molecule (44). If this solvent property is relevant for the system under study, other empirical measures of Lewis basicity should be used cf. Section 7.7. 

In a similar way, another set of solvent Lewis-basicity parameters, i phOH) based on band shifts of the O—H stretching vibration of phenol in tetrachloromethane induced by hydrogen-bond formation with added HBA solvents B, was used by Koppel and Paju [88] to classify 198 solvents, according to Eq. (7-36) (5phOH = 0 for CCI4) ... 

Another remarkable IR spectroscopic parameter of solvent Eewis basicity has been introduced by Laurence et al. [239], using the so-called infrared comparison method analogous to Kamlet and Taft s solvatochromic comparison method [224]. The band maxima wavenumbers of the C=0 stretching vibration of the two homomorphs CCI3CO2H and CCI3CO2CH3 have been measured in the gas phase as well as in non-HBD and non-HBA solvents in order to establish a reference line that conforms to Eq. (7-37) cf Fig. 7-3. 

Another important treatment of multiple interacting solvent effects, in principle analogous to Eq. (7-50) but more precisely elaborated and more generally applicable, has been proposed by Kamlet, Abboud, and Taft (KAT) [84a, 224, 226], Theirs and Koppel and Palm s approaches have much in common, i.e. that it is necessary to consider non-specific and specific solute/solvent interactions separately, and that the latter should be subdivided into solvent Lewis-acidity interactions (HBA solute/HBD solvent) and solvent Lewis-basicity interactions (HBD solute/HBA solvent). Using the solvato-chromic solvent parameters a, and n, which have already been introduced in Section 7.4 cf. Table 7-4), the multiparameter equation (7-53) has been proposed for use in so-called linear solvation energy relationships (LSER). 

In addition to the p parameter of solvent Lewis basicity, the coordinate covalency parameter has been found to be useful in correlating certain types of so-called family-dependent solute basicity properties [226, 267]. Family-independent (FI) basicity properties are defined as those which have a linear relationship with p when all solute bases are considered together. Family-dependent (ED) basicity properties are those which exhibit a linear relationship with p only when different families of solutes having similar HBA sites are considered separately. Thus, ED properties can be correlated to FI properties if an empirical coordinate covalency parameter is used in correlation equa-... 

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