Solvent, acceptor number

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

A solvent may play a decisive role when a reaction chooses the ion radical pathway. For instance, the solvent effect on the thermodynamic contribution to the activation free energy causes an increase in the cleavage rate constant for the chloroanthracene anion radical with an increase in the solvent acceptor number. There are examples of similar solvent effects in a review by Jaworski (1998). 

Belser et al.204 studied the solvent dependence of the properties of several cyano-complexes, [Ru(AB)2(CN)2]. They noted a good correlation between absorption band energy and solvent acceptor number, the CT band energy increasing with this parameter. Decreased electron density at the metal center due to cyanide-solvent interactions was the explanation for their observations. Quantum yields were only reported in three solvents for this group of compounds204 and no correlation with the acceptor number was apparent. 

AN solvent acceptor number, based on P NMR chemical shift of Et3PO (Gutmann and Meyer) ... 

Another scale for measuring solvent acidity was formulated by Mayer et al. [43]. It is called the solvent acceptor number (AN) and is based on the relative values of the NMR chemical shifts produced by a given solvent with a strong Lewis base, triethylphosphine oxide (fig. 4.13). The data were normalized so that the acceptor number of hexane is zero and that for the 1 1 adduct with the strong Lewis acid, SbCls, 100 when dissolved in 1-2 dichloroethane. The attractive feature of this scale is that it varies over a wide range for the polar solvents con-... 

FIGURE 7.3 Plots of (a) rNO, IR stretching frequency, and (b) i mlct. lowest energy visible band, for (TBA)2[Fe(CN)5NO], against solvent acceptor number in the Gutmann s scale. A = acetone B = acetonitrile C = methanol D = water. Ref. 24... 

Although the success of the empirical solvent parameters has tended to downgrade the usefid-ness of the dielectric approach, there are correlations that have succeeded as exemphfied by Figure 13.1.1. It is commonly held that the empirical solvent parameters are superior to dielectric estimates because they are sensitive to short-range phenomena not captured in dielectric measurements. This statement may not be generahzed, however, since it depends strongly on the chemical reaction investigated and the choice of solvents. For instance, the rate of the Menschutkin reaction between tripropylamine and methyl iodide in select solvents correlates better with the log e function than with the solvent acceptor number. ... 

The solvent acceptor number and other polarity scales include appreciable, perhaps predominant, contributions from solvent structure changes rather than merely measuring anion solvation. 

Using the empirical parameter of the solvent polarity Z based on the molar energy of the transition in E, in kilocalorie per mole (kcal/mol) for the CT band, the Ep values of sodium salt of l-methyl-4-[2-(4-hydroxyphenyl)ethenyl)]pyridinium] hydrogensquarate are shown in Table 6.3. These data illustrate the analogy between the Z values of the sodium salt of l-methyl-4-[2-(4-hydroxyphenyl)ethenyl)]pyridinium] hydrogensquarate with Ep(30), which is the empirical solvent polarity parameter, based on the intramolecular charge transfer absorption of a pyridinium-N-phenolate betaine dye. The Z values are practically equal in the solvents acetone, pyridine, and cyclohexane, which means that for a difference in the values of 8, solvent donor number (DN) and solvent acceptor number (AN) of 35.3, 33.1, and 18.9 (kcal/mol), the... 

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