Kosower Z values

DR. JOHN BRAUMAN (Stanford University) I have a question about the empirical correlations for quantities like charge transfer band energies versus parameters such as the Kosower Z-value. There is a very large literature of that type and there are many, many good correlations for a variety of parameters. You obtain straight lines with your simple dielectric continuum model. It seems to me, however, that one ought to be able to derive these types of relationships directly from the model. And it doesn t seem to be very helpful to say that these relationships are simply empirical and, therefore, not worth the attention. What you want to do is derive the equations and see whether they, in fact, all reduce to the same terms. 

Product selectivities were also reported for solvolyses of p-mcthoxybcnzoyl chloride in aqueous MeOH, EtOH, 2,2,2-trifluoroethanol, PrOH, IVOil, and Bu OH at 25, 35, and 45 °C. The S values were small and depended significantly on the alcohol cosolvent, varying from 1.3 in MeOH to 0.1 in Bu OH, but depended only slightly on the solvent composition and temperature. As S adjusts the product ratios for changes in bulk solvent compositions, it was concluded that preferential solvation by either alcohol or water at the reaction site was not a major factor influencing rates or products. Logarithms of rate of solvolyses of p-mcthoxybcnzoyl chloride correlated well with Kosower Z values (based on solvatochromism).24... 

Contrary to the Z-forms of protonated azobenzenes, the Z-forms of donor/acceptor di-substituted azobenzenes isomerize very quickly at room temperature. To investigate this reaction, the Z-form is produced by flash excitation. The isomerization of these pseudo-stilbenes is strongly dependent on the polarity of the solvent. " For instance, 4-diethyIamino-4 -nitroazobenzene isomerizes with k g = 0.007 s in hexane, but with kz, g = 600 s in N-methylformamide. 4-Anilino-4 -nitroazobenzene in cyclohexane obeys the relation In kz, E = 22( 3) - 72( 9)T0V(8.314 T). Schanze et al. " have established two linear relationships between the free-activation enthalpy of isomerization and the Kosower Z values of aprotic and protic solvents. Sanchez and de Rossi report a strong pH sensitivity of... 

The Kosower Z value is the wavelength of the maximum of the charge-transfer band for the above-mentioned ion pair, given in units of kcal/mole ... 

Determination of the original Kosower Z value was hindered by the insufficient solubility of the model compound in non-polar solvents. Later, therefore, l-ethyl-4-carbo-t-butoxypyridinium iodide, which is readily soluble also in non-polar solvents, was used as the reference donor [Ko 68]. The Z values obtained with the two different references are in excellent agreement. 

Kosower initially reported Z values for 21 solvents and 35 solvent mixtures (interpretation of the data relating to the solvent mixtures is, however, often uncertain because of the overlapping of the individual effects of the components). The Kosower scale was later extended to 45 solvents [Le 66, Be 62, Go 65, Fo 69], and Griffiths and Pugh [Gr 77] determined the Kosower Z values of a further 40 solvents. 

As a model system for confirmation of the correctness of the above correlation Brownstein used l-ethyl-4-carbomethoxypyridinium iodide, as proposed by Kosower. By definition, the R value relating to this is 1.00, and the S value relating to absolute ethanol is 1.00. The Kosower Z values served as the k values here. On the basis of the experimental data of Kosower, Brownstein calculated S and R for many systems, and found good correlations. Accordingly, he assumed that his equation was of general validity and suitable for the description of the solvent dependence not only of spectral data, but of any solvent-dependent experimental parameter. 

In actual fact, this consideration is only true for systems in which one of the many factors influencing the solvent effect is predominant and the others are negligible in comparison. The reason why this correlation was suitable for a description of the Kosower Z values was that the spectral shifts serving as its basis are determined predominantly by the acceptor strength of the solvent. The experiments of Brownstein can actually be regarded as a confirmation of the Kosower acceptor scale. 

The Gutmann acceptor numbers (AN) of some solvents are listed in Table 4.9, together with the Kosower Z values and the Dimroth-Reichardt Ej values characterizing the acceptor strengths. 

By presenting the joint occurrence of the double (donor and acceptor) effects of the solvents, this model also explained why it was not possible to describe solvent effects in these systems by means of a single empirical parameter. It is obvious that the standard AG values for dissolution, for example, could not give a linear correlation with parameters predominantly reflecting a single type of effect, e.g., the Kosower Z value, the Reichardt Ej value, the DN value, the AN value, the relative permittivity of the solvent alone. Even in systems where one of the effects is so predominant that the system can be described more or less with a single parameter, only calculations in which the other parameters are also taken into... 

Used in determination of solvent polarity (Kosower Z values). Violet-black cryst. -h 2H2O (MeOH aq.). Mp 205-276° dec. Highly solvatochromic. 

The polar character of the transition state for the decomposition of 3 is also manifested in a remarkably large dependence on solvent polarity. The fraction of radicals scavengeable varies with solvent in the range from 30 to 98 at 40 fc, but not in a manner related to solvent polarity. The rate of decomposition of 3 in a variety of solvents is directly correlated with mea-siures oT solvent polarity such as the Kosower Z values (19) with a sensitivity to solvent polarity about 64fl of that observed for the ionization of -methoxyneophyl tosylate ( ). It also shows a kinetic salt effect with a rate dependence of radical formation on the concentration of lithium perchlorate in tetrahydrofuran which provides strong evidence for appreciable transition state polarization. 

Used in detn. of solvent polarity (Kosower Z values). 

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