Hammett substituent constants, calculation

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

To estimate the and constants for unsubstituted pyridyl groups, Pasternak and Tomasik attempted to use the dependence of the frequencies of symmetric (vj and asymmetric (VaJ NO2 stretching vibrations in substituted nitrobenzenes (2) on the Hammett substituent constants (75BAP923). The pyridyl group constants obtained from the data of the IR spectra of the nitrophenyl pyridines in CHClj and CHBrj have considerable scatter in their values depending on the solvent. Most of the and values estimated from Vs-NOa Jiff widely from the respective values obtained from v noj even for the same solvent. Four of the seven Hammett-type relationships used by the above authors to calculate the a values have a low correlation coefficient (r = 0.949-0.965). The method in question, just as polarography, is suitable only for a rough estimation of constants. 

The original Hammett substituent constants [Hammett, 1937 Hammett, 1970] measuring the overall electronic effect of the meta- and paro-substituents of benzene derivatives having the functional group in the side chain. They were originally calculated from the variation of the acid dissociation constant of substituted benzoic acids (m-, P-XC6H4COOH) in water at 25°C, with respect to the unsubstituted compound (i.e. benzoic acid) ... 

A substituent effect on electrochemical and xerographic properties of triarylamines has recently been reported16. The oxidation potentials correlate with both the Hammett substituent constants and the calculated HOMO energies of the molecules under investigation, and this approach was used16 to rationalize xerographic properties, including transport phenomena of solid-state solutions in polycarbonate-Z. 

From the pKa values, the value of the Hammett substituent constant can be calculated to show whether the side-chain R is electron withdrawing or electron donating. 

Some data on substituent effects for the reaction of trifluoroacetanilides with methoxide ion in methanol and methanol-O-d are given below. Calculate the isotope effect for each system. Plot the rate data against appropriate Hammett substituent constants. What facets of the data are in agreement with a normal addition-elimination sequence passing through a tetrahedral intermediate ... 

Hammett substituent constants may be found in the literature. McDaniel and Brown [3] give the most reliable figures and indicate their approximate precision. A larger list has been published by Jaffe [4] these are mainly calculated indirectly from reactions and equilibria other than the ionisation of substituted benzoic acids. Useful tables of Hammett substituent constants and most of the other parameters to be described in this article, may be found in a review by Tute [5]. 

The formation of r-type complexes between trimethoxybenzene and para-substituted phenylchlorocarbenes in pentane has been studied by UV-Vis spectroscopy . Results of LFP experiments and DFT calculations on the equilibrium exhibit a good correlation with the Hammett substituent constants of the aryl substituents. 

Although the FMMF method dramatically reduces the number of substituent constants that one must consider, it suffers from the disadvantage that it is not possible to visually compare two reactions with a three-parameter equation, i.e., one cannot simply plot logs of rates or equilibrium constants against a single parameter as one does in the normal Hammett treatment. It is possible, however, to calculate Hammett substituent constants for any position using the FMMF parameters and the following relationship ... 

Fujita et al. were the first to develop a calculation method that was based, analogously to the Hammett approach, on substituent constants r[6] (see Eq. (7))... 

These constants, K toK/, may be estimated by use of the Hammett equation. Estimation of 1 and K 4 involves application of the methods outlined in Section II, A, i.e., application of substituent constants for and N+H to the Hammett equation for the acid-base equilibria of benzoic acids. Estimation of A2 and involves application of the method used in Section III,A, i.e., the p-value for the basicity of substituted pyridines, with cr-values for COOH and COO . Provided the necessary a- and p-values are known, this procedure permits the calculation of four independent, or virtually independent, estimates of Krp. A check on the method is available from the relationships shown in Eq. (16) which is readily obtained by multiplication of Eq. (12) and (14) and of Eq. (13) and (15). 

Substituent effects as evaluated on the basis of the Hammett equation and its extended forms, are - this has to be emphasized again — empirical results. Nevertheless, it is very soothing to know that theoretical approaches, i. e., calculations of substituent effects using ab initio molecular orbital theory (Topsom, 1976, 1981, 1983 Taft and Topsom, 1987, STO-3G and 4-31G level), give results that are consistent with the experimental data. However, it is not recommended to use only theoretically calculated substituent constants and values for F, R, and other parameters for the interpretation of experimental data. 

The diazonio group is a somewhat more complex substituent for such evaluations because it is charged, in contrast to the majority of substituents on which the Hammett treatment is based. The electrostatic interaction of the diazonio and other charged groups was calculated by Hoefnagel et al. (1978) and by Exner (1978). The substituent constants they obtained, including the effects of coulomb interactions, are only slightly different from those of Lewis and Johnson (1959). 

Values of e are correlated by the Op constants in accord with eq. (61) (119, 120). Kawabata, Tsuruta, and Furukawa (121) have proposed a revised form of the Price-Alfrey equation based on the definition e = 0 for styrene. On the basis of this redefinition, they have calculated a new set of Q values. These Q values are linear in the Hammett 0 constants. It is not clear from their paper whether this linear relationship is for substituents directly bonded to the carbon-carbon double bond or whether it is applicable only to Q values for substituted styrenes. Charton and Capato (119) were unable to obtain significant correlations between any a constants and the q values of Schwan and Price (117). Zutty and Burkhart (122) have proposed a redefinition of the Price-Alfrey equation based on ethylene as the reference system with Qo and e defined as 1 and 0, respectively. 

Finally, in this account of multiparameter extensions of the Hammett equation, we comment briefly on the origins of the a, scale. This had its beginning around 1956 in the a scale of Roberts and Moreland for substituents X in the reactions of 4-X-bicyclo[2.2.2]octane-l derivatives. However, at that time few values of o were available. A more practical basis for a scale of inductive substituent constants lay in the o values for XCHj groups derived from Taft s analysis of the reactivities of aliphatic esters into polar, steric and resonance effects . For the few o values available it was shown that o for X was related to o for XCHj by the equation o = 0.45 <7. Thereafter the factor 0.45 was used to calculate c, values of X from o values of XCH2 . ... 

The Hammett treatment provides a correlation of much experimental data. Tables 26-6 and 26-7 contain 38 substituent constants and 16 reaction constants. This means that we can calculate relative k or K values for 608 individual reactions. To illustrate, let us suppose that we need to estimate the relative rates of Reaction 16 of Table 26-7 for the para-substituents R = OCH3 and R = CF3. According to the p value of 4.92 for this reaction and the cr values of p-OCH3 and p-CF3 in Table 26-6, we may write... 

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