Correlations Hammett parameters

H-nmr chemical shifts of N-1—H and N-3—H signals have been used as a criterion for distinguishing between N-l-substituted and N-3-substituted hydantoin derivatives (22). They can often be related to electronic properties, and thus good linear correlations have been found between the shifts of N—H and Hammett parameters of the substituents attached to the aryl group of 5-arylmethylenehydantoins (23). 

Lindberg et al.17 have shown that the carbon Is shifts of some substituted benzenes are linearly correlated with the Hammett a parameters of the substituents. However, Hammett parameters are of rather limited applicability a given set of a values can be used to correlate data only for similar chemical systems. It has recently been shown18 that a wider variety of core binding energy shifts can be correlated by the four-parameter relation... 

FIGURE 1. Hammett plots [o (a) and cr+ (b), respectively] for the formation of semimercaptals from GSH and substituted nitrosobenzenes. Log k /k values were calculated from the rate constants reported for pH 7.4, 37 °C18 22 35 and pH 7.49, 25 °C, ionic strength 1 M30. Correction for pH is unnecessary as k and k ° are affected in the same manner. Correction for the different temperatures has been revealed to be insignificant. (The Hammett parameter p appears to vary with 1 IT while a seems to be independent of temperature54. Hence, log k /k 0 has been assumed to correlate with 1 IT.) For want of a for the complex nitrosochloramphenicol substituent (20), a of 4-CHOHMe was used55... 

Next, a quantitative model, referred to as the E and C equation, is presented for predicting and correlating the enthalpies of adduct formation. The use of this equation and the interpretation of the parameters which result is discussed. Exceptions to the correlation are considered and the valuable insight about intermolecular interactions that can be gained by firmly establishing these exceptions is demonstrated. The parameters we obtain and valid transformations of these parameters are considered in the light of both the HSAB model and Donor Strength model of acid-base chemistry. Both of these concepts are shown to be at best incomplete models of coordination. The relationship between our parameters and the a—q Hammett parameters is quantitatively demonstrated. 

The most-studied reaction at pyridine nitrogen is protonation, and correlation of pK values with Hammett parameters has received extensive investigation.7,47 102-114 Quite clearly, electronic interactions of a-substituents are tempered by general steric effects,47,102 and there is also doubt as to the mode by which electronic effects are conveyed from the /l- and y-positions. Groups in the 4-position of potential (-I-R) character... 

Other properties of pyridine 1-oxides have been investigated with regard to applicability of Hammett parameters. The type of parameter required and the degree of correlation achieved varies, but again the need for a sliding o scale is generally indicated. Such measurements... 

The first adequate examination of the applicability of the Hammett equation to aromatic substitution was made by Roberts and his associates (1954). New data on the nitration of the halobenzenes together with the results for other substituted benzenes (Ingold, 1953) revealed that the rates for nitration in the meta position were, indeed, correlated satisfactorily by the Hammett rr-constants. Para substituents, in particular groups directing to the ortho and para positions, exhibit important deviations from the line defined by the meta reactivities (Fig. 3). It was suggested that these deviations were the consequence of significant resonance interactions in the transition state. At the same time, de la Mare (1954) examined the application of the Hammett parameters for the correlation of the relative rate data for non-catalytic chlorination. The poor agreement achieved (Fig. 4) prompted his conclusion that variable resonance interactions in the... 

The elimination of one unknown is accomplished by the assumption that om Me = o-m-Me (— 0.069). Ample evidence is available to substantiate this assignment. Experimentally, the meta reactivities even in electron-deficient reactions are adequately correlated by the Hammett parameters, as illustrated for aromatic nitration (Fig. 3) and solvolysis (Fig. 10). Presumably this is the consequence of the unimportance of resonance interactions between the meta substituents and the reaction site. Solution of the equation (25) with Me as —0.069 yields cr+Me as —0.280. The assignment of a value to oJ.Me allows the extension of the treatment to all other substituents through relations (23) and (24). 

Gustowski, D. A., Gatto, V. J., Mallen, J., Echegoyen, L., Gokel, G. W., Direct correlation of cation binding strengths to Hammett parameters in substituted TV-benzylaza-15-crown-5 lariat ether and A 7V-dibenzyl-4,13-diaza-18-crown-6 BiBLE derivatives. J. Am. Chem. Soc. 1987, 52, 5172-5176. 

In many cases, experimental data have been discussed in terms of variations of the paramagnetic contribution, nuclear shielding constant (Equation (2)). Empirical correlations with parameters related to the electronic properties of other substituents in the molecule (electronegativities, Hammett substituent constants etc.) have been found. This kind of investigation has provided useful information about the electronic structure of the sulphur atom in different bonding situations and most of all about the electronic properties of the S-O bond, which is still a rather controversial matter. 

The earlier work on the reaction of the OH radical with substituted benzenes has shown the correlation of second-order rate constants with the Hammett parameter The Hammett treat-... 

In the reaction of SO with halobenzenes the correlation of the rate constants with the Hammett parameters gave values of -1.2 and -1.6 with halotoluenes. Studies on product distribution in substituted toluenes have shown that H atom abstraction from -CH3 group is an additional reaction pathway, especially in toluenes containing substituent at tw-positions (e.g. 4-chlorotoluene and 3,4- dichloro toluene). 

The Hammett-Taft analysis of the redox potentials gave a good correlation using parameters. Also, the dependence was additive for the disubstituted ferrocenes,... 

In addition to the study on R. harrisii, Brinckman et al. found a good correlation between a series of triorganotins mAA.falcatu using TSA values as the descriptor. A similar finding was observed for the uptake of trialkyltin chloride by Escherichia coli In addition to the TSA correlation, the results indicated that the uptake also correlated well with the Hansch 7rgnd Taft-Hammett parameters. 

The one-bond 7( C, H) coupling constant in mono-substituted benzenes does not correlate with the electronegativity, but can be correlated to a combination of Hammett parameters . The lack of strict correlations can also be seen from 2-hydroxynaphthalene in which 7( C-1, H-1) is decreased slightly compared to naphthalene, whereas 7( C-3, H-3) is increased slightly . For 1-hydroxynaphthalene, an increase is seen for the C-8,H-8 coupling constant of the peri bond . For a general introduction see elsewhere . ... 

Halophenols, photooxidation of 1084 Hammett constants 71 Hammett parameters 350 Hammett plots 999 Hammett sigma values 522 Hammett-type stmcture-acidity correlations 506... 

The interfacial hypothesis has been also proposed independently and very recently by Smyth et al. (ref. 41) who correlate the rate constants with the Hammett parameters (p and o ) and found that the solubilization of KF cannot be the rate-limiting step since p is as large as + 6.4. 

Carbon-13 shifts, measured by the double-resonance technique, of the carbonyl group of weto-substituted benzaldehydes correlate with Hammetts parameter. In the case of the para-substituted benzaldehydes, there was no correlation between the chemical shift and a, presumably because, in contrast to substituted benzenes, resonance contributions for both electron-attracting and releasing groups will not significantly affect the electron density at the carbonyl carbon. 

Star, A., Han, T. R., Gabriel, J. C. P., Bradley, K. and Grtiner, G. (2003), Interaction of aromatic compounds with carbon nanotubes correlation to the Hammett parameter of the substituent and measured carbon nanotube FET response . Nano Letters, 3,1421-3. 

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