Hammett parameter correlation with

C-nmr data have been recorded and assigned for a great number of hydantoin derivatives (24). As in the case of H-nmr, useful correlations between chemical shifts and electronic parameters have been found. For example, Hammett constants of substituents in the aromatic portion of the molecule correlate weU to chemical shifts of C-5 and C-a in 5-arylmethylenehydantoins (23). Comparison between C-nmr spectra of hydantoins and those of their conjugate bases has been used for the calculation of their piC values (12,25). N-nmr spectra of hydantoins and their thio analogues have been studied (26). The N -nmr chemical shifts show a linear correlation with the frequencies of the N—H stretching vibrations in the infrared spectra. 

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... 

Relative activation enthalpies (Aif) in Table 2 were converted to o% kx k ) at 298 K, and were plotted against Hammett a constants. Here, we used enthalpies, because the size of the entropy and hence the free energy depend much on low frequencies, which are less reliable than higher frequencies, especially for compounds with weak interactions such as TS (8). The use of free energy (AG ) gave similar correlations with more scattered points. As for the Hammett o constant, we used dual-parameter o constants in the form of the Yukawa-Tsuno equation (LArSR equation) (9) as defined in eq 3. Here, the apparent a constant (aapp) has a variable resonance contribution parameter (r), which varies depending on the nature of the reaction examined for t-cumyl... 

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... 

At constant pH, the parameter p is dependent on the electronic effects of the nitrosoarene substituent(s). Using only definite acceptor substituted nitrosoarenes, a Hammett correlation on the a scale was obtained25. Separate investigation of fc2RSH and fc2(rearr) for a wider selection of nitrosoarenes reacting with GSH revealed reasonable correlation with Hammett a constants only for fc2RSH (p = +1.4). However, Utrearr) was found to fit better on the Hammett a+ scale (p+ = —3.5)30 [for further discussion of Utrearr) see Section II.D.l.b]. 

The broad applicability of LFERs for heterogeneous catalytic reactions has been demonstrated independently by Kraus (23) and Yoneda (24-27). The first author concentrated mostly on the established relationships such as the Hammett and Taft equations, whereas Yoneda has concentrated particularly on correlations with reactivity indices and other quantities. Since then, LFERs have been widely applied to heterogeneous catalytic reactions, and experience has been gained as to the suitability of each different type. An important step has been made toward an interpretation of the slopes of linear correlations (parameter a in Eq. 3) as the quantities that are closely connected with reaction mechanisms. 

Much work has been done on the quantitative correlation of the basicity of pyridines with Hammett substituent constants. The best single parameter correlation for 4-substituents is with ap... 

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... 

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