Taft resonance parameters

In the form of treatment developed by Taft and his colleagues since 195660-62, the Hammett constants are analyzed into inductive and resonance parameters, and the sliding scale is then provided by multiple regression on these. Equations 5 and 6 show the basic relationships. 

Taft and Topsom151 have fairly recently written an extensive review of the electronic effects of substituents in the gas phase. This article includes a tabulation of substituent inductive and resonance parameters. The inductive parameters (designated Op) are based on measured spectroscopic properties in either the gas phase or in hydrocarbon or similar solvents. The resonance parameters were arrived at through the treatment of 38 gas-phase reactivity series by iterative multiple regression, using the cr values of Bromilow and coworkers155 as the starting point. The of value for NO2 was found to be 0.65 (quoted... 

Positive slopes in partial correlations of small series for which the Taft equation is suitable. For a resonance parameter r = 0.8. 

While good <5ES correlations within a-substituted series were found in mechanistic studies, the reason for nonapplicability of Es parameters to a / -series in the quaternization of thiazoles was not experimentally determined. There are two possible reasons for this nonapplicability. (1) The Taft Es parameters are not solely a measure of steric effects, but rather also contain inductive, resonance, and other effects and must be corrected and more precisely redefined. (2) The Taft Es parameters do measure steric effects, but,... 

Carbon-13 NMR data for wCsHsFe (CO)2X appear in Table XL. In this investigation (100), correlations between Taft ai parameters and SCO were observed. This correlation showed that as the electron demand of X increased the CO resonance was found at higher field. The chemical shift of the carbonyl... 

T-parameter of X, a better fit was obtained with a combination of Taft inductive and resonance parameters. The effect is more dramatic in tram-FC6H4PtX(PEt3)2 compounds. The shielding parameters of the m-and p-fluorophenyl derivatives permit a more direct evaluation of the electronic interaction between an anionic ligand and the metal (75). In both the titanium and platinum compoimds, the metal atom appears to release electron density to the aromatic ring. 

Addition to a terminal alkene shows only a P effect. Both electron withdrawing and electron donating groups increase the rate of radical addition when in this position, because the orbital resulting after radical addition is lower in energy due to delocalization of the radical by either inductive, hyperconjugative, or resonance effects. The sensitivity to the Taft steric parameter is relatively small (S = 0.28), meaning that sterics play very little role in the p-position. 

The Hammett cr-values contain contributions from both inductive/field effects and the resonance effect. The cr-constant can be separated quantitatively into a resonance component R, which operates mainly in the para position, and an inductive component F, which is assumed to be equal in the meta and para positions. Hansch, Leo and Taft have calculated the F and R values of MesSi to be 0.01 and —0.08, respectively, as quoted in Table 1. These values seem somewhat at odds with experimentally determined values for the inductive and resonance parameters, which give mean values of —0.08 and 0.06, respectively. These values confirm the generally accepted view that MesSi is electron-supplying by inductive effects and electron-withdrawing by resonance effects. 

In SAR work, the biological activity of compounds is usually expressed by the values of IC50 or ED50. Physico-chemical properties used in SAR can be broadly classified into three general tjq)es electronic parameters, steric parameters and hydrophobic parameters. Electronic parameters include Hammett constants o ,(7, <7 ), Swain and Lupton field parameter (F), Swain and Lupton resonance parameter (R), etc. Steric parameters include Taft s steric parameter (Eg), molecular volume (Vm), molecular surface area (Area), molecular weight (MW), van der Waals radius (r), molar refractivity (MR), parachor (Pr), etc. Hydrophobic parameters include partition coefficient (LogP), distribution coefficient (LogD), substituent constant ( ), solubility... 

Taft inductive and resonance parameters were calculated for a series of monomers from data and used to identify activating forces for the monomers. NMR data were compared with calculated net atomic charges. Relative reactivity studies were also performed in order to verify the utility of this fast and convenient NMR probe of monomer reactivity. 

In the related paper on screening parameters of para-substituted fluorobenzenes in relation to resonance effects, a few measurements for SOMe and S02Me were recorded but no use was made of them for calculation of o-R-type parameters . However, some years later, Sheppard and Taft used these data (carbon tetrachloride solution) to calculate values through equation 11 ... 

More advanced scale was proposed by Kamlet and Taft [52], This phenomenological approach is very universal as may be successfully applied to the positions and intensities of maximal absorption in IR, NMR (nuclear magnetic resonance), ESR (electron spin resonance), and UV-VS absorption and fluorescence spectra, and to many other physical or chemical parameters (reaction rates, equilibrium constant, etc.). The scale is quite simple and may be presented as ... 

Taft and Topsom s article151 and also Topsom s171 should be consulted for details of the setting up of the scales of substituent parameters. The equation has been applied to a wide range of gas-phase reactivities. (In the multiple regressions an intercept term is often permitted, but usually this turns out to be indistinguishable from zero, as it should be if equation 20 is valid.) For aliphatic and alicyclic saturated systems the resonance term is duly negligible. The roles of field, resonance and polarizability effects are discussed and the interpretat of the various p values is attempted. 

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