Charton’s steric parameter

Charton s steric parameter, v,(5) was examined as an alternative to DICARB for bulk at Ri and R2 Eiraryl, for the substituents on the aryl ring. Preliminary regression equation results achieved using such alternatives were similar but not superior to those outlined below the carbon counts for DICARB and ESCARB have the further advantage of conceptual simplicity and computational accuracy. 

These can be broadly divided into those that apply to sections of the molecule and those that involve the whole molecule. The former include parameters such as van der Waals radii, Charton s steric constants and the Verloop steric parameters. The latter range from relative molecular mass (RMM) and molar volumes to surface area. They have all been used to correlate biological activity to structure with varying degrees of success. 

In this equation ctj is Charton s electronic parameter for inductive effects of the substituents. The positive coefficient indicates that electron-attracting substituents are favourable. It is demonstrated in the paper that tt values are not position independent. Besides an intrinsic hydrophobic factor, intramolecular steric and hydrogen-bonding components are also included. The final equation including all compounds is equation (6). 

Two other steric parameters are independent of any kinetic data. Charton s v values are derived from van der Waals radii/ and Meyer s values from the volume of the portion of the substituent that is within 0.3 nm of the reaction center. The V values are obtained by molecular mechanics calculations based on the structure of the molecule. Table 9.7 gives v and value.s for some groups. As can be seen in the table, there is a fair, but not perfect, correlation among the Ei, v, and values. Other sets of steric values, (e.g., and have also been proposed. ... 

While the development of the Taft parameter is similar to that of Hammett and Hansch, / )-val ucs are based on rate constants instead of equilibrium constants. The Taft parameter is a measure of changes in activation energy, not standard free energy. Of the Hammett, Hansch, and Taft parameters, the Taft parameter is utilized the least in QSAR studies. Other steric parameters have been developed over time, and like the Taft parameter, all have shortcomings. One alternative steric parameter was developed by Marvin Charton of Pratt Institute in New York. Charton s parameter is based on the van der Waal radius of a substituent.6 Another alternative steric model is the STERIMOL parameter set developed by Arie Verloop of Philips-Duphar in Holland.7 Unlike Taft and Charton, Verloop... 

For the purpose of correlation analysis or linear FREE-ENERGY RELATIONS, various scales of steric parameters have been proposed, notably A values, Taft s Es, and Charton s u scales. 

Lall, R.S. (1982) Steric effect I — Charton s parameter vs topological index. Curr. Sci. -India, 51, 775—777. 

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