Sigma-plus values

Fig. 3. Plot of Carbonyl 170 NMR chemical shift values(ppm) for ArCOX versus rho-plus values for chemical shift sigma-plus correlations(see ref 15).

Fig. 11. Plot of pK for aromatic acids (ArC02H2 ArCOjH + H ) versus sigma-plus. Data from K. Yates (1960) J. Am. Chem. Soc. 82, 4059. Open and filled circles represent Hammett sigma values, open circles sigma-plus parameters.

Sigma R Plus Series. Fits to Eq. (1) with Use of Values... 

The sigma values of or fc-substituents present problems similar to those of />am-substituents plus, very often, those introduced by steric distortion and by hydrogen-bonding. The most determined attack on this multifactorial problem was made by Fujita and Nishioka (1975) who set (Tq as equal to dp, plus a fraction of the Swain and Lupton field constant (to deal with the greater proximity), plus a fraction of Taft s steric factor see p. 627). When hydro-... 

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