Estimates of Hammett's Sigma Values from Quadrupole Resonance Studies

摘要

A linear correlation exists between the Cl35pure quadrupole resonance frequency in substituted chlorobenzenes and the Hammett parameter sigma for the substituent. The relation is expressed asfequals;1.024sgr;plus;34.826thinsp;Mcps.For the 52 frequencies used in determining this relation, the correlation coefficient isr= 0.96 and the roothyphen;meanhyphen;square deviation of the experimentally determinedfvalues from those predicted by the equation is 360 kcps.A proper ortho sigma parameter for use in thef vssgr; correlation is apparently the value derived from the Hammett relation sgr; = lnK/K0in whichKandK0are the equilibrium constants for ionization of the substituted and unsubstituted benzoic acid in water at 25deg;C. This parameter is linearly proportional to the polar substituent sgr;*of Taft for the ortho substituent.In the many multiply substituted chlorobenzenes investigated, the proper total sigma value has been taken as the sum of the individual parameters. Few cases are apparent in which the linear relation offto sgr; is seriously violated. Additivity of the electron displacement properties of substitution, suggested by Jaffe and indicated by studies of the magnetic shielding in fluorobenzenes, is apparently confirmed.Sigma values for 29 substituents have been estimated from the Cl35pure quadrupole resonance frequency measured in the substituted chlorobenzene.