Estimates of Hammett's Sigma Values from Quadrupole Resonance Studies

A linear correlation exists between the Cl³⁵ pure quadrupole resonance frequency in substituted chlorobenzenes and the Hammett parameter sigma for the substituent. The relation is expressed as $$\text{f=1.024σ+34.826\hspace{0.17em}}\mathrm{Mcps}.$$ For the 52 frequencies used in determining this relation, the correlation coefficient is r = 0.96 and the root‐mean‐square deviation of the experimentally determined f values from those predicted by the equation is 360 kcps. A proper ortho sigma parameter for use in the f vs σ correlation is apparently the value derived from the Hammett relation σ = lnK/K₀ in which K and K₀ are the equilibrium constants for ionization of the substituted and unsubstituted benzoic acid in water at 25°C. This parameter is linearly proportional to the polar substituent σ^* 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 of f to σ 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 Cl³⁵ pure quadrupole resonance frequency measured in the substituted chlorobenzene.