Spectroscopic inversion of the mesomeric effect of the four halogens in ring-substituted acetophenone derivatives

Abstract

The absolute Raman intensity ( I R) and the i.r. intensity ( A i.r.) of the ν CO band of ring substituted acetophenones have been measured in carbontetrachloride. The √( A i.r.) values of the para derivatives are correlated with the resonance substitution constants (ϱ R + = − 2.98) and the √( I R) values depend on resonance (ϱ R + = − 3.05) and inductive (ϱ I = − 1.30) substitution constants. An inversion effect was however observed for the halogens. From the experimental values of this work and from the values of the scattering coefficients determined by Michel, the computed spectroscopic σ R + values of the four halogens are: −0.02 ( p-F), −0.31 ( p-Cl), −0.46 ( p-Br) and −0.69 ( p-I). These σ R + values are linearly related to the polarizability of the halogen atoms. Further, the frequency shift of the 1 L a band of monosubstituted benzene and of the 1 B band of p-acetophenones are in very good agreement with these new σ R + values. The depolarization ratio seems also to increase when the electron releasing power of the substituent becomes greater.