Vibrational spectra dihalogenated benzenes. I. In-plane vibrations

Abstract

Infrared and Raman spectra of D₄-ortho, meta and para fluorochloro and fluorobromo benzenes are presented and assignments are given. Some of the bands of H₄-p-fluorochloro benzene assigned earlier as fundamentals are shown to be due to impurities and alternate assignments are given. The observed frequencies of the H₄ and D₄ molecules are combined in a normal coordinate analysis to obtain a 33-parameter modified valence force field, for the in-plane vibrations, with an average difference of ±0.8% between observed and calculated frequencies. The normal coordinate analysis shows that many of the previous assignments of the ring modes ν₁, ν₆, and ν₁₂ for disubstituted benzenes are not very reliablie. Revised assignments with good correlation with the corresponding benzene modes are given.