CH Stretching Overtone Spectra of Fluorine Substituted Toluenes

Abstract

We have measured and analyzed the vapor phase CH stretching overtone spectra of 3,5-difluorotoluene, 2,3,5,6-tetrafluorotoluene, and 2,3,4,5,6-pentafluorotoluene. The aryl bands are complex due to Fermi resonance. We have used a two-level system model to analyze the Fermi resonance within the aryl bands. The aryl regions of the overtone spectra are “corrected” for Fermi resonance and interpreted as uncoupled stretching oscillators. The methyl band profiles are complex due to the coupling between CH stretching and methyl torsion. The methyl band profiles are simulated with a CH stretching methyl torsion model. The CH stretching modes are described by a harmonically coupled anharmonic oscillator local mode model, and the methyl torsion is described by a rigid rotor model. Simulation parameters are obtained from ab initio calculations with the HF/6-31G(d) method. Relative intensities of aryl and methyl contributions in the CH stretching overtone spectra in the region ΔvCH = 2−5 are measured and calculated for these molecules. We have found that the methyl bands of 3,5-difluorotoluene are very similar to corresponding bands in toluene, 4-methylpyridine, p-xylene, and m-xylene. The methyl bands of 2,3,5,6-tetrafluorotoluene and 2,3,4,5,6-pentafluorotoluene are virtually identical to those of 2,6-difluorotoluene. Thus, the main factor that determines the structure in methyl CH stretching overtone spectra of these molecules is the nature of the groups adjacent to the methyl group. The aryl bands of these molecules are primarily affected by the nearest substituents.