FT-IR spectroscopy in liquid xenon solution, ab initio calculations, normal coordinate analysis, and vibrational assignments of meso-2,4-dichloropentane and racemic-2,4-dichloropentane

Abstract

The infrared spectra of meso-2,4-dichloropentane and racemic-2,4-dichloropentane dissolved in liquid xenon have been obtained at 170 K in the range 400–4000 cm−1. The absorptions were observed with a low-temperature cell and a Fourier transform infrared spectrophotometer. The 2,4 dichloropentane sample was synthesized and its meso and racemic forms were separated with a spinning band distillation column. Ab initio molecular orbital calculations were performed on meso and racemic 2,4-dichloropentane to obtain the equilibrium geometry, vibrational frequencies, force fields, and infrared intensities. The calculations were done at the Hartree-Fock level using the 3-21G basis set. The Cartesian force fields from ab initio calculations have been converted to the force field in symmetry coordinates and scaled. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. Potential energy distributions (PED) were also calculated. Vibrational normal modes of the most stable rotamer of symmetry C1 of meso-2,4-dichloropentane and the most stable rotamer of symmetry C2 of racemic 2,4-dichloropentane have been assigned to infrared absorption bands observed in liquid xenon solution. The assignments were based on calculated frequencies and PEDs. Variable temperature studies of the infrared spectrum of meso and racemic 2,4-dichloropentane dissolved in liquid argon were done between 165 and 195 K. Other rotamers could not be observed from the temperature dependent studies of the absorption band profile. The drastic decrease in the congestion of the spectra of the liquid xenon solution compared with the gas phase, room temperature spectra, combined with the observed small frequency shifts, allows the determination of reliable band positions.