Improved assignments of the vibrational fundamental modes of ortho-, meta-, and para-xylene using gas- and liquid-phase infrared and Raman spectra combined with ab initio calculations: Quantitative gas-phase infrared spectra for detection

Abstract Xylenes contain a blend of the ortho -, meta -, and para - isomers, and all are abundant contaminants in the ground, surface waters, and air. To better characterize xylene and to better enable its detection, high quality quantitative vapor-phase infrared spectra of all three isomers over the 6500 – 540 cm −1 range are reported. All fundamental vibrational modes are assigned based on these vapor-phase infrared spectra, liquid-phase infrared and Raman spectra, along with density functional theory (DFT), ab initio MP2 and high energy-accuracy compound theoretical model (W1BD) calculations. Both MP2 and DFT predict a single conformer with C 2v symmetry for ortho -xylene, and two conformers each for meta - and para -xylene, depending on the preferred orientations of the methyl groups. For meta -xylene the two conformers have C s and C 2 symmetry, and for para -xylene these conformers have C 2v or C 2h symmetry. Since the relative population of the two conformers is approximately 50% for both isomers and predicted frequencies and intensities are very similar for each conformer, an arbitrary choice to discuss the C s conformer for meta -xylene and the C 2v conformer for para -xylene is made. Integrated band intensities for all isomers are reported. Using the quantitative infrared data, the global warming potential values of each isomer are determined. Potential bands for atmospheric monitoring are also discussed.