The limits of rovibrational analysis: The severely entangled ν1 polyad vibration of dichlorodifluoromethane in the greenhouse IR window

Abstract

Five intense bands of dichlorodifluoromethane (CFC-12, or R12) in the infrared atmospheric window help make it a major greenhouse contributor. These include the ν1 fundamental at 1101.4 cm-1 and the ν2 + ν3 combination at 1128.6 cm-1. High-resolution spectra measured using the Australian Synchrotron Far-Infrared beamline were analyzed, and transitions of C35Cl2F2 were assigned to ν1, ν2 + ν3, and the ν3 + 2ν5 combination at 1099.7 cm-1. The (ν3 = 1; ν5 = 2) state couples indirectly to ν1 = 1 via Fermi resonances linking both states with ν2 = ν3 = 1. The ν1 = 1 rotational levels are further riddled with perturbations and avoided crossings due to Coriolis resonance with the upper vibrational states of ν2 + ν9 at 1102.4 cm-1 and (indirectly) ν2 + ν7 at 1105.8 cm-1. A global treatment of all these states fits the observed line positions and satisfactorily accounts for the significant intensity of ν2 + ν3. Spectral simulations elucidate resonance perturbations that affect the distribution of IR absorption in the CF stretch region, and consequently the global warming potential of R12. Combination levels derived from rovibrational analysis lead to reassessment of the gas phase wavenumber values for the ν3 (458.6 cm-1), ν7 (437.7 cm-1) and ν9 (436.9 cm-1) fundamentals of C35Cl2F2, consistent with a cold, vapor phase far IR spectrum and previously published solid state spectra. B3LYP and MP2 anharmonic frequency calculations provide further support. At the MP2/aug-cc-pVTZ level, the root mean square (r.m.s.) error for unscaled anharmonic fundamentals is 6.2 cm-1, decreased to 1.7 cm-1 if only considering the seven lowest wavenumber modes, and integrated band intensities according with experimental literature values. Smaller basis sets produce band strengths that are too high. Low-resolution band assignments are reported for C35Cl37ClF2, C37Cl2F2, and 13C35Cl2F2. (Figure Presented).