The Limits of Rovibrational Analysis: The Severely Entangled nu(1) Polyad Vibration of Dichlorodifluoromethane in the Greenhouse IR Window

摘要

Five intense bands of dichlorodifluoromethane (CFC-12, or R12) in the infrared atmospheric window help make it a major greenhouse contributor. These include the nu(1) fundamental at 1101.4 cm(-1) and the nu(2) + nu(3) combination at 1128.6 cm(-1). High-resolution spectra measured using the Australian Synchrotron Far-Infrared beamline were analyzed, and transitions of (CCl2F2)-Cl-35 were assigned to nu(1), nu(2) + nu(3), and the nu(3) + 2 nu(5) combination at 1099.7 cm(-1). The (v(3) = 1; v(5) = 2) state couples indirectly to v(1) = 1 via Fermi resonances linking both states with v(2) = v(3) = 1. The v(1) = 1 rotational levels are further riddled with perturbations and avoided crossings due to Coriolis resonance with the upper vibrational states of nu(2) + nu(9) at 1102.4 cm(-1) and (indirectly) nu(2) + nu(7) 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 nu(2) + nu(3). 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 nu(3) (458.6 cm(-1)), nu(7) (437.7 cm(-1)) and nu(9) (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 (CClClF2)-Cl-35-Cl-37, (CCl2F2)-Cl-37, and (CCl2F2)-C-13-Cl-35.