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 C(35)Cl2F2 were assigned to ν1, ν2 + ν3, and the ν3 + 2ν5 combination at 1099.7 cm(-1). The (v3 = 1; v5 = 2) state couples indirectly to v1 = 1 via Fermi resonances linking both states with v2 = v3 = 1. The v1 = 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 C(35)Cl2F2, 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 C(35)Cl(37)ClF2, C(37)Cl2F2, and (13)C(35)Cl2F2.

IR Band Profiling of Dichlorodifluoromethane in the Greenhouse Window: High-Resolution FTIR Spectroscopy of ν2 and ν8

“…Rigorous incorporation of the ν 1 state via a combined fit is no trivial undertaking because ν 1 itself is coupled to ν 3 + 2ν 5 , ν 2 + ν 3 , ν 2 + ν 7 , and ν 2 + ν 9 . This will be further elucidated in the work to be published on ν 1 . However, the effects of this a -axis Coriolis interaction may be approximated by adding an empirically determined Herman–Wallis correction term .…”

“…Evaluation of the force field of R12 has been performed by Giorgianni et al and Davis et al on the basis of low-resolution infrared spectra. , Table outlines the wavenumber values and symmetry of the vibrational bands of both C 35 Cl 2 F 2 and C 35 Cl 37 ClF 2 that have been examined to date. The ν 3 , ν 7 , and ν 9 fundamentals in the 400–500 cm –1 region are reassessed in light of solid state IR and Raman spectra measured by LeBlanc and Anderson, and of high-resolution analysis of the ν 1 , ν 3 + 2ν 5 , ν 2 + ν 3 , ν 2 + ν 7 , and ν 2 + ν 9 region around 1100 cm –1 supported by MP2/aug-cc-PVTZ anharmonic frequency calculations . It can be seen that only a few bands have been studied using high-resolution techniques, whereas the remainder have been studied using medium to low resolution.…”

“… a C 35 Cl 37 ClF 2 has C s symmetry: A′ = A 1 + B 1 ; A″ = A 2 + B 2 . b Reassessment of ν 3 , ν 7 , and ν 9 values based on solid state IR and Raman data of LeBlanc et al, and on MP2/aug-cc-PVTZ anharmonic frequency calculations and high-resolution band assignments for ν 2 + ν 3 , ν 2 + ν 7 , and ν 2 + ν 9 along with the far IR spectrum of R12 at 155 K …”

Far-Infrared Synchrotron Spectroscopy and Quantum Chemical Calculations of the Potentially Important Interstellar Molecule, 2-Chloroethanol

“…Torsional bands are well suited for investigation by synchrotron spectroscopy since they have relatively low energies that fall within well suited regions of the power spectra of far-infrared synchrotron sources. The analysis of the far-infrared synchrotron spectra of chlorine containing molecules presents a challenge due to the increased number of bands that can lead to somewhat congested spectra; − this arises from the two naturally occurring isotopes of chlorine ( 35 Cl/ 37 Cl ≈ 3). In the following we analyze the ν 19 fundamental for both isotopologues of 2CE, and complement the results with ab initio calculations.…”

Study of the Vibrational Spectra and Absorption Cross Sections of 1-Chloro-1-fluoroethene by a Joint Experimental and Ab Initio Approach

“…High-resolution spectroscopy is able to yield the requested molecular constants, − but the rovibrational analysis of the spectra of halocarbons is often complicated due to the congestions of the absorptions coming from low vibrational states and different isotopologues, as well as the presence of anharmonic and Coriolis interactions. Reliable band assignments combined with accurate modeling of vibrational couplings and high-level ab initio calculations provide the detailed basic data that, − coupled to the different experimental methods developed for reducing the spectral congestion (for example the use of cooling cells, − and supersonic jet expansion − ) and with the aid by sophisticated data-processing techniques, − are necessary to guide and support the rovibrational analysis. In addition to the spectroscopic interest due to the environmental issues related to the presence of haloethenes in the atmosphere, the potential threat to the stratospheric ozone layer related to their photodissociation and corresponding release of halogen atoms motivated many studies on the elimination mechanisms and distributions of kinetic energy, − the kinetics of their reactions with O­( 3 P), and their photochemistry. , …”