Gas-Phase Reaction between CF2O and CF3C(O)OH: Characterization of CF3C(O)OC(O)F

The thermal decomposition of trifluoroacetic acid and carbonyl fluoride (CF₂O) has been extensively studied because of their importance in the oxidation of hydrochlorofluorocarbons in the atmosphere. We hitherto present the study of the thermal reaction between these two molecules. The reaction mechanism was studied using Fourier transform infrared spectroscopy in the temperature range of 513-573 K. The reaction proceeds homogeneously in the gas phase through the formation of a reaction intermediate, here characterized as CF₃C(O)OC(O)F (detected for the first time in this work), the major final products being CF₃C(O)F, HF, and CO₂. We demonstrate that the reaction is first-order with respect to each reagent, second-order global and the mechanism consists of two steps, the first being the rate-determining one. The E_a = 110.1 ± 6.1 kJ mol^-1 and A = (1.2 ± 0.2) × 10^-12 cm³ molec^-1 s^-1 values were obtained from the experimental data. The low activation energy is explained by the hydrogen-bond interactions between the -OH group of the acid and the F atom of the CF₂O. First-principles calculations at the G4MP2 level of theory were carried out to understand the dynamics of the decomposition. Thermodynamic activation values found for this reaction are as follows: Δ H^⧧ = 105.6 ± 6.4 kJ mol^-1, Δ S^⧧ = -88.6 ± 9.7 J mol^-1 K^-1, and Δ G^⧧ = 153.7 ± 13.5 kJ mol^-1. The comparison between theory and experimental results showed excellent similarities, thus strengthening the proposed mechanism.

Our changing atmosphere: evidence based on long-term infrared solar observations at the Jungfraujoch since 1950

The Institute of Astrophysics of the University of Liège has been present at the High Altitude Research Station Jungfraujoch, Switzerland, since the late 1940s, to perform spectrometric solar observations under dry and weakly polluted high-mountain conditions. Several solar atlases of photometric quality, extending altogether from the near-ultra-violet to the middle-infrared, were produced between 1956 and 1994, first with grating spectrometers then with Fourier transform instruments. During the early 1970s, scientific concerns emerged about atmospheric composition changes likely to set in as a consequence of the growing usage of nitrogen-containing agricultural fertilisers and the industrial production of chlorine-bearing compounds such as the chlorofluorocarbons and hydro-chlorofluorocarbons. Resulting releases to the atmosphere with ensuing photolysis in the stratosphere and catalytic depletion of the protective ozone layer prompted a worldwide consortium of chemical manufacturing companies to solicit the Liège group to help in clarifying these concerns by undertaking specific observations with its existing Jungfraujoch instrumentation. The following pages evoke the main steps that led from quasi full sun-oriented studies to priority investigations of the Earth's atmosphere, in support of both the Montreal and the Kyoto Protocols.

Infrared measurements of HF and HCl total column abundances above Kitt Peak, 1977-1990: seasonal cycles, long-term increases, and comparisons with model calculations

Series of high-resolution (approximately 0.01 cm-1) solar absorption spectra recorded with the McMath Fourier transform spectrometer on Kitt Peak (altitude 2.09 km, 31.9 degrees N, 111.6 degrees W) have been analyzed to deduce total column amounts of HF on 93 different days and HCl on 35 different days between May 1977 and June 1990. The results are based on the analysis of the HF and H35Cl (1-0) vibration-rotation band R(1) lines which are located at 4038.9625 and 2925.8970 cm-1, respectively. All of the data were analyzed using a multilayer, nonlinear least squares spectral fitting procedure and a consistent set of spectroscopic line parameters. The results indicate a rapid increase in total HF and a more gradual increase in total HCl with both trends superimposed on short-term variability. In addition, the total columns of both gases undergo a seasonal cycle with an early spring maximum and an early fall minimum, with peak-to-peak amplitudes equal to 25% for HF and 13% for HCl. In the case of HF, the changes over the 13 years of measurement are sufficiently large to determine that a better fit is obtained assuming a linear rather than an exponential increase with time. For HCl, linear and exponential models fit the data equally well. Referenced to calendar year 1981.0 and assuming a sinusoidal seasonal cycle superimposed on a linear total column increase with time, HF and HCl increase rates of (10.9 +/- 1.1)% yr-1 and (5.1 +/- 0.7)% yr-1 and total columns of (3.17 +/- 0.11) x 10(14) and (1.92 +/- 0.06) x 10(15) molecules cm-2 (2 sigma) are derived, respectively; the corresponding best fit mean exponential increase rates are equal to (7.6 +/- 0.6)% yr-1 and (4.2 +/- 0.5)% yr-1 (2 sigma). Over the 13-year observing period, the HF and HCl total columns increased by factors of 3.2 and 1.8, respectively. Based on HF and HCl total columns deduced from measurements on the same day, the HF/HCl total columns ratio increased from 0.14 in May 1977 to 0.23 in June 1990. Short-term temporal variations in the HF and HCl total columns are highly correlated; these fluctuations are believed to be caused by dynamical variability in the lower stratosphere. The results of this investigation are compared with previously reported measurements and with time-dependent, two-dimensional model calculations of HF and HCl total columns based on emission histories and photo-oxidation rates for the source molecules.