Density Functional Theory Calculations on Atmospheric Degradation Reactions of Fluorinated Propenes

Scientists from NCSR “Demokritos”University of CreteNational Oceanic and Atmospheric Administration and  University of Colorado  have used the HellasGrid Infrastructure and the EGI Grid infrastructure in order to solve problems coming from the area physics-chemistry.

Halogenated organic compounds are being effectively utilized as industrial solvents, fire-suppressants and refrigeration media for several decades, the first and best known class being ChloroFluoroCarbons (CFC). However, CFCs are harmful to the protective layer of stratospheric ozone, attributed to the presence of chlorine atoms. Thus, new classes of CFC alternatives containing no chlorine atoms are appropriately devised, which must additionally possess short atmospheric lifetimes and a minimal contribution to global warming. An extensive investigation of their reactivity is primarily performed by experimental studies, in order to assess their impact in atmospheric quality. Since experiments may not be able to provide all the answers, assistance is addressed to molecular quantum mechanical calculations.

The atmospheric reactivity of two hydrofluoro-olefins (HFOs), CF3CF=CH2 (2,3,3,3-tetrafluoropropene, HFO-1234yf) and (Z)-CF3CF=CHF (1,2,3,3,3-pentafluoropropene, HFO-1225ye), was investigated both experimentally and theoretically. Theoretical calculations were performed by Density Functional Theory (DFT) in order to elucidate several aspects of HFOs chemistry in the atmosphere.

Theoretical calculations yielded equilibrium molecular structures along with vibrational frequencies and the absolute electronic energies at reliable levels of theory comprising of the B3P86 functional with the aug-cc-pVDZ and aug-cc-pVTZ basis sets, respectively. The data were subsequently fed into statistical thermodynamics formulas to compute the reaction enthalpies. The most likely sites for the Cl, OH and NO3 addition to the double bond of each HFO were determined, and the energetics of the initial adduct formation pathways helped to explain the experimentally observed pressure dependence difference between Cl and OH reactions. Furthermore, the calculated exothermicities for the degradation reactions of peroxy radicals in the presence of O2, NO and HO2 yielded the most likely atmospheric degradation products of HFOs, in support of the experimental results.

Figure 1 - Enthalpy diagram (298.15 K) for the reaction of Cl, NO3, and OH with CF3CF=CH2 and formation of peroxy adducts calculated at the B3P86/aug-cc-pVTZ level of theory.
As, a large amount of calculations was needed, due to the number of molecular species involved in this work, the computing power of the South-Eastern European Virtual Organization (SEE-VO) was effectively utilized to accomplish the task. Automation of the job submission and data retrieval processes was performed by shell scripts using commands of the gLite middleware.

Future plans include the examination of the degradation mechanism of halogenated organic molecules as harmful and toxic pollutants in aqueous  environments is currently being investigated using Density Functional Theory (DFT).

Contacts

  • Dr. Yannis G. Lazarou, Institute of Physical Chemistry, NCSR “Demokritos”, Aghia Paraskevi, Attiki, Greece, lazarou (at) chem.demokritos.gr
  • Dr. Vassileios C. Papadimitriou, Department of Chemistry, University of Crete, Heraklion, Greece, bpapadim (at) chemistry.uoc.gr
  • Dr. James B. Burkholder, ESRL, CSD, National Oceanic and Atmospheric Administration (NOAA), Boulder, Colorado, USA, James.B.Burkholder (at) noaa.gov
  • Dr. Ranajit K. Talukdar, CIRES, University of Colorado, Boulder, Colorado, USA, Ranajit.K.Talukdar (at) noaa.gov

Reference

  1. Atmospheric Chemistry of CF3CF=CH2 and (Z)-CF3CF=CHF: Cl and NO3 Rate Coefficients, Cl Reaction Product Yields, and Thermochemical Calculations” V.C. Papadimitriou, Y.G. Lazarou, R.K. Talukdar, J.B. Burkholder, J. Phys. Chem A2011, 115, 167 – 181.