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J. Phys. Chem. A 2007, 111, 11608-11617
Kinetic Study of the Reactions of Cl Atoms with CF3CH2CH2OH, CF3CF2CH2OH, CHF2CF2CH2OH, and CF3CHFCF2CH2OH Vassileios C. Papadimitriou,† Dimitrios K. Papanastasiou, Vassileios G. Stefanopoulos, Aristotelis M. Zaras, Yannis G. Lazarou,‡ and Panos Papagiannakopoulos* Laboratory of Photochemistry and Kinetics, Department of Chemistry, UniVersity of Crete, 71003 Heraklion, Crete, Greece ReceiVed: June 18, 2007; In Final Form: August 20, 2007
The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF3CH2CH2OH (1), CF3CF2CH2OH (2), CHF2CF2CH2OH (3), and CF3CHFCF2CH2OH (4), were studied in the gas phase over the temperature range of 273-363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm3 molecule-1 s-1): k1 ) (4.42 ( 0.48) × 10-11 exp(-255 ( 20/T); k1(303) ) (1.90 ( 0.17) × 10-11, k2 ) (2.23 ( 0.31) × 10-11 exp(-1065 ( 106/ T); k2(303) ) (6.78 ( 0.63) × 10-13, k3 ) (8.51 ( 0.62) × 10-12 exp(-681 ( 72/T); k3(303) ) (9.00 ( 0.82) × 10-13 and k4 ) (6.18 ( 0.84) × 10-12 exp(-736 ( 42/T); k4(303) ) (5.36 ( 0.51) × 10-13. The quoted 2σ uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (RF-CHO; RF ) -CH2CF3, -CF2CF3, -CF2CHF2, and -CF2CHFCF3), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the R-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2-C5 ethers and C2-C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm3 molecule-1 s-1) ) (5.8 ( 1.4) - (1.56 ( 0.13) × (ionization potential (in eV)).
1. Introduction The negative impact of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) in stratospheric ozone depletion1,2 and global warming1 has led to international agreements (Montreal Protocol), which have imposed restrictions on their production and have stressed the need for their replacement with more friendly compounds. In particular, hydrofluorocarbons (HFCs) have been suggested as potential substitutes because they do not contain chlorine or bromine atoms and therefore have no contribution to the stratospheric ozone destruction.3 However, their strong infrared absorbance in the atmospheric window, due to the C-F bonds, and their relatively long atmospheric lifetimes classify them among the anthropogenic greenhouse gases,1 which are regulated by the Kyoto Protocol, and therefore need to be replaced by environmentally friendly alternatives. Partially fluorinated alcohols (HFAs) are considered among the most promising CFC substitutes because they are expected to have higher chemical reactivities than HFCs due to the presence of the -OH group and therefore shorter atmospheric lifetimes. HFAs are already used as cleaning agents4 as well as in a variety of industrial applications5, and therefore it is essential * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Present address: Earth System Research Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, CO 80305-3328. ‡ Present address: Institute of Physical Chemistry, National Centre for Scientific Research “Demokritos”, Aghia Paraskevi 153 10, Attiki, Greece.
to assess their impact on global warming and climate change. At first it is very important to determine their atmospheric lifetimes by measuring their rate coefficients with major atmospheric reactants, such as OH radicals and Cl atoms, which initiate their chemical degradation in the troposphere. Although the reaction with OH radicals is the main removal pathway of HFAs in the troposphere, the reactions with less abundant Cl atoms may also contribute to their removal due to the higher reactivity of Cl atoms. Concentrations of chlorine atoms on the order of 104 molecule cm-3 have been detected over the marine boundary layer6-9 as well as during ozone episodes in urban atmospheres.10 Moreover, the reaction kinetics of HFAs with chlorine atoms will provide (a) information about the relationship between structure (position and degree of fluorination) and chemical reactivity of HFAs and (b) comparison with the reactivity of other CFC alternatives. In addition, it will provide information about the primary reaction pathways of HFAs, and therefore about the degradation mechanism of HFAs in the troposphere. Reaction kinetics of several HFAs with Cl atoms have been studied in the past by three groups,11-14 in addition to the study of their tropospheric degradation mechanisms. The aim of this work is to measure the absolute rate coefficients for reactions 1-4 over the temperature range of 273-363 K by employing the very low-pressure reactor mass spectrometry (VLPR-MS). Furthermore, the subsequent oxidation mechanism was also investigated by mass spectrometric end-product analysis.
10.1021/jp074728e CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007
Reactions of Cl Atoms with HFAs
J. Phys. Chem. A, Vol. 111, No. 45, 2007 11609
Cl + CF3CH2CH2OH f HCl + products
(1)
Cl + CF3CF2CH2OH f HCl + products
(2)
Cl + CHF2CF2CH2OH f HCl + products
(3)
Cl + CF3CHFCF2CH2OH f HCl + products
(4)
Finally, quantum mechanical calculations were performed to determine the bond strengths of C-H and O-H bonds and the ionization potentials of the title compounds to investigate the relationship between structure and reactivity for straight-chain fluorinated alcohols.
Cl and HCl, respectively. The ratio aHCl/aCl (see eq III) was also determined by performing accurate titration experiments using the reaction of Cl atoms with n-C6H14 and was found to be 1.3 ( 0.1. Thus the flow rate FCl was obtained by using eq II
FCl )
2∆FCl2 1 + IHCl/(1.3 × ICl)
The steady-state concentration of all species in the reactor was determined by using the expression
[M] )
2. Experimental Methods The absolute rate coefficients were measured by using the VLPR-MS apparatus, which has been presented in detail previously.15 However, a brief description of the apparatus and the experimental procedures followed in the kinetic measurements are presented below. The reactants enter the Knudsen reactor (Vcell ) 107 cm3) through separate capillary inlets to suppress back-streaming. The double-wall reactor is thermostated, and its internal surface is coated with a thin Teflon (Du-Pont Teflon 121A) layer to inhibit wall reactions. The reaction mixture escapes from the reactor through a 5 mm orifice into the first stage of a differentially pumped system. A conical skimmer and a tuning fork chopper located in the second vacuum chamber result in the formation of a 200 Hz modulated effusive molecular beam that is collimated into the ionization region of a quadrupole mass spectrometer (Balzers QMG511). The modulated component of the mass spectrometric signal is discriminated and amplified by a lock-in amplifier and subsequently transferred with an A/D card into a microcomputer, where it is stored for subsequent analysis (Figure 1). Chlorine atoms were produced by flowing a 5% mixture of Cl2 in helium through a quartz tube, which was enclosed in a 2.45 GHz Evenson microwave cavity operating at 35 W. The quartz tube was coated with a dried slush of a fresh phosphoric acid-boric acid mixture (H3PO4/H3BO3) to inhibit the recombination of chlorine atoms on the walls. The dissociation of Cl2 was almost complete, converted into Cl atoms and HCl inside the quartz tube. The absence of any remaining Cl2 inside the reactor was verified by monitoring the mass peak at m/z 70 in the absence of other reactants. All the kinetic measurements were carried out by setting the ionization electron energy at 19 eV to suppress HCl fragmentation and to avoid its contribution at m/z 35 (8C) fluorotelomer alcohols pose a threat to the environment,39,40 because they consist of a likely source for the potentially toxic perfluorinated carboxylic acids (PFCAs). Acknowledgment. This work is part of the project “Impact of Fluorinated Alcohols and Ethers on the Environment” and has received support from the CEC Environment and Climate Program (through Contract No. ENVK2-1999-00099). It was also supported by the Greek Secretariat of Research and Technology within the project TROPOS (2005-2006). Computational work was partially supported by the “Excellence in the Research Institutes” Program, Action 3.3.1, which was funded by the Greek Ministry of Development. We thank Dr. D. Levi of DuPont de Nemours International for providing us with the Teflon FEP 121A aqueous dispersion. Supporting Information Available: Optimized structural parameters of all species and their harmonic vibrational frequencies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) World Meteorological Organization. Scientific Assesment of Ozone Depletion: 2002; Global Ozone Research and Monitoring Project, Report No. 47; World Meteorological Organization: Geneva, Switzerland, 2003. (2) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (3) Wallington, T. J.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. J. Chem. Soc., Faraday Discuss. 1995, 100, 55. (4) Tokutei Furon, Tokutei Furon Siyousakugen Manyuaru; Fluorocarbon Manufacturers Association: Tokyo, Japan, 1990.
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