Location of the first excited states of fluorinated ethers, E143a (CH3OCF3), E134 (CHF2OCHF2), and E125 (CHF2OCF3)
Introduction
Because of CFCs ability to deplete stratospheric ozone, many applications have begun to employ HFCs (hydrofluorocarbons) as working fluids. HFCs contain no chlorine and thus are given ozone depletion potentials of essentially zero. Attention is now focused on global warming, with the goal of reducing greenhouse gas emissions. Air conditioners, heat pumps, and refrigeration devices that use refrigerants also use energy. They contribute to global warming both by the release of the refrigerant and by the emission of carbon dioxide and other greenhouse gases in powering the devices [1]. Some HFCs have fairly significant global warming potentials 2, 3. Leading CFC replacements such as HFC-125, HFC-134a and HFC-143a have global warming potentials not much lower than the CFCs they replace [2]. For this reason research is still devoted toward the exploration of novel CFC alternatives, third generation CFC alternatives.
One such third generation CFC alternative is the fluorinated ether series. These species are analogous to HFCs with the addition of an ether linkage. Generally, four main pathways lead to the destruction of organic molecules in the atmosphere, reaction with OH radicals, reaction with chlorine atoms, reaction with O(1D), and UV photolysis.
Zhang et al. [4] measured the rate of reaction of fluorinated ethers with hydroxyl radical using a flash photolysis resonance fluorescence technique. E143a (CH3OCF3), E134 (CHF2OCHF2), and E125 (CHF2OCF3) were found to have rate constants of 2.14×10−14, 2.53×10−14, and 3.38×10−15, cm3 molecule−1 s−1. Lifetimes on the order of 3.0, 2.6, and 19 years respectively were calculated [4]. Garland et al. [5] in 1993 measured the rate of reaction of OH radicals with E134 using pump- and probe-laser induced fluorescence of OH. A rate expression of 5.4×10−13e−1560/T was determined. At 298 K the rate is found to be 3.0±0.7×10−15 cm3 molecule−1 s−1. A lifetime on the order of 24 years was determined [5]. Huie et al. measured the rate of E134 reaction with OH and determined the 298 K rate constant to be 5.1×10−15 cm3 molecule−1 s−1 (reported by Garland et al. [5]). Hsu et al. used a relative rate technique to measure the 298 K OH rate constants of E143a, E134 and E125 to be 1.03×10−14, 2.27×10−15, and 4.16×10−16 cm3 molecule−1 s−1[6]. For E134 the results of Garland, Huie, and Hsu are in reasonable agreement 5, 6. Good et al. [7] using a 2D chemical transport model and the rate data of Hsu et al. [6] determined the lifetimes of E143a, E134 and E125 to be 5.7, 29.0, and 165.0 years respectively. These results suggest that these ethers have much longer atmospheric lifetimes than previously suspected and that penetration of these ethers into the upper stratosphere is plausible [7].
Kambanis et al. [8] measured the temperature dependant rate of reaction between E134 and chlorine atoms to be 1.03±0.19×10−12e−867/T cm3 molecule−1 s−1. At 298 K the rate constant is thus 5.61×10−14 cm3 molecule−1 s−1. This rate is approximately 24 times faster than the rate or reaction between E134 and hydroxyl radicals as measured by Hsu et al. [6]. Kambanis et al. [8] using a tropospheric chlorine atom concentration of 104 molecules cm−3 determined an atmospheric lifetime of approximately 61.6 years. Christensen et al. [9] using a relative rate technique measured the 298 K rate of reaction between chlorine atoms and E143a to be 1.4×10−13 cm3 molecule−1 s−1. An atmospheric lifetime of 22.6 years with respect to loss due to reaction with chlorine atoms can be estimated. This can be compared to the lifetime with respect to loss from reaction with OH of 5.7 years. The rate of reaction between E125 and chlorine atoms has to date not been measured.
Good et al. [10] determined the rate of chemical reaction of E143a, E134 and E125 with excited oxygen atoms, O(1D). Temperature independent rate constants of 1.0×10−10, 2.5×10−11, and 1.6×10−11 cm3 molecule−1 s−1 were determined for E143a, E134, and E125, respectively. It was suggested that reaction with O(1D) may contribute to the loss of E125.
To the best of our knowledge no work on the photolysis has been reported in the literature. In order to comment on the feasibility of photolysis as a removal mechanism for these ethers, it is necessary to know the location of the first excited state of each fluorinated ether. No experimental spectra have been reported in the literature for these species. This work presents ab initio studies of the first excited state of E143a, E134, and E125 are presented.
Section snippets
Methodology
Ground state geometries were optimized using the B3LYP method [11]. The large 6-311++G(3df,3pd) basis set was used and with this methodology a frequency calculation was performed. All frequencies were positive indicating that the structure was indeed a local minimum. The structure of each ether has been described in previous work [12]. With these geometries, a calculation of the first excitation energy was performed. A state-averaged CASSSCF calculation with equal weights for the states studied
Results and discussions
On the ground state, the highest occupied molecular orbital (HOMO) of each of these molecules involves one of the lone pairs of electrons on the oxygen atom. The transition from the ground state to the first excited state corresponds to a transition from the lone pair of oxygen to a virtual molecular orbital, which has Rydberg character. When hydrogen is replaced by fluorine in these molecules, the energy difference between the HOMO and the lowest lying virtual orbital increases. The result is
Atmospheric significance
There are several factors that limit the effectiveness of photolysis as a removal mechanism for these ethers in the upper atmosphere. The first is that the concentrations of these ethers in the upper atmosphere will be significantly less than concentrations in the lower atmosphere. Good et al. [10] using 2D chemical transport modeling estimated the volume mixing ratio for each ether taking into consideration reaction with hydroxyl radical. Using this data along with the known atmospheric
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