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A: Kinetics, Dynamics, Photochemistry, and Excited States
Photo Oxidation Reaction Kinetics of Ethyl Propionate with Cl Atom and Formation of Propionic Acid Ramya Cheramangalath Balan, and Balla Rajakumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05215 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018
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Photo Oxidation Reaction Kinetics of Ethyl Propionate with Cl Atom and Formation of Propionic Acid Ramya Cheramangalath Balan and B. Rajakumar * Department of Chemistry, Indian Institute of Technology Madras, Chennai-600036, India *corresponding author:
[email protected] http://chem.iitm.ac.in/faculty/rajakumar/ http://www.profrajakumar.com Abstract Temperature dependent rate coefficients for the photo oxidation reaction of ethyl propionate with Cl atom was investigated experimentally using relative rate technique. Gas chromatography with flame ionization detector (GC-FID), Gas chromatography-mass spectrometry (GC-MS) and GC-Infrared spectroscopy (GC-IR) were used to follow the concentrations and identification of reactants and products. The kinetics of ethyl propionate with Cl atoms was investigated over the temperature range of 263-363 K at atmospheric pressure, relative to C2H6 and C2H4. Theoretical calculations were also performed at CCSD(T)/6-311++G(d,p)//BHandHLYP/6-311G(d,p) level of theory, and the rate coefficients for H-abstraction reactions were calculated using canonical variational transition state theory (CVT) with interpolated single point energies (ISPE) method over the temperature range of 200-800 K. The temperature dependent rate coefficient for the reaction of ethyl propionate with Cl atom was obtained both experimentally as well as theoretically and are kExpt (T) = [(6.88 ± 1.65) × 10-24] T4.5exp [(1108 ± 87)/T] cm3 molecule-1s-1 and kTheory (T) = (6.73 × 10-19) T2.74exp [(571)/T] cm3 molecule-1s-1 respectively. Based on product analysis on the title reaction and the computational studies, we have proposed the atmospheric degradation mechanism and various pathways for Cl atom-initiated photo-oxidation of EP. Propionic acid is identified as the major product in the degradation of ethyl propionate on 1 ACS Paragon Plus Environment
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reaction with Cl atom. The thermochemistry, branching ratios and cumulative lifetime of ethyl propionate are calculated and presented in the manuscript. 1. Introduction Esters are volatile oxygenated organic compounds that are released globally via natural emissions and human-made activities. Atmospheric oxidation of ethers will also produce esters in the atmosphere1. Once these compounds get emitted into the Earth’s atmosphere, they react with atmospheric oxidants such as OH radical, Cl atoms, NO3 radicals etc. and serves as secondary source for many volatile organic compounds. Even though, the globally averaged concentration of OH radical (1 x 106 radical cm-3)2 is quite higher than the globally averaged concentrations of Cl atoms (1 x 103 atoms/ cm3)3, the rate coefficients of the esters with Cl atoms are higher than that of OH radicals. In marine boundary layer, the reaction of VOC with Cl atom is competitive with the respective reactions of OH radicals (1 x 105 atoms / cm3)4. Ethyl propionate (EP), C5H10O2, has a sweet odour like pineapple due to which it is widely used in food and perfume industries5. Fruits like kiwis and strawberries are the natural sources of EP6. Moreover, EP has wide applications as biodiesel compound7-8. The future dearth of petroleum oil is also looking for eco friendly renewable sources like ethyl propionate. The use of these molecules in various fields will directly emit EP into the atmosphere. Considering the potent applications of EP, the complete atmospheric fate of this molecule must be studied thoroughly. Therefore, to know the atmospheric fate of EP, the kinetic parameters for their reactions with oxidizing species such as OH radicals, NO3 radicals, Cl atoms and O3 molecule are required to be known. Langer et al.9 reported the rate coefficient of EP with NO3 radicals using discharge flow – visible absorption technique at room temperature, to be k
(EP+NO3)
= (3.30 ± 0.40) × 10-17 cm3
molecule-1 s-1. Wallington et al.10 used flash photolysis- resonance fluorescence technique and 2 ACS Paragon Plus Environment
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reported the rate coefficient at 298K, to be k
(EP+ OH)
= (2.14 ± 0.30) × 10-12 cm3 molecule-1 s-1.
Campbell et al.11 reported the rate coefficient for the reaction of EP with OH radical at 292 K and 100 Torr as, k (EP+ OH) = (1.76 ± 0.25) × 10-12 cm3 molecule-1 s-1. Cometto et al.12 reported the temperature dependent rate coefficient for the reaction of EP with OH radicals using pulsed laser photolysis laser-induced fluorescence technique in the temperature range of 243-372 K and 100104 Torr pressure as k (T) = [(0.59 ± 0.21) × 10-13] exp [(1064 ± 98)/T] cm3 molecule-1 s-1. To the best our knowledge, only two rate coefficients at room temperature are reported for the reaction of Cl atoms with EP at atmospheric conditions. Cometto et al.12 studied the room temperature rate coefficient for the reaction of EP with Cl atoms with respect to methanol and ethane by relative rate technique along with gas chromatography (GC). They have reported the rate coefficient for the title reaction at 298K to be k (EP+ Cl) = (3.70 ± 0.10) × 10-11 cm3 molecule-1 s-1. Andersen et al.13 measured the rate coefficient for the reaction of EP with Cl atoms relative to ethyl chloride, and ethylene as reference compounds at 980 mbar and 293 K and reported it to be k
(EP+ Cl)
= (3.11±0.35) ×10-11 cm3 molecule-1 s-1. They have used relative rate (RR) technique
along with FTIR spectrometer to monitor the concentration of the reactants. No temperature dependence of the reaction of Cl atoms with EP is available till date. Therefore, the temperature dependent photo-oxidation reaction kinetics for the reaction of Cl atom with EP was studied in the present study, using the relative rate technique over the temperature range of 263−363K. Product analysis has been carried out for the title reaction and propionic acid, acetic acid, acetone, vinyl propionate, HCl and CO2 were identified as products. To complement the experimental results, computational methods were used to calculate the rate coefficients for the reaction of Cl atoms with ethyl propionate using Canonical Variational Transition state theory (CVT) with Small-Curvature Tunneling (SCT) at CCSD(T)/cc3 ACS Paragon Plus Environment
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pVDZ//BHandHLYP/6-311g (d, p) level of theory. Based on the computational calculations, degradation mechanism was proposed for the reaction of EP with Cl atoms. The formation of products was found to be in line with the predictions by the present theoretical study. Moreover, the cumulative lifetimes, branching ratios and thermo-chemistry for the title reaction were calculated and reported in this manuscript.
2. Experimental Section Rate coefficients were measured for the Cl atom-initiated photo-oxidation reaction of EP over the temperature range of 263-363K, relative to ethane and ethylene. A Pyrex glass reaction chamber of 2L volume was used to carry out the relative rate experiments. The temperature inside the reaction chamber was controlled by circulating heated or cooled fluid in the double jacketed walls and was measured using a calibrated thermocouple with an accuracy of ±2 K. The ends of the chamber were mounted with 2-inch diameter fused silica windows. A KrF excimer (Coherent Compex Pro, 248 nm) laser was used for the generation of Cl atoms from (COCl)2 insitu. The laser fluence was maintained at ~6-10 mJ cm-2 pulse-1 during the experiments. The complete experimental procedure and additional information are available in our previous publications14,15.
Before starting the experiments, some preliminary tests were performed to confirm that there were no secondary reactions happening during the reaction. The reaction mixture consisting of the reactant (EP) and reference compound (either Ethylene or Ethane) was prepared 4 ACS Paragon Plus Environment
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in the reaction chamber and allowed to mix for about 4 hours in the absence of photolysis beam. The concentration of each compound was monitored by GC (Agilent Technologies 7890B) equipped with a Flame Ionization Detector (FID), at different time intervals during this process. HP plot Q capillary column was used for the analyses and the column was maintained at 160ºC. N2 was used as carrier gas. It was observed that there was no considerable reduction in the initial concentration of reactant and reference compounds, indicating the absence of dark reactions and wall losses (
