Photo Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Photo Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2Butanone with Cl Atoms and OH Radicals in the Gas Phase Ramya Cheramangalath Balan, and Balla Rajakumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00995 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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The Journal of Physical Chemistry

Photo Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2Butanone with Cl atoms and OH radicals in the Gas phase Ramya Cheramangalath Balan and B. Rajakumar* Department of Chemistry, Indian Institute of Technology Madras, Chennai-600036, India *corresponding author: [email protected]

Abstract The temperature dependent rate coefficients for the gas phase reaction of 4-hydroxy-2butanone (4H2BN) with Cl atoms and OH radicals were explored experimentally using relative rate technique and computational methods. The concentrations of the reactants as well as products were followed using Gas Chromatography with Flame Ionization Detector (GC-FID), Gas Chromatography/Mass Spectrometry (GC-MS) and Gas Chromatography/Infrared spectroscopy (GC-IR) as analytical techniques. Formaldehyde was obtained as the major product during the title reaction. The kinetics of 4H2BN with Cl atoms/OH radicals were measured over the temperature range of 298-363 K at 760 Torr in N2 atmosphere using C3H8, C2H4, Iso-propanol and n-propanol as reference compounds. The temperature dependent rate coefficient for the reaction of 4H2BN with Cl atom/OH radical were obtained as, kExpt(T) = [(1.52±0.86) × 10-26] T5 exp [(2474±450)/T] cm3 molecule-1 s-1 and k (T) = [(2.09±0.24) × 1012]

exp [-(409±15)/T] cm3 molecule-1 s-1. Theoretical calculations were carried out at M062X/6-

31G(d,p) and M06-2X/6-31+G(d,p)

level of theories, and the rate coefficients for H-

abstraction reactions were evaluated using Canonical Variational Transition State (CVT) Theory with the inclusion of Small-Curvature Tunneling correction (SCT) over the temperature range of 200-400 K. The rate coefficients obtained over the studied temperature range were used to fit the data and the Arrhenius expression was obtained to be: kCl(Theory) (200-400 K) = (6.10 × 10-25) T4.42 exp(2397/T) cm3 molecule-1 s-1, kOH(Theory) (200-400 K) = (1.13 × 10-19) T2.27 1 ACS Paragon Plus Environment

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exp(1505/T) cm3 molecule-1 s-1 respectively for the reactions of Cl atoms and OH radicals with 4H2BN. The possible reaction mechanism proposed based on the obtained products for the title reaction, thermochemistry, branching ratios, atmospheric implications as well as cumulative lifetime of 4H2BN were also explored in this study. 1. Introduction Ketones are Volatile Organic Compounds (VOCs), getting emitted as gases from various sources (biogenic and anthropogenic) into the atmosphere. The VOCs released into the atmosphere will be removed from the Earth’s atmosphere by several processes like dry and wet deposition, vegetation, photolysis, deposition on bio-matters and clouds. Transformation of compounds via chemical processes also takes place in the atmosphere. The chemical processes include their reactions with atmospheric oxidants such as hydroxyl (OH) and nitrate (NO3) radicals, Cl atoms and O3 molecules.1,2 These reactions lead to the formation of a series of oxidation products in the atmosphere, which will directly/indirectly affect the composition of Earth’s atmosphere. The reactions of VOCs with these oxidants in the troposphere leads to the production of Secondary Organic Aerosols (SOAs), secondary pollutants such as photochemical smog, acid rain and fog. The formed SOAs are harmful to health and can act as cloud condensation nuclei distressing cloud properties as well as the hydrological cycle in the atmosphere.3,4 The studies on atmospheric chemistry of VOCs are in demand in the present era to address the global environmental issues such as noxious and carcinogenic human health effects and also enhancement of the overall greenhouse effect, which are caused by the release of VOCs into the troposphere. Therefore, a detailed description of VOCs and their reactivity with atmospheric oxidants needs to be understood. In the troposphere, OH radicals and Cl atoms are the most predominant oxidizing agents and consequently, they are considered as major atmospheric sinks for the removal of VOCs.

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The globally averaged OH radical concentrations (1 × 106 radical cm-3)5 is quite higher than that of the globally averaged Cl atoms concentrations (1 × 103 atoms/ cm3)6. In marine boundary layer, the reaction of VOC with Cl atoms is competitive with the respective reactions of OH radicals (1 × 105 atoms / cm3)7. 4H2BN belongs to the class of hydroxy carbonyls, and these are formed as the reaction products in the atmospheric oxidation of VOCs8 especially unsaturated hydrocarbons. Moreover, hydroxy carbonyls are formed from the atmospheric degradation of oxygenated compounds. The degradation of branched alcohols and the photooxidation of diols lead to the formation of 4H2BN and 1-hydroxy-2-butanone (1H2BN) with yields 50% and 66%, respectively9. Furthermore, photo-oxidation of alkenes in presence of O2 leads to the formation of β-hydroxy carbonyls in the atmosphere10,11. Field measurements have shown the presence of significant quantity of hydroxy ketones in the troposphere.12 The presence of hydroxy carbonyls was also observed in snow with concentrations equivalent to that of carboxylic acids (0.9 to 53.8 g/L)13. The atmospheric degradation of these compounds influences the aerosol formation and also forms the precursors of other oxidants such as ozone, peroxy acyl/acetyl nitrates (PANs) and nitric acid14. It is important to know, the atmospheric life time of hydroxy carbonyls such as4H2BN, because of these oxygenated volatile organic compounds are the leading attractive replacements for the traditional chlorinated and aromatic solvents. Therefore, the atmospheric photo oxidation of these multifunctional hydroxy carbonyls intermediates is important because of their major role to the formation of free radicals which are in turn responsible for the oxidation of hydrocarbons15. To understand the atmospheric fate of hydroxy carbonyls, we need better knowledge of their environmental impact. Due to their photo-oxidation, they might contribute to some extent, to the creation of photochemical air pollution in regional as well as urban areas. Exact kinetic data as well as mechanistic evidence on the atmospheric fate of alternative oxygenated solvents are essential and hence needs to be investigated. 3 ACS Paragon Plus Environment


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The kinetic study available on the reaction of Cl atoms with 4-hydroxy 2-butanone at 298K was reported by Messaadia et al.16. They have investigated the kinetics using relative rate method and reported the rate coefficient at 298 K as (1.45±0.15) ×10-10 cm3 molecule-1 s1.

To the best of our knowledge, only few kinetic studies on the gas-phase reaction of 4H2BN

with OH radicals have been reported in the literature so far. Among these, two studies were carried out at room temperature and atmospheric pressure conditions using relative rate technique by Aschmann et al.17, Baker et al.18 and the third one using an absolute rate method was carried out by Gisele et al.19. Aschmann et al.17 studied the gas-phase reactions of selected hydroxy carbonyls including the present test molecule 4H2BN with OH and NO3 radical and O3 molecules. The obtained rate coefficient at 296±2K (in cm3 molecule-1 s-1) was reported to be kOH = (8.1±1.8) ×10-12. This result showed that, the gas-phase reaction of 4H2BN with the OH radical is the major tropospheric loss process for this molecule. Baker et al.18 studied the gas phase reactions of two C4-hydroxyketones including 4H2BN, which were identified as the products of the reactions of hydroxyl aldehydes with OH radicals and reported the rate coefficient of the 4H2BN at 298K to be (13.9±2.8) ×10-12 cm3 molecule-1 s-1. The first absolute rate coefficients were measured using pulsed Laser Photolysis–Laser Induced Fluorescence (PLP-LIF) for the reaction of 4H2BN with OH at 294K by using the vapor pressure measurements of 4H2BN by Gisele et al.19 and the reported rate coefficient was (4.8±1.2) × 10-12 cm3 molecule-1 s-1. 4H2BN was obtained as a product of the OH oxidation of 1,3butanediol with the yield (50±9) % as stated by Bethel et al9. Messaadia et al.20 have reported that hydroxy carbonyls can be removed mainly due to photolysis. Messaadia et al.16 carried out the temperature dependence study using relative rate method (FTIR as analytical technique) for the reaction of 4H2BN with OH radicals over the temperature range of 278-338 K and the rate coefficient obtained at 298 K to be; (1.31±0.30) × 10-11 cm3 molecule-1 s-1. They carried out the experiment at four different temperatures (278, 298, 313 and 338 K) with only

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one reference molecule. The rate coefficients obtained by Bouzidi et al.13 using the PLP-LIF technique for the reaction of 4H2BN with OH radicals as (4.79±1.20) × 10-12 cm3 molecule-1 s-1 was found to be 70% lower than that of the value reported by Messaadia et al.16 Yu A.Y21 computationally calculated and reported the temperature dependent rate coefficient for the reaction of 4H2BN with OH radicals over the temperature range of 200-1000 K as k = 8.75 × 10-21 T2.69 exp(323/T) cm3 molecule-1 s-1. They employed BHandHLYP/6-31G(d,p) level of theory for their calculations and the energies were further refined with CCSD(T)/6-311G(d,p) method. The corresponding pre-reactive complexes for each reaction path were also calculated using the same theory. Literature reports showed large deviations for rate coefficients for the reaction of 4H2BN with OH radical reaction. Therefore, in this work, the temperature dependent photo-oxidation reaction kinetics of Cl atom and OH radical with 4H2BN over the temperature range of 298−363 K was measured using the relative rate technique. To complement experimental measurements and to further comprehend the reaction mechanism of 4H2BN with Cl atoms and OH radicals, the rate coefficients were computed in the temperature range of 200−400 K with Canonical Variational Transition State (CVT) Theory with the inclusion of Small-Curvature Tunneling correction (SCT)22,23. Product analysis was performed for the title reaction. Formaldehyde, acetaldehyde, acetone, CO2 and HCl were obtained as products. Based on the computational studies and the product analysis of the title reaction, we have predicted the plausible atmospheric degradation reaction mechanism for Cl atom/OH radical initiated photo-oxidation of 4H2BN. Moreover, the cumulative atmospheric lifetimes, branching ratios and thermo-chemistry for the title reaction were computed and described in the manuscript. O

298-363K /760 Torr N2 (COCl)2/(H2O2)248 nm HO

Products

200-400K (Theory) 5 ACS Paragon Plus Environment


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2. Experimental Section Pyrex reaction chamber of 2L volume with fused silica windows (2-inch diameter) at both the ends were used to conduct the relative rate experiments. The temperature inside the chamber was upheld by circulating the cooled or heated fluid inside the double jacketed walls. The temperature inside the cell was measured by using the calibrated thermocouple with a precision of ± 2K. The precursor used to produce the Cl atoms and OH radicals in the present study were (COCl)2 and H2O2 respectively. A 248 nm KrF-Excimer laser (Coherent Compex Pro) was used to produce Cl atoms and OH radicals from their respective precursors. During the course of the reaction, laser fluence was upheld between 6 and10 mJ cm-2 pulse-1. The experiments were carried out at 760 Torr over the temperature range of 298-363 K. The reaction mixture containing a reference compound (either propane or ethylene and Iso-propanol or n- propanol), reactant (4H2BN) and precursor were prepared in the reaction chamber and allowed for homogeneous mixing for ~2-3 hours before laser photolysis. The concentration of the compounds (reference and sample) was analysed using GC (Agilent Technologies 7890B) coupled with a Flame Ionization Detector (FID), at particular time intervals during the reaction. It was observed that, there was no substantial decrease in the initial concentration of both the reactant and reference compounds, indicates that dark reactions and wall losses are negligible (

Photo Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2

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