Cl-Atom-Initiated Atmospheric Degradation of Saturated Cyclic

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Cl Atom Initiated Atmospheric Degradation of Saturated Cyclic Hydrocarbons – Kinetic and Mechanistic Investigation Ramya Cheramangalath Balan, and Balla Rajakumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b02225 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 6, 2019

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The Journal of Physical Chemistry

Cl Atom Initiated Atmospheric Degradation of Saturated Cyclic

Hydrocarbons

–

Kinetic

and

Mechanistic

Investigation. Ramya Cheramangalath Balan and B. Rajakumar* Department of Chemistry, Indian Institute of Technology Madras, Chennai, India - 600036 *E-mail: [email protected] Abstract The temperature dependent reaction kinetics of methyl cyclohexane (MCH) and methyl cyclopentane (MCP) with Cl atoms were experimentally explored via relative rate technique. Gas chromatography coupled with Flame Ionization Detector (GC-FID) was employed to follow the reactant as well as the reference compounds concentrations during the course of reaction. Gas chromatography coupled with mass spectrometry (GC-MS) as well as Gas Chromatography coupled with Infrared spectroscopy (GC-IR) were used as the diagnostic tools for the detection and identification of the products in the title reaction. The rate coefficients for the reaction of Cl atoms with methyl cyclohexane and methyl cyclopentane were measured in the temperature range of 283-363 K at 760 Torr using isoprene and propylene as reference compounds. To counterpart the experimental results, computational calculations were performed over the temperature range of 200-400 K using Canonical Variational Transition State Theory incorporated with Small Curvature Tunneling (CVT/SCT), Conventional Transition State Theory (CTST) coupled with Wigner tunneling corrections and CVT coupled with Wigner using MP2 level of theory with 6-31G(d,p) as basis set. The temperature dependent rate coefficient measured for the reaction of methyl cyclohexane and methyl cyclopentane with Cl atoms to be k (MCH) = [(4.48±0.75) × 10-11] exp [(604±25)/T] cm3 molecule-1 s-1 and k (MCP) = [(5.71±0.66) × 10-12] exp [(1819±669)/T] cm3

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molecule-1 s-1 respectively. The measured rate coefficients at 298 K for the reactions of Cl -10 cm3 atoms with methyl cyclohexane and methyl cyclopentane are 𝑘𝑀𝐶𝐻 298𝐾= (3.36±0.34) ×10 -10 cm3 molecule-1 s-1 respectively. The conceivable molecule-1 s-1 and 𝑘𝑀𝐶𝑃 298𝐾= (2.25±0.24) ×10

atmospheric degradation mechanism for the reaction of methyl cyclohexane as well as methyl cyclopentane with Cl atoms was projected based on the products obtained during the reaction. Atmospheric implications, cumulative atmospheric lifetimes, thermochemistry, ozone formation potentials and branching ratios of these molecules were also calculated and reported in this manuscript. 1. Introduction Hydrocarbons are one of the major components found in diesel, jet fuels and gasoline, vehicular discharges, and in urban areas.1-3 These are getting released in large scale to the atmosphere due to its world-wide use in fuel. They enter into the atmosphere either directly or by the incomplete combustion of other hydrocarbons. Cyclic hydrocarbons are volatile organic compounds (VOCs) that are released into the troposphere from various natural as well as anthropogenic sources including combustion of materials like wood, oil products, meat etc. Methyl cyclohexane (MCH) and methyl cyclopentane (MCP) are used as solvents in organic synthesis. It will convert naphtha reformers to toluene. In industry, MCH and MCP have got variety of applications such as adhesives and sealant chemicals, fuels and fuel additives.4-6 Moreover, MCH and MCP are used as a jet fuel surrogate blends and also used in rubber, paint, varnish industries as well as a grease extraction solvent.7 Once these compounds enter into the atmosphere, they undergo series of processes such as wet and dry deposition and reaction with tropospheric oxidants such as OH radicals, NO3 radicals and Cl atoms.8,9 These reaction of cyclic hydrocarbons with atmospheric oxidants leads to the formation of secondary organic aerosols (SOAs), tropospheric ozone, photochemical smog and acid rain in the atmosphere. The formed SOAs are harmful to human health and also


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disturb the properties of clouds as well as the hydrological system in the atmosphere10,11. Among the atmospheric oxidants the rate coefficients for the reactions of Cl atoms with cyclic hydrocarbons are larger than that of the other tropospheric oxidants. Chlorine atom reactions play a vital role in the polluted areas and in marine boundary layers (MBL) since the concentrations are especially high in this region (1.3 × 105 atoms cm−3) compared with the respective concentrations in other regions.12,13 Therefore, the chemistry of the selected molecules with Cl atoms in the troposphere is essential14-16 for the better understanding of the atmospheric chemistry of these molecules. There are limited reported rate coefficients available in the literature for the reaction of MCH with Cl atoms as well as OH radicals at room temperature. Ballesteros et al.17 reported the rate coefficients for the reaction of MCH with Cl atoms and OH radicals at 298 K to be k (MCH+Cl) = (3.11±0.16) × 10-10 and k(MCH+OH) = (1.18±0.12) × 10-11 cm3 molecule-1 s-1 respectively. They have employed relative rate (RR) method coupled with FTIR to follow the reactants concentrations. Aschmann et al.18 measured the rate coefficient using RR method for the reaction of MCH with Cl atoms at 298K and reported as (3.47±0.12) × 10-10 cm3 molecule-1 s-1. Bejan et al.19 reported the rate coefficient for the reaction of Cl atoms with MCH as k(MCH+Cl) = (3.45±0.37) × 10-10 cm3 molecule-1 s-1 using RR technique. Sprengnether et al.20 reported the rate coefficient for the reaction of OH radicals with MCH using absolute technique at 298 K as k(MCH+OH) = (9.42±0.12) × 10-12 cm3 molecule-1 s-1. Kramp et al.21 measured the rate coefficient for the reaction of MCH with OH radicals via RR method and reported the rate coefficient at 296 K as k

(MCH+OH)

= (9.40±0.06) × 10-12 cm3 molecule-1 s-1.

However, only one study was reported on the reaction of MCP with Cl atoms by Anderson et al.22 so far. They have reported the rate coefficients for the reaction of Cl atoms with MCP at 298K using relative rate method as (2.82±0.11) ×10-10 cm3 molecule-1 s-1.



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There are no previous studies reported on the temperature dependent reaction kinetics of MCH as well as MCP with Cl atoms till date. Therefore, in the current study, the temperature dependent rate coefficients for the reaction of Cl atom with MCH and MCP were studied over the temperature range of 283−363 K by employing the RR technique. In addition to the experimental findings, the rate coefficients were calculated computationally in the temperature range of 200−400 K using Canonical Variational Transition State Theory combined with Small Curvature Tunneling (CVT/SCT) as well as Conventional Transition State Theory (CSTS). Furthermore, the products obtained for the reaction of MCH and MCP with Cl atoms were analysed in presence of O2 at 298 K and 760 Torr of N2 using GC-MS and GC-IR. HCl, cyclohexanone, 1-methyl cyclohexanol and CO2 were obtained as products for the reaction of MCH with Cl atoms, while in the case of MCP with Cl atoms HCl, cyclo pentanone, cyclo pentanol, 1-methyl cyclopentanol and CO2 were obtained as products. Based on the product analysis, the probable tropospheric degradation mechanism was proposed for the Cl atom-initiated photo oxidative reaction of MCH and MCP. Additionally, the atmospheric cumulative lifetimes, ozone formation potentials, branching ratios and thermochemistry for the reaction of MCH and MCP with Cl atoms were also calculated and presented in this manuscript.

+

Cl

283-363K/760 Torr N2 (COCl)2/ 248 nm 200-400K (Theory)

products

1

products

2

methyl cyclohexane

+

Cl

283-363K/760 Torr N2 (COCl)2/ 248 nm 200-400K (Theory)

methyl cyclopentane


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2. Experimental Section Relative rate (RR) experimental method was used to measure the kinetic parameters for the title reactions. The error associated with this method depends on the error in the measurement of the reference reaction kinetic parameters. RR experiments for all the temperatures were carried out in a dual layered pyrex reaction chamber of 100 cm length and ~2L volume. Reaction chamber was closed with fused silica windows of 2-inch diameter (Thorlabs, WG42012) at both the ends to let the photolysis beam inside the chamber. The temperature inside the cell was maintained by circulating fluid (heated/cooled) in the double jacketed walls. The temperature inside the chamber was measured using a calibrated K-type thermocouple (±2 K), during the course of the reactions. A KrF excimer laser (Coherent, Compex-Pro) was used to produce 248 nm radiation beam, which was used for the photolysis of precursor (COCl)2 to produce Cl atoms insitu. The laser fluence throughout the experiments was maintained at ~5-10 mJ cm-2 pulse-1. (COCl)2

248 nm

2 CO + 2 Cl

3

The experiments were conducted at 760 Torr over the studied temperature range of 283-363 K. The reaction blend consisting of a reference compound (either propylene or isoprene) and the reactant (MCH or MCP) were prepared and were allowed for uniform mixing (~3 hours) inside the chamber in the absence of laser beam (photolysis beam). The concentration of each compound was scrutinized using GC equipped with FID, at particular time intervals during the course of experiments. It was observed that there was no sizable reduction in the initial concentration of both the compounds (reactant and the reference compounds), which indicates that the dark reactions as well as wall losses are negligible throughout the experiments. The reaction mixture was photolyzed in the absence of precursor (COCl)2 for about 30 minutes with 10 Hz repetition rate and observed that, there were no considerable



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drop in the concentration of any of the reactants and reference compounds. This confirms that, the effect of direct photolysis at 248 nm is unimportant. The wall loss was less than 3% over the course of 5 hours. Further probable errors include the temperature measurement, and the transfer from the reaction cell to GC. Compiling these errors, the uncertainty in the VOC concentration should have at least 5% systematic error on each data point. The relative rate plot passing through origin indicates that, the secondary chemistry is negligible throughout the experimental investigations. The initial concentration of the reactants, reference compounds and the precursor were optimized in such way that, the minimum reproducible area under the GC- peak should be present even after the last photolysis. Furthermore, we have carried out the experiments at 298K using the same experimental conditions and in presence of O2 and observed that the obtained rate coefficient with and without oxygen is almost equal within the experimental uncertainties. The broad explanation for the experimental procedure and all other information are presented in our prior publications.23-26 The experiments were performed after adding ~3 Torr of sample (MCH/MCP), ~6.0 Torr of reference compound (isoprene and propylene), ~11 Torr of (COCl)2 and then the reaction cell was pressurized to 760 Torr with N2 as diluent gas. The reaction mixtures were photolyzed using 1000, 2000, 3000 pulses (the laser was operated at 10 Hz) over regular time intervals. Subsequently, the reaction mixture was kept for homogeneous mixing using oil free diaphragm pump for few minutes. The samples were taken out from the cell using a gas-tight syringe and then injected into the GC for further analysis. After each photolysis, the sample was analyzed in GC-FID to measure the concentration of MCH, MCP and reference compounds. Using RR method, kinetic data were attained, in which the relative loss of the reactant with respect to the relative loss of the reference compound, due to their reactions with oxidizing species will be followed and measured using the equation shown below. 4 ACS Paragon Plus Environment

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(

𝑙𝑛

)

[𝑠𝑎𝑚𝑝𝑙𝑒]0 [𝑠𝑎𝑚𝑝𝑙𝑒]𝑡

=

𝑘𝑠𝑎𝑚𝑝𝑙𝑒

(

𝑙𝑛

𝑘𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒

)

[𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒]0 [𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒]𝑡

Where, [sample]0 and [Reference]0 are the primary concentrations of samples (MCH/MCP) and reference, [sample]t and [Reference]t are subsequent concentrations of samples and reference at time “t”. A plot of ln ([sample]0/[sample]t) versus ln([Reference]0/[Reference.]t) displays a straight line with a zero intercept gives slope which is equal to ksample./kReference.. The products were separated for the reactions of MCH and MCP with Cl atoms, and the formed products were analyzed using GC (7890B)-IR (Thermo scientific (IS50 GC-IR)) and GCMS (5977A MSD Agilent Technologies) vide infra. 2.2. Materials Chemicals: methyl cyclohexane (99.5%, Aldrich), methyl cyclopentane (99.5%, Aldrich), isoprene (99.5%), propylene (99.5%). All the liquid chemicals are degassed via several freeze-pump-thaw cycles before its use in the above experiments. 2.3. Computational Section GAUSSIAN 09 program suite27 was used for the optimization of all the electronic structure calculations. The geometries of all the stationary points including reactants (MCH and MCP), transition states (TS) and products were optimized using the second-order perturbation theory [MP2] (Møller−Plesset28) incorporated with the Pople’s basis set 631G(d,p).29,30 The vibrational frequency calculations were performed at every stationary point to characterize the minimum energy equilibrium structures, which shows all real frequencies except the transition states, which possess one imaginary frequency along with other real frequencies. Furthermore, the same level of theory was employed for the intrinsic reaction coordinate (IRC)31,32 calculations to confirm that the TSs found were connected to



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desired reactants and products. The minimum energy pathways (MEP) were also constructed using the above level of theory. Kinetic analysis Rate coefficients for the reaction of MCH and MCP with Cl atoms were calculated via the CVT with SCT corrections using the POLYRATE 2008 program33 as well as the CTST as shown in below equation.

( ) (

𝑘𝐵𝑇 𝑞 ‡

𝑘(𝑇) = 𝜎𝑟

ℎ

𝑞𝑅

𝑒𝑥𝑝

―∆𝐸0‡ 𝑅𝑇

)

5

Where “𝜎𝑟" denotes the reaction path degeneracy,” ‡ ” denotes the transition state, kB and h are Boltzmann and Planck’s constant respectively, T is temperature in Kelvin, q‡, qR are the partition functions for the transition states as well as the reactants respectively. ∆𝐸0‡ is the barrier height, and “R” is the gas constant. In the CVT, the rate coefficient kCVT(T) was calculated by minimizing the general rate coefficient (kGT (T, s)) with respect to “s” along the minimum energy path., kGT(T,s) can be minimalized by changing the transition state's separating surface along the reaction coordinate (s) to acquire canonical variational transition state rate coefficient. 𝑘𝐶𝑉𝑇(𝑇) = min 𝑘𝐺𝑇(𝑇,𝑠) = 𝑘𝐺𝑇(𝑇,𝑠𝐶𝑉𝑇(𝑇))

(

) (

6

𝑘𝐵𝑇 𝑄𝐺𝑇(𝑇,𝑠) ― 𝑉𝑀𝐸𝑃(𝑠) 𝑘𝐺𝑇(𝑇,𝑠) = 𝜎 𝑒𝑥𝑝 ℎ 𝑘𝐵𝑇 ∅𝑅(𝑇)

)

7

Where kCVT is the rate constant obtained from CVT calculations, sCVT is the reaction coordinate (s) at which the Canonical Variational Transition state dividing surface was found. ‘QGT and ØR are the partition functions. VMEP(s) is the classical potential energy of generalised TS at “s”. The minimum energy path is obtained via straight dynamics for a range of the reaction path: −1.0 Å

Cl-Atom-Initiated Atmospheric Degradation of Saturated Cyclic

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