Photooxidation Reactions of Ethyl 2-Methylpropionate (E2MP) and Ethyl 2,2-Dimethylpropionate (E22DMP) Initiated by OH Radicals: An Experimental and Computational Study

Article abstract of DOI:10.1021/acs.jpca.0c00903

The relative rate (RR) technique was used for the measurement of OH-initiated photooxidation reactions of ethyl 2-methylpropionate (E2MP) and ethyl 2,2-dimethylpropionate (E22DMP) in the temperature range of 268-363 K at 760 Torr. In addition to this, the

Full text of DOI:10.1021/acs.jpca.0c00903

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A: Kinetics, Dynamics, Photochemistry, and Excited States
Photo-Oxidation Reactions of Ethyl 2-Methyl Propionate
(E2MP) and Ethyl-2,2- Dimethyl Propionate (E22DMP) Initiated
by OH Radicals: An Experimental and Computational Study
Revathy Kaipara, and Balla Rajakumar
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.0c00903 • Publication Date (Web): 24 Mar 2020
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Photo-Oxidation Reactions of Ethyl 2-methyl propionate (E2MP) and Ethyl-
2,2-dimethyl propionate (E22DMP) Initiated by OH radicals: An Experimental
and Computational study
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Revathy Kaipara and B. Rajakumar*
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Department of Chemistry, Indian Institute of Technology Madras, Chennai - 600036, India
*Address for correspondence: rajakumar@iitm.ac.in
http://chem.iitm.ac.in/faculty/rajakumar/
ABSTRACT
Relative Rate (RR) technique was used for the measurement of OH-initiated photo-oxidation
reactions of Ethyl 2-methyl propionate (E2MP) and Ethyl-2,2-dimethyl propionate (E22DMP) in
the temperature range of 268-363 K at 760 Torr. In addition to this, the thermodynamic and kinetic
parameters for the title reactions were theoretically investigated using CCSD(T)/cc-pVTZ//M06-
2X/6-311++G(2d,2p) level of theory in the temperature range of 200-400 K using Canonical
Variational Transition State theory (CVT) in combination with Small Curvature Tunneling (SCT)
method. The rate coefficients at (298±2) K were measured to be kE2MP + OH = (2.71±0.79) × 10-12
cm3 molecule-1 s-1 and kE22DMP + OH = (2.58±0.80) × 10-12 cm3 molecule-1 s-1. The degradation
mechanisms for the title reactions were investigated in the presence of O2 using Gas
Chromatography with Mass Spectrometry (GC-MS) and Gas Chromatography with Infrared
Spectroscopy (GC-IR). From the recognized products, the possible product degradation
mechanisms were predicted. In addition to this, the atmospheric lifetimes (ALs), lifetime-corrected
Radiative Forcing (RF), Global Warming Potential (GWPs) and Photochemical Ozone Creation
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Potentials (POCPs) were calculated to further understand the environmental impact of these
molecules on the Earth’s troposphere.
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1.Introduction
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In the Earth’s atmosphere, Oxygenated Volatile Organic Compounds (OVOCs) are emitted from
both anthropogenic as well as biogenic sources.1 OVOCs undergoes innumerable chemical and
physical processes, leading to their conversion or exclusion from the atmosphere. Biogenic
production of OVOCs along with global anthropogenic release of all nonmethane VOCs is about
200 Tg yr-1.2,3 Removal of OVOCs happens via photolysis and wet/dry deposition.4,5 Majority of
the OVOCs have low Henry’s law constants (below 103 M atm−1), signifying that, they will
partition into the gas phase.6 Ethyl 2-methyl propionate (E2MP) and ethyl-2,2-dimethyl propionate
(E22DMP), categorized as OVOCs are fatty acid derivatives of methyl alkyl ester. They are
present in alcoholic beverages and fruits such as apple, banana, orange, wine grape, strawberry
and nectarine.7 A major concentration of these esters was also detected in the food flavoring
agents.8 In Earth’s atmosphere, these esters are generated in situ via the oxidation of ethers.9-11
Therefore, the emission of these OVOCs into the Earth’s atmosphere from various sources,
produces ground level pollutants, tropospheric ozone and secondary organic aerosols (SOAs).12-16
The formation of SOAs and tropospheric ozone during the atmospheric oxidation of organic
compounds are assessed globally at 20-380 Tg yr-1 and 4500 Tg yr-1 respectively.18 This causes a
significant rise in the concentration of emitted greenhouse gases, which in turn affect the global
climate. Hence, the widespread use of these OVOCs in the various applications inevitably results
in fugitive emission of these molecules into the Earth’s atmosphere. The most of OVOCs are
oxidized in the Earth’s atmosphere via chemical reactions.19 The deterioration of saturated OVOCs
are initiated by their reaction with atmospheric oxidants. The degradation of OVOCs in the
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troposphere leads to the production of secondary pollutants, such as ozone, VOCs, peroxyacyl
nitrates, and SOA.20 Therefore, the oxygen containing compounds play a significant role in
defining the oxidizing capacity of the troposphere both on a local and a worldwide scale.
The emitted OVOCs are mainly removed via the photo-oxidation reactions with cleansing agents
(like OH and NO3 radicals, Cl atoms and O3 molecules).21 Although, their Cl-initiated kinetics is
an order of magnitude higher than their reaction towards OH radicals. The low global
concentration of Cl atoms (1× 103 atoms cm-3)22-28, when compared to that of OH radicals (1×
106 molecules cm-3) reduces the atmospheric significance of Cl atom-initiated reactions.29
However, in coastal areas, Cl atom-initiated kinetics of OVOCs is significant. Moreover, in the
troposphere, OH radical is considered to be the dominant oxidizing agent.30,32 Therefore, OH-
initiated photo-oxidation reactions mainly determine the atmospheric fate of these molecules. The
production cycle for the tropospheric OH radical is because of the reaction of O(1D) formed from
the UV-photolysis of O3 with water vapor.33-38 Till date, the kinetics of E2MP and E22DMP with
OH radicals has not been investigated, so far. Therefore, this is the first investigation on the
temperature-dependent kinetics of E2MP (R1) and E22DMP (R2) towards OH radicals in T = 268-
363 K and at N2 pressure of 760 Torr using RR technique coupled with Gas–
Chromatography/Flame Ionization Detector (GC-FID). In addition to this, the kinetics was also
calculated in T = 200-400 K using the geometries and energies obtained at CCSD(T)/cc-
pVTZ//M06-2X/6-311++G(2d,2p) level of theory. The involved H-abstraction reaction pathways
are depicted below, where a, b and c are the respective H-abstraction from the same reactive site.
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O
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O
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E2MP
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OH R1
C5[a,b,c]
-H2O
O
C2[a,b,c]
C4[a,b]
-H2O
C1
C3[a,b,c]
-H2O
-H2O
-H2O
O
O
O
O
O
O
O
O
O
Scheme 1a. H-abstraction reaction channels include in the reaction of E2MP with OH radicals.
O
O
E22DMP
OH R2
C7[a,b,c]
C9[a,b]
-H2O
C6[a,b,c]
C10[a,b,c]
-H2O
C8[a,b,c]
-H2O
-H2O
-H2O
O
O
O
O
O
O
O
O
O
O
Scheme 1b. H-abstraction reaction channels include in the reaction of E22DMP with OH radicals.
The product analyses were studied using GC with Mass Spectrometer (GC-MS) and Infrared
Spectroscopy (GC-IR). To further comprehend their tropospheric impact, atmospheric lifetimes,
RFs, Global Warming Potential (GWPs) and Photochemical Ozone Creation Potentials (POCPs)
were calculated.
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2. Methodology
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2.1 Experimental details
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Kinetics for R1 and R2 were measured in between 268-363 K and at 760 Torr of N2. A doubled
walled Pyrex glass cell of 100 cm length and 2.4 L volume was used to perform all the experiments
over various temperatures, 268 K, 283 K, 298 K, 313 K, 343 K and 363 K. Temperature inside the
reaction cell was maintained using Julabo (a thermostat), which circulates heated or cooled fluid
inside the double jacked wall. Calibration of the cell temperature was performed using a K-type
thermocouple and a ±2 K in the uncertainty was found. Photolysis of H2O2 (a precursor for OH
radicals) was done using a KrF Excimer laser with λmax = 248 nm (Coherent COMPEX Pro 102F).
The concentration of H2O2 in the reaction cell was varied in the range of (4-6) × 1017 molecule
cm-3. The detailed experimental set up and the related procedure are available in our previous
publications.39,40 Reference compounds used were butane (n-C4H10) or iso-butane (i-C4H10)
respectively.
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Reaction chamber consists of a mixture of sample molecule (E2MP or E22DMP), H2O2 and a
reference compound. The initial concentrations of the sample and reference compounds were
varied in the range of (3-6) × 1016 molecule cm-3. Concentration of the reaction mixture was
monitored using GC-FID (Agilent technologies, 7890 B). Separation of the gaseous species was
acquired using HP-PLOTQ capillary column (30 m × 0.32 mm × 20 μm). The GC-oven
°
temperature and the flow rate were maintained at 220 C and 3 mL min-1. To further verify the
influence of secondary reactions or dark reactions, the reaction mixture ((E2MP/E22DMP) +
reference + H2O2) was kept for 2-3 hours with no photolysis source. After a certain time, the
concentrations were analyzed and was found to depict no change in the concentrations. Hence,
dark reactions do not contribute to a significant extent. In addition to this, to check the interference
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of secondary reaction in the absence of H2O2, the reaction mixture (E2MP/E22DMP) + reference)
was photolyzed and kept for 2-3 hours. After that, the analysis of the concentration led to the
conclusion that, the concentrations of the sample molecules or the reference compounds did not
show significant variation. The Figures of these control experiments are shown in Figure S1 of the
Supporting Information (SI). Thus, the intrusion of any secondary chemistry inside the reaction
cell was insignificant and it was also confirmed that, the decay in the concentration of the sample
and reference is merely due to their reactivity towards OH radicals.
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•OH + E2MP/E22DMP → Products
•OH + n-C4H10/i-C4H10 → Products
(1)
(2)
Measurement of the temperature-dependent kinetics was procured from the instantaneous loss of
the sample and the reference using the following expression:




{Ester}0
kEster
kref
{ref }0








(3)
ln

ln
{Ester}
{ref }
t
t




where, [Ester]0, [ref]0, [Ester]t, and [ref]t are the concentrations of E2MP, E22DMP and reference
compounds at times t0 and t, and kEster and kref are the rate coefficients of the sample molecules and
퐸푠푡푒푟)0
ln ((
푟푒푓)0
(퐸푠푡푒푟)푡 ) ln ((
reference compounds. Plot of
vs
) is expected to yield
(푟푒푓)푡)
a straight line with zero intercept, indicating the absence of secondary chemistry on the measured
kinetics over the studied temperature range. The ratio (푘푘 ) obtained can be multiplied by
,
퐸푠푡푒푟
푘푟푒푓
푟푒푓
which would yield the 푠푎푚푝푙푒 at a particular temperature.
푘
2.2. Product analysis
The product analyses were studied using GC-MS (GCMS-5977A-MSD, Agilent technologies) and
GC-IR (GC (7890B, Agilent technologies)-IR (Thermo scientific IS50)-MCT). HP-PLOTQ
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column was used for the separation of the eluted products. The oven temperature of the two
instruments was varied from 30 °C to 250 °C with a temperature rate of 5 °C min-1. The source and
the MS Quad temperature were kept at 150 °C and 200 °C respectively. All the GC-IR experiments
were obtained with a spectral resolution of 4 cm-1.
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Purity of the chemicals as listed by the manufactures is given here, E2MP and E22DMP (99%,
Aldrich), H2O2 (98%, Spectrochem), n-C4H10, i-C4H10, N2 and O2 (98%, Bhuruka gases) and He
(99.999%, Praxair). The esters were purified before their use through several cycles of freeze-
pump-thaw.
2.3. Computational Methodology
A meta-hybrid functional, M06-2x (with 54 % HF exchange)41-43 coupled with Pople’s basis set
6-311++G(2d,2p) (MG3S)44,45 was used for the optimization of the reactants (E2MP and
E22DMP), transition states (TSs) and products (Pi). The use of M062x method in combination
with MG3S basis set is known to have the best performance, and it is selected for use for direct
dynamics calculations46-51. Individually, both the M06-2x method as well as the MG3S basis set52
are known to be good for the computation of accurate barrier heights and thermodynamic
parameters. Details of the computational methodology are specified in Section 1 of the SI.
3. Results and discussion
3.1. Kinetics Measurements
Rate coefficients (k in cm3 molecule-1 s-1) were measured with respect to n-butane (n-C4H10) and
iso-butane (i-C4H10). The reported rate coefficients of the reference compounds at 298 K are, kn-
C4H10+OH = (2.36±0.47) × 10-12 and ki-C4H10 + OH = (2.12±0.42) × 10-12.53 Relative rate plots for R1
and R2 are given in Figures 1a and 1b. The slopes obtained from the relative plots and the average
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rate coefficient (kavg) calculated at each temperature are listed in Tables 1a and 1b. The errors
quoted are the 2σ statistical errors obtained by fitting the kinetic data using the linear least squares
method and the reported uncertainties in the reference rate coefficients. The details about the error
analysis are given in Section -2 of the SI.
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The kinetics (in units of cm3 molecule-1 s-1) was fitted to the linear least squares method and the
Arrhenius expression were obtained as, 268 ― 363 퐾 = [(4.15±0.87) × 10-13 exp (568.3±64.5)/T] and
푘푅1
268 ― 363 퐾
= [(4.24±0.92) × 10-13 exp (548±66)/T]. A negative dependence of the kinetics on the
푘
푅2
temperature was observed and the respective Arrhenius plots are shown in Figures 2a and 2b. The
298 퐾
298 퐾
298 K rate coefficients were obtained as,
× 10-12 respectively.
(2.71±0.79) × 10-12 and
(2.58±0.80)
=
푘푅1
=
푘
푅2
3.2. Computational section
3.2.1. Reaction energetics
Optimized stationary points obtained at the studied level of theory for both the reactions R1 and
R2 as well as the respective Potential Energy Surfaces (PESs) are shown in Figure 3, Figures 4a
and 4b. The calculated thermochemical parameters (such as ΔG° and ΔH° in the units of kcal mol-1)
for both the reactions R1 and R2 are listed in Tables 2a and 2b. It can be understood from the
Tables 2a and 2b that, all the H-abstraction reaction channels in E2MP (C1[a,b,c] – C5[a,b,c]) and
E22DMP (C6[a,b,c] – C10[a,b,c]) were found to be exothermic (ΔH°< 0) and spontaneous (ΔG°
< 0). The reaction channel C1 (ΔH°=-28.5 and ΔG° = -29.8) in E2MP and C9[a, b] (ΔH°= -20.1
and ΔG° = -21.2) in E22DMP were found to be thermodynamically more facile, when compared
to other H-abstraction reaction routes. Schematic representations of all the TSs involved in the
reactions R1 and R2 are shown in Scheme 1c and 1d.
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Scheme 1c
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TS5
TS1
H1
TS9
H9
TS8
H8
TS10
H5
O
TS6
TS7
H6
H7
H10
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C
C


TS11
C
O
C
H11
H12


C
H4
H2
TS12
H3
TS4
TS2
TS3
E2MP
Scheme 1c. Plausible H-abstraction TSs in the reaction of E2MP with OH radicals.
It is clear from the PES in Figure 4a that, the abstraction of H1 atom by OH radical from the α-
carbon in C1 via TS1 was found with a ΔEǂ value of 3.0 kcal mol-1. The three methyl hydrogens
(H2, H3 and H4) attached to the α-carbon were non-equivalent, due to their spatial arrangement.
Therefore, TS2, TS3 and TS4 were found with ΔEǂ values (in units of kcal mol-1) of 6.2, 3.3 and
4.9. Similarly, the three H-atoms (H5, H6 and H7) at the β-carbon were non-equivalent. Hence, TS5,
TS6 and TS7 were observed with ΔEǂ values (in units of kcal mol-1) of 4.3, 3.3 and 6.5. The two
H-atoms (H8 and H9) at the α'-carbon via TS8 and TS9 with ΔEǂ values of 2.5 kcal mol-1 were found
to be equivalent. The three H-atoms (H10, H11 and H12) at the β'-carbon were non-equivalent, due
to their 3D-orientation. Therefore, TS10, TS11 and TS12 were identified with ΔEǂ values (in units
of kcal mol-1) of 7.7, 6.0 and 4.6. All the TSs (TS1-TS12) were located via the elongation of the
breaking C-H bond by 12-14% and a compression of O-H bond distances by 39-43%. To further
comprehend the nature of the TSs, L-parameter54 values were calculated and are shown in Table
2a, suggested that, all the TSs depict reactant-like nature or are early TSs.
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Scheme 1d
5
6
7
TS1
TS10 TS11
8
9
TS12
TS2
TS3
TS4
H1
H2
H3
O
H11
C
H10
O
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H12
H13
H14
C
TS13
TS14
C


H4
H5

C

C
H6
TS5
H9
H7
H8
TS6
TS7
TS8 TS9
E22DMP
Scheme 1d. Plausible H-abstraction TSs in the reaction of E22DMP with OH radicals.
From the PES displayed in Figure 4b and the Scheme 1b revealed that, one of the H-atoms (H1) at
the -CH3 group attached to the α-carbon was different from H2 and H3, due to their stereographical
orientation. Hence, TS1 was located with a ΔEǂ value of 0.6 kcal mol-1, while TS2 and TS3 were
equivalent with ΔEǂ values of 5.4 kcal mol-1. Similarly, H6 attached to the -CH3 group at the β-
carbon was dissimilar to H4 and H5. Hence, TS4 and TS5 were equivalent with ΔEǂ value of 0.8
kcal mol-1, whereas TS6 was located with a ΔEǂ value of 2.8 kcal mol-1. Furthermore, H7, H8 and
H9 at the -CH3 group attached to the α-carbon were non-equivalent, due to their stereographical
orientation. Therefore, TS7, TS8 and TS9 were located with ΔEǂ values (in units of kcal mol-1) of
0.5, 0.8 and 2.2. H10 and H11 at the α'-carbon were equivalent and with ΔEǂ value of -0.2 kcal mol-1.
H13 attached to the β'-carbon was dissimilar to H12 and H14. Therefore, TS12 and TS14 were
equivalent with ΔEǂ value of 3.3 kcal mol-1, whereas TS13 was observed with a ΔEǂ value of 4.7
kcal mol-1. All the TSs (TS1-TS14) were located along the MEP with a C-C bond elongation of
10-12% and an equivalent O-H bond length compression by 39-44%. Moreover, it was evident
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from Table 2b that, L-parameter54 values for all the TSs were found to be less than one. Thus, all
the TSs are early TSs or exhibit reactant-like nature.
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3.2.2. Rate coefficient calculations
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The kinetics for the reactions R1 and R2 were calculated using CVT/SCT method and are listed in
Table 3. In reaction R1, C4[a,b] via TS8 and TS9 was found to be kinetically labile reaction
channel.
The global rate coefficient for the reaction R1 was obtained by adding the distinct rate coefficients
obtained for individual H-abstraction reaction channels (listed in Table S5) as shown below,
푡표푡푎푙
푘
= 푘퐶1 + 푘퐶2[푎,푏,푐] + 푘퐶3[푎,푏,푐] + 푘퐶4[푎,푏] + 푘퐶5[푎,푏,푐]
(4)
푅1
The Arrhenius expression was acquired by performing a non-linear fit to the calculated rate
coefficients (k in cm3 molecule-1 s-1) and the respective Arrhenius plot is shown in Figure 2a. The
298 퐾
overall rate coefficient for R1 at 298 K was calculated to be
= 3.01×10-12 and the
푘
푅1
temperature-dependent expression was obtained as, 200 ― 400 퐾= 6.15×10-21 T2.70 exp[(1384/T)]
푘푅1
cm3 molecule-1 s-1. The rate of the reaction decreases in magnitude, as the temperature increases.
It is evident from the Figure 2a that, the computed rate coefficient at 298 K was found to be in
good agreement with the measured value ( 298 퐾, 푚푒푎푠푢푟푒푑 = (2.71±0.79)×10-12 under the
푘푅2.3
experimental error limits.
In the case of the reaction R2, reaction channel C9[a,b] via TS10 and TS11 was both
thermodynamically favored and kinetically feasible. The total rate coefficient was calculated by
the addition of the individual rate coefficients (given in Table S6) involved in reaction R2 using
the following formula,
푡표푡푎푙
푘
= 푘퐶6[푎,푏,푐] + 푘퐶7[푎,푏,푐] + 푘퐶8[푎,푏,푐] + 푘퐶9[푎,푏] + 푘퐶10[푎,푏,푐]
(5)
푅2
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A nonlinear fit to the computed rate coefficients was performed to obtain the Arrhenius plot given
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in Figure 2b. The total rate coefficient for R2 at 298 K was obtained as, 298 퐾= 2.82×10-12 and the
푘푅2
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8
9
Arrhenius expression was obtained as, 200 ― 400 퐾= 3.88×10-27 T4.93 exp[(1806/T)] cm3 molecule-
푘푅2
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1
s-1. Overall, the rate decreased, as the temperature rises from 200 to 400 K. It is evident from
Figure 2b that, the computed room temperature rate coefficient was in good agreement with the
measured rate coefficient ( 298 퐾, 푚푒푎푠푢푟푒푑= (2.58±0.80) × 10-12), under the experimental error
푘푅2
limits.
3.3. Branching ratios (BRs)
BRs for the title reactions were estimated to understand the impact of individual reaction channel
(ki) to the total rate of the reactions (ktotal) and are calculated using the following expression,
ki
100
Branching ratios (%) =
(6)
ktotal
The role of each reaction route to the global kinetics of the reactions R1 and R2 are shown in
Figures 5a and 5b. It is obvious from the Figure 5a that, the contribution of C4[a,b] via TS8 and
TS9 was found to decrease, as temperature increases from 200 K (45 %) to 400 K (31 %). On
contrary, the contributions of other TSs increased with an equivalent rise in the temperature.
Therefore, these TSs were minor contributors to the total reactivity of E2MP towards OH radicals,
when compared to the major reaction channel. Hence, reaction channel C4[a,b] was the dominant
contributor to the total reaction.
Moreover, it can be understood from the Figure 5b, the BR of the TS10 and TS11 via reaction
channel C9[a, b] was the major contributor to the total reactivity. Further, it can be seen that, the
contribution of TS10 and TS11 decreased from 67% to 20% with an increase in temperature from
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200 to 400 K. On the other hand, the contribution of TS13 increased with an increase in the
temperature. After 313 K, TS13 was the major contributor to the global reactivity. Furthermore,
the BRs of other TSs increased with an increase in the temperature. However, these TSs were
minor contributors to the total reactivity, when compared to the dominant TSs. Therefore, reaction
channel C9[a,b] is the major contributor in the reactivity of E22DMP towards OH radicals.
3.4. Reactivity trends
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Based on the available literature, it was corroborated that, no prior kinetic study was carried out
for reactions R1 and R2. Hence, the current work is the first attempt to investigate these reactions
and thus, direct comparison to the available literature cannot be attained. However, the kinetics
measured in this work were compared with the rate coefficient of the analogues molecules (MP,
M2MP and EP). The rate coefficients measured in this work and the reported rate coefficients of
the analogues molecules at 298 K are given in Table 4. As seen from the Table 4 that, the reactivity
of M2MP towards OH radical was found to be higher than that of the reactivity of MP towards
OH radicals. This difference in the reactivity can be better explained in terms of the radical
stability, deactivating group and hyperconjugation effects. Mendes et al.55 theoretically
investigated the kinetics for the reactions of MP and M2MP with OH radical and explained that,
the H-abstraction from the α-position attached to the ester moiety (RC(O)OR') happens in a similar
way, due to the electron delocalization of RC(O)OR' group. So, the effect of deactivating group
was found to be insignificant. Furthermore, the radical stability increases in the order of -
CH3<1°<2°<3°group. Moreover, higher the number of alkyl substituents on the radical bearing
carbon, more is the stabilization energy, due to the hyperconjugation effect. Hence, the H-
abstraction by OH radical from M2MP formed the 2° radical, whereas, in the case of MP, the
formation of 1° radical was noted to be the major reaction channel. This in turn explained the
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increase in the reactivity of the M2MP, when compared to the reaction of MP towards OH radicals.
Similarly, the reactivity of EP56 and E2MP towards OH radical can also be explicated. However,
it is important to know that, the dominant reaction sites for the reaction of E22DMP with Cl atoms
and OH radicals were the abstraction of the hydrogen atoms attached to the acetoxy side (-OCH3-).
Therefore, the reactivity of E22DMP is comparable, even though the OH-based kinetics was an
order of magnitude different, when compared to the respective Cl-initiated kinetics. Ifang et al.57
investigated the kinetics for the reaction of E22DMP with Cl atom and observed a small
discrepancy in the overall kinetics, due to the reactivity of -CHx- (x = 1,2,3) moiety, which
influences both the α and β positions attached to the acyl group (-C(O)OR). It is also well-known
that, for the reaction of OH radical with oxygen-rich compounds, ester moiety can alter the
reactivity of the -CHx- groups on both the sides, up to δ position.13
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In addition to this, the reactivity of the studied esters towards OH radical were compared based on
the SAR method given by Kwok and Atkinson.58 The authors assumed that, the rate constants for
the H-abstraction from the C-H bond of the molecules is based on the estimation of the group rate
constants from the -CH3, -CH2 and -CH< moieties and are defined as,
k(CH3-X) = kprim F(X),
k(X-CH2-Y) = ksec F(X) F(Y), and
k(X-CH-Y(Z)) = ktert F(X) F(Y) F(Z),
where, kprim, ksec and ktert are the rate coefficient of -CH3, -CH2 and -CH<; X, Y and Z and F(X),
F(Y) and F(Z) are the substituent group and factors, where F(-CH3) = 1.00, F(-CH2) = F(-CH<) =
1.23, F(C(O)OR) = 0.74, and F(-C(O)O-) = 1.6. Based on this, k (in cm3 molecule-1 s-1) for the
298 퐾, 푆퐴푅
reactions R1 and R2 were calculated as,
= 3.80×10-12 cm3, and 298 퐾,푆퐴푅= 1.72×10-12
푘푅2
푘
푅1
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298 퐾,푚푒푎푠푢푟푒푑
respectively. From this kinetic data, it was concluded that, the measured ones (
=
푘푅1
5
6
298 퐾,푚푒푎푠푢푟푒푑
(2.71±0.79)×10-12 and
= (2.58±0.80)×10-12), and the calculated kinetic data (
푘
푅2
7
8
9
298 퐾, 푐푎푙푐푢푙푎푡푒푑
298 퐾, 푐푎푙푐푢푙푎푡푒푑
= 3.01×10-12 and
= 2.82×10-12) were in reasonable agreement
푘
푘
푅2
푅1
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with the SAR-based rate coefficients.
3.5. Product analysis
The product analyses for the reactions R1 and R2 were studied using GC-MS and GC-IR. The
obtained chromatograms are shown in Figures 6a and 6b. Based on the product analyses, possible
product degradation mechanisms were predicted and are shown in Figures 7a and 7b. The
individual MS and IR-absorption spectra of all the products are given in Figures S4 and S5.
In the case of reaction R1, reaction channel (C4[a,b]) was found to be kinetically dominant
pathway. Hence, CH3CHO formed from this channel was found to be the major degradation
product. This product was formed due to the interaction of OH radical towards the α'-carbon in the
presence of O2, followed by recombination reaction to form the (CH3)2CHC(O)OCH•CH3 radical.
Further, this alkoxy radical undergoes Cα'-O decomposition to form CH3CHO and
(CH3)2CHC(O)O• radical. Later, this radical undergoes Cα -C(O) bond scission to form CO2 and
1-propene (CH3CH=CH2) as products. Cavalli et al.59 explained the formation of peroxy radical in
their product study for the reaction of methyl propionate with OH radical due to the reaction of
alkyl radical with O2. Author also illustrated the formation of alkoxy radical due to the self-reaction
of peroxy radical followed by its decomposition to yield various products. There are several other
literatures also explained the formation of alkoxy radical and its further degradation to form
various products.60-62 The formation of these products was confirmed with m/z ratios at 32, 37, and
44 for CH3CHO; and 29, 39, 40, 42 and 43 for CH3CH=CH2. The IR- peak at 2850 cm-1 (C-H
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stretch) and 1725 cm-1 (C=O stretch) for CH3CHO; and 3083 cm-1 (=C-H stretch), 2966-2960 cm-1
(C-H stretch in alkane), 1654 cm-1 (C=C bond) and 675-990 cm-1 (bending =C-H stretch) and 1465
cm-1 (C-H scissoring) for CH3CH=CH2 also confirmed the formation of these products.
Furthermore, CH3CHO and ethyl formate (HC(O)OCH2CH3) were also observed as one of the
degraded products, due to the elimination of methyl group in the presence of O2, followed by the
recombination reaction and C-C bond fission of the corresponding radical. HC(O)OCH2CH3 was
confirmed with m/z ratios at 31, 38, 43, 48, 56 and 74; and IR-peaks at 2967 cm-1 (C-H stretch),
1726 cm-1 (C=O stretch), 1206 cm-1 (C-O stretch) and 842 cm-1 (O-C(O)). In addition to this, other
minor products were observed, due to the approach of OH radical towards α-carbon in reaction
channel (C1). (CH3)2C•C(O)OCH2CH3 radical was formed from the H-abstraction by OH radical
via C1. After that, this radical reacts with O2 and formed (CH3)2COO•C(O)OCH2CH3 radical. This
unstable alkyl peroxy radical undergoes self-reaction to produce alkoxy (CH3)2CO•C(O)OCH2CH3
radical. Later, this alkoxy radical undergoes Cα-C(O) decomposition to form CH3C(O)CH3 and
•C(O)OCH2CH3 radial. Further, this radial undergoes O-Cα' cleavage in the presence of O2 to form
HCHO and CH3OH as products. Ifang et al.57 studied the product analysis of methyl-2-methyl
propionate with Cl atom in the presence of O2 and explained the formation of acetone by the
decomposition of corresponding alkoxy radical. The formation of these products was recognized
with m/z ratios at 29, 28 and 30 for HCHO; 43, 58, 38 and 30 for CH3C(O)CH3; and 15, 30, 31
and 32 for CH3OH. The IR- peaks at 2964 cm-1 (C-H stretch), 1659 cm-1 (C=O stretch) and 842
cm-1 (O-C(O) stretch) for HCHO; and 1716 cm-1 (C=O stretch) for CH3C(O)CH3; and 3200-3600
cm-1 (strong and broad O-H stretch), 3510-3678 cm-1 (strong and sharp O-H stretch) and 1055-
1148 cm-1 (C-O stretch) for CH3OH also confirmed the formation of these products.
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In the case of reaction R2, acetaldehyde was formed due to the interaction of OH radical towards
the α'-carbon (C9[a,b]) in the presence of O2, followed by the self-reaction to form corresponding
alkoxy radical ((CH3)2CHC(O)OCHO•CH3). Further, this radical undergoes decomposition
reaction to form CH3CHO and (CH3)2CHC(O)O• radical. Later, this radical undergoes -C-C(O)
cleavage to form 2-methyl 1-propene ((CH3)2CH=CH2) and CO2. The products were identified
with m/z ratios at 32, 37 and 44 for CH3CHO; and 37, 38, 39, 41, 42, 55 and 56 for (CH3)2CH=CH2.
The IR-peaks at 2850 cm-1 and 2750 cm-1 (C-H stretch) and 1725 cm-1 (C=O stretch) for
CH3CHO; and 3010-3100 cm-1 (medium =C-H stretch), 1620-1680 cm-1 (variable C=C bond) and
675-990 cm-1 (bending =C-H stretch) for (CH3)2CH=CH2 also confirmed the formation of these
products. Moreover, some minor products were also observed from the product analysis, due to
the approach of OH radical towards the methyl group attached to the α-carbon. HCHO and
(CH3)2O•CHC(O)OCH2CH3 radical were formed, due to the H-abstraction by OH radical from the
methyl group attached to the α-carbon in the presence of O2. HCHO was confirmed with m/z ratios
at 29, 28 and 30; and IR- peaks at 2964 cm-1 (C-H stretch),1659 cm-1-1518 cm-1 (C=O stretch) and
842 cm-1 (O-C(O) stretch). Further, the -C-C(O) bond scission from the
(CH3)2O•CHC(O)OCH2CH3 radical leads to the formation of acetone (CH3C(O)CH3) and
•C(O)OCH2CH3 radical. Wallington et al.63 in their product study on the reaction of Cl atom with
methyl-2,2dimethyl propionate and reported that the alkyl radical formed in their system was
further oxidized to acetone. The formation of CH3C(O)CH3 was recognized with m/z ratios at 43,
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•
58, 38 and 30; and IR-peaks at 1716 cm-1 (C=O stretch). After that, C(O)OCH2CH3 radical
undergoes two different decomposition channels to give different products. First, the
•
•
decomposition of C(O)OCH2CH3 radical leads to the formation of CO2 and CH3CH2 radical.
•
Later, methanol (CH3OH) and HCHO were formed from the further decomposition of CH3CH2
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radical in the presence of O2. CH3OH was identified with m/z ratios at 15, 30, 31 and 32; and IR-
peaks at 3200-3600 cm-1 (strong and broad O-H stretch), 3510-3678 cm-1 (strong and sharp O-H
stretch) and 1055-1148 cm-1 (C-O stretch). The other pathway involves the formation of ethyl
formate (HC(O)OCH2CH3) and further reaction with OH radical followed by the HO2 radical
elimination to form acetic formic anhydride (HC(O)OC(O)CH3). This product was recognized
with m/z ratios at 31, 38, 43, 48, 56 and 74; and was not identified in GC-IR, because the
concentration may be below the detection limit of the MCT detector of IR spectrometer.
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3.6. Atmospheric implication
The kinetics measured in this work were used to determine the atmospheric lifetime of the studied
esters. The atmospheric lifetimes (τ) with regard to OH radical were estimated using the
expression, τ =1/k[X], where ‘k’ is the rate coefficient for the reaction of E2MP and E22DMP with
OH radical at 298K and ‘X’ is the global concentration of OH radical (1×106 molecule cm-3).64 By
using this equation, the atmospheric lifetimes were estimated to be 92 hours for E2MP and 105
hours for E22DMP. In addition to this, to further understand their impact on the atmosphere, RFs,
GWPs and POCPs were calculated and are shown in Table 5. RF was calculated using the method
recommended by Pinnock et al.65 They have assumed that the molecules are well-mixed in the
Earth’s atmosphere. However, the residence time of these studied molecules are not long enough
for the vertical mixing. Hence, Hodneberg et al.66 suggested a correction factor to determine the
lifetime-corrected RF using the expression,
푎휏푏
1 + 푐휏푑
(7)
푓 (휏) =
where, a, b, c and d are constants with values of 2.692, 0.9312, 2.994 and 0.9302. Based on this
explanation, the lifetime-corrected RF of E2MP and E22DMP were assessed to be 0.04 and 0.20.
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The positive RF revealed that, these studied molecules do not contribute towards any significant
warming in the Earth’s atmosphere.67 Moreover, by using the estimated value for the lifetime-
corrected RF, GWPs for the studied esters were calculated using the following equation,
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푎∫푇0퐻푒푥푝 ― 1 푑푡
[
]
휏
(8)
(9)
퐺푊푃 =
퐴퐺푊푃퐶푂
2
∑
( )
푎 = 푘퐴푘퐹 푣푘
where, TH is the time horizon, a is the total infrared radiative forcing,
is the global
AGWP
CO2
warming potential for CO2. Further, to determine the GWPs, vibrational frequencies (νk)68 and
intensities (Ak) of the studied molecules were obtained at the B3LYP/6-31+G(d, p) level of theory
and were calculated in the time horizon of 20,100 and 500 years.69, 70 As can be seen from the
Table 5 that, the GWPs of the E2MP was found to be negligible, as compared to the GWPs of
E22DMP. In the case of E22DMP, the GWPs in the time horizon of 20 years was calculated to be
3.9. According to the Intergovernmental Panel on Climate Change (IPCC) suggested that, the
GWPs of the molecules were typically calculated with respect to CO2 (GWPs =1).15 Hence, the
emission of E2MP into the Earth’s atmosphere does not cause any significant change in the global
warming, when compared to E22DMP.
3.7. Photochemical Ozone Creation Potentials (POCPs)
The POCPs for the studied esters were determined based on the method suggested by Jenkin et
al.71 and Derwent et al72 and are listed in Table 5. The authors explained that, the POCP of a
particular VOCs is determined by enumerating the consequence of a small incremental increase in
its emanation on the calculated amount of ozone formed, comparative to an identical increase in
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the O3 release, due to a reference VOC (ethene). The expression used for the calculations is as
follows,
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훽
(
)
(10)
푃푂퐶푃 = 훼1 × 훾푠 × 훾푅 1 ― 훼2 × 푛푐
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where, α1, β and α2 are constants, γs is the structure-based ozone formation index, γR is the
reactivity-based ozone formation index, and nc is the number of carbon atoms. Further, the γs and
γR for the molecules were calculated using the following equation,
푛퐵
28
(11)
훾 =
푠
×
( 6 )
(푀)
푘푂퐻
6
(12)
훾 =
푅
×
(
( 푛 )
)
푒푡ℎ푒푛푒
푘
푂퐻
퐵
Where,
is the number of C-C and C-H bonds, M is the molecular weight,
is the rate
푘푂퐻
푛퐵
coefficient for the reaction of molecules with OH radical at 298 K, 푒푡ℎ푒푛푒 is the rate coefficient
푘푂퐻
for the reaction of ethene with OH radical at 298 K and was reported to be 8.64 × 10-12.71 Based
on these values, the POCP of E2MP and E22DMP were calculated to be 33.9 and 30.9. These
values suggested that, there will be a minimal formation of ozone in the Earth’s troposphere from
these studied molecules, when compared to ethene (100).
4. Conclusions
OH radical-initiated photo-oxidation reactions of E2MP and E22DMP were investigated, both
experimentally and computationally. The computed thermodynamic parameters illustrated that,
the H-abstraction by OH radical from the α'-position towards the acetoxy side (-C(O)OCH-) was
found to be kinetically labile reaction channel for both the studied esters. The kinetic data revealed
that, the reactivity of E2MP and E22DMP towards OH radical was found to be comparable with
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each other. In the OH-initiated oxidation of studied esters in the presence of O2, carbonyl
compounds (such as HCHO, CH3C(O)CH3 and CH3CHO) were formed as major oxidation
products. Moreover, when compared to E22DMP, E2MP is short-lived and has low RF, which
leads to low GWPs. Hence, the emission of E2MP into the Earth’s troposphere is expected to have
a minor impact on global warming and climate change.
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Supporting Information
The cartesian coordinates and vibrational frequencies obtained for all the stationary points are
listed in Tables S1 to S4, Control experiments to verify dark reactions are shown in Figure S1,
IRC plots are shown in Figures S2 and S3, rate coefficients obtained for all the individual reaction
pathways involved in the title reactions are given in Tables S5 and S6, the mass and IR spectra for
all the products obtained from the degradation of title reactions are shown in Figures S4 and S5
and in Section-1 a complete explanation of computational methodology is described.
Acknowledgements
BR shows gratitude to DST, Government of India for research grants. A special thanks to HPCE-
IITM for high-performance computational resources. RK thanks IIT-M for research fellowship.
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