Balla Rajakumar

The kinetics of the Cl-initiated reactions of isobutyl acetate (IBAc) and 2-methyl allyl acetate (2MAAc) were elucidated as a function of temperature, employing experimental and computational methodologies. The relative rate method, with the aid of gas chromatography with a flame ionization detector, was adopted to determine rate coefficients at various temperatures. Computational calculations were performed over 250–400 K using CVT/SCT with interpolated single-point energies at the CCSD(T)/aug-cc-pVDZ//MP2/6–31+g(d,p) and CCSD(T)/6–31+g(d,p)//MP2/6–31+g(d,p) levels of theory for the IBAc + Cl and 2MAAc + Cl reactions, respectively. Experimental and computational results revealed an inverse temperature dependence for both reactions. The experimentally determined Arrhenius expressions include k(288–353 K) = (2.70 ± 0.68) × 10–11 exp{(405 ± 80)/T} and k(288–363 K) = (7.13 ± 1.58) × 10–11 exp{(429 ± 71)/T} cm3 molecule–1 s–1 for IBAc + Cl and 2MAAc + Cl reactions, respectively. The rate coefficients for the 2MAAc + Cl reaction were found to be three times higher than those of the IBAc + Cl reaction. Computationally calculated rate coefficients also followed this trend, and the most feasible pathways were determined using the obtained site-specific reaction kinetics and branching ratios. Analysis of the postreaction mixture via gas chromatography with mass spectrometry facilitated the identification of degradation products, enabling the proposal of a mechanism consistent with the obtained products. The comparison among the room-temperature rate coefficients of various acetates provided the relation between molecular structure and kinetics. The atmospheric lifetimes of IBAc and 2MAAc were calculated based on these rate coefficients, and the implications of the generated products on the Earth’s atmosphere are discussed in detail.

The sulfur-containing volatile organic compounds (VOSCs) have a pronounced potential to impact the atmosphere. Hence, studying their photooxidation reactions with tropospheric oxidants has substantial relevance. Allyl methyl sulfide (AMS), which falls under this category, is found to be released from various sources. Hence, the present study explored the kinetics of AMS + OH and AMS + Cl reactions over 283-363 K, employing both experimental and computational methodologies. Pulsed laser photolysis─laser-induced fluorescence (PLP-LIF) technique was utilized to determine the rate coefficients of the AMS+OH reaction, whereas the kinetics of the AMS + Cl reaction were assessed using the relative rate (RR) technique, aided by gas chromatography with a flame ionization detector (GC-FID). The reaction products formed during the oxygen-assisted AMS + Cl reaction were separated and identified by gas chromatography-mass spectrometry (GC-MS). The Arrhenius expressions determined using experimental methodologies include k_AMS+OH(283-363 K) = (3.28 ± 1.07) × 10^-12 exp{(534 ± 104)/T} and k_AMS+Cl(283-363 K) = (9.14 ± 1.34) × 10^-11 exp{(426 ± 47)/T} cm³ molecule^-1 s^-1. Computational studies were performed to complement the experimental findings and confirmed the addition pathways as the major pathways in both reactions. An inverse correlation with the temperature was observed for the rate coefficients of AMS + OH and AMS + Cl reactions. At the same time, the AMS + OH reaction kinetics were found to be independent of the pressure, and a positive correlation with the pressure was revealed for the AMS + Cl reaction. Sulfur dioxide, a key product of the AMS+Cl reaction, can lead to various atmospheric issues that can cause serious human health problems.

Kinetic investigations of two isomers of methyl cyclohexene (1-methyl cyclohexene, 1MCHXE, and 4-methyl cyclohexene, 4MCHXE) with OH radical over the temperature range 263-363 K employing experimental and theoretical methodologies were performed in this study. The pulsed laser photolysis-laser-induced fluorescence technique was employed for the experimental studies. The obtained temperature-dependent rate coefficients for the OH-initiated reactions of 1MCHXE and 4MCHXE are k(263-363 K) = (2.09 ± 0.65) × 10^-11 exp{(388 ± 96)/T} and k(263-363 K) = (6.49 ± 1.34) × 10^-12 exp{(462 ± 63)/T} cm³ molecule^-1 s^-1, respectively. Computational studies were conducted using CVT/SCT/ISPE with the aid of Gaussian 16 and POLYRATE 17-C software. 1MCHXE exhibited higher reactivity than 4MCHXE, both experimentally and theoretically. The reason behind this increase in the rate coefficient is discussed in terms of the position of the methyl group with respect to the unsaturated double bond. The thermodynamic and kinetic feasibilities of different reaction pathways were investigated in detail. Branching ratios and Fukui functions for radical attack were calculated to support the site-specific reactivity followed by the OH-initiated reaction pathways. Since methyl cyclohexene is released from a variety of natural and manmade sources and serves as a model for a large number of biogenic terpene compounds, atmospheric implication parameters such as lifetime and photochemical ozone creation potential of its isomers were determined.

Temperature- and pressure-dependent kinetic study of the reactions of the Criegee intermediate (CH₂OO) with 2-furaldehyde and 3-furaldehyde was performed by using computational methodologies. The geometry optimization and thermochemical parameters calculations were performed using B3LYP/6-311G+(2df,2p) theory, whereas the barrier heights in the potential energy diagram for the reaction pathways were calculated at the CCSD(T)-F12b/cc-pVTZ-F12//B3LYP/6-311 + G(2df,2p) level of theory. The room-temperature high-pressure-limit rate coefficients were calculated using canonical variational transition state theory in conjugation with small curvature tunneling (CVT/SCT) to be 7.20 × 10^-12 cm³ molecule^-1 s^-1 and 3.04 × 10^-12 cm³ molecule^-1 s^-1 for 2-furaldehyde and 3-furaldehyde, respectively. The room-temperature rate coefficients calculated from the MESMER were 2.62 × 10^-12 cm³ molecule^-1 s^-1 and 1.59 × 10^-12 cm³ molecule^-1 s^-1, respectively. The predicted products of the title reactions were formic acid and formaldehyde, along with 2-furoic acid for the 2-furaldehyde reaction and 3-furoic acid for the 3-furaldehyde reaction. The large value of atmospheric lifetimes (>400 days) of the furaldehydes due to their reactions with CH₂OO suggests that the title reactions do not have any significant effect on the net atmospheric furaldehyde concentrations.

Chlorinated hydrocarbons have wide applications in the industry and are likely to be emitted to the environment, where they can influence the atmosphere. Hence, the kinetics of the degradation of isobutyl chloride (IBC) initiated by OH radicals and Cl atoms was determined both experimentally and theoretically in this work. The rate coefficients were experimentally measured from 268 to 363 K, using pulsed-laser photolysis-laser-induced fluorescence (PLP-LIF) for the reaction with OH radicals and using the relative-rate (RR) method for the reaction with Cl atoms. The room temperature rate coefficients (in cm³ molecule^-1 s^-1) for both reactions were measured to be k_IBC+OH^298K = (1.45 ± 0.10) × 10^-12 and k_IBC+Cl^298K = (5.36 ± 1.31) × 10^-11, respectively. The rate coefficients showed a dependence on temperature given by the Arrhenius equations k_IBC+OH^268-363K = (1.22 ± 0.32) × 10^-12 exp[(39.2 ± 81.4)/T] cm³ molecule^-1 s^-1 and k_IBC+Cl^268-363K = (1.42 ± 0.64) × 10^-11 exp[(417.2 ± 136.6)/T] cm³ molecule^-1 s^-1. Rate coefficients were also calculated theoretically to understand the site-specific reaction kinetics at the CCSD(T)/aug-cc-pVDZ//BHandHLYP/6-31+G(d,p) and CCSD(T)/6-311++G(d,p)//BHandHLYP/6-31+G(d,p) levels for the reaction of IBC with OH radicals and Cl atoms, respectively. Parameters were calculated to assess the impact of IBC release on the atmosphere using the obtained kinetics. The products formed in these reactions were also analyzed to comment on the detailed degradation mechanism of IBC.

When volatile organic compounds undergo atmospheric oxidation, they can form various oxygenated organic compounds, including unsaturated ketones. Unsaturated ketones in the atmosphere can impact air quality and human health. The kinetics for the oxidation of 5-hexen-2-one (3-buten-1-yl methyl ketone, 3BMK) and 4-hexen-3-one (ethyl-1-propenyl ketone, E1PK) with OH radicals were investigated experimentally and computationally. The experiments were carried out using pulsed laser photolysis - laser-induced fluorescence technique over 263-358 K and over 26-30 Torr. At 298 K, the rate coefficients for the reactions of 3BMK and E1PK were (4.72 ± 0.12) × 10^-11 and (8.45 ± 0.13) × 10^-11 cm³ molecule^-1 s^-1, respectively. The temperature-dependent rate coefficients were expressed using the Arrhenius equations to be k_3BMK+OH^T=(263-358)K = (2.45 ± 0.68) × 10^-12 exp{-(-948 ± 85)/T} and k_E1PK+OH^T=(263-358)K = (1.16 ± 0.81) × 10^-12 exp{-(-1241 ± 212)/T} cm³ molecule^-1 s^-1. The purpose of the computational study was to improve the comprehension of the kinetics and thermochemistry. The atmospheric lifetime for both compounds were calculated to be ∼4 and ∼2 days for 3BMK and E1PK, respectively. Gas chromatography-mass spectrometry (GC-MS) was used to examine the secondary organic products that resulted from the reactions at 298 K and 760 Torr. 2-Hydroxyacetaldehyde and acetaldehyde were detected as the products of the degradation of 3BMK and E1PK.

Temperature-dependent kinetics for the reaction of C2-Criegee intermediates (CH₃CHOO) with acetaldehyde (CH₃CHO) was studied at 268-313 K and 50 Torr using cavity ring-down spectroscopy with single-wavelength (360 nm) probing. The measured rate coefficients are expected to have contributions from both the anti- and syn-conformers of CH₃CHOO. Negative T dependence was observed for the title reaction, and the corresponding Arrhenius equation is k₃(T = 268 - 313 K) = (1.34 ± 0.07) × 10^-13 × exp{(1.71 ± 0.03) kcal mol^-1/RT}. The room temperature rate coefficients measured at 50 and 100 Torr are (2.43 ± 0.17) × 10^-12 and (2.56 ± 0.20) × 10^-12 cm³ molecule^-1 s^-1, respectively. Theoretical calculations were performed at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p) level of theory to obtain the high-pressure limit rate coefficients for the reaction of anti- and syn-CH₃CHOO with CH₃CHO. The high-pressure limit rate coefficient for syn-CH₃CHOO is approximately 3 orders of magnitude smaller than that of the anti-conformer, the latter being closely aligned with the experimental value. The rate coefficients for anti-CH₃CHOO + CH₃CHO at 50 Torr using the master equation solver (MESMER) are in agreement with the experimental values in the studied temperature range. MESMER also predicted CH₃COOH to be the major product for both anti- and syn-CH₃CHOO reactions by comparing the rate coefficients for the product formation pathways. A dramatic dependence of the pressure on stabilization of the SOZs was also observed for both conformers at different pressures.