Canonical Variational Transition Research Articles

Experimental and Theoretical Studies of Trans-2-Pentenal Atmospheric Ozonolysis

We investigated the kinetics, mechanism and secondary organic aerosols formation of the ozonolysis of trans-2-pentenal (T2P) using four different reactors with Fourier Transform InfraRed (FTIR) spectroscopy and Gas Chromatography (GC) techniques at T = 298 ± 2 K and 760 Torr in dry conditions. The rate coefficients and branching ratios were also evaluated using the canonical variational transition (CVT) state theory coupled with small curvature tunneling (CVT/SCT) in the range 278–350 K. The experimental rate coefficient at 298 K was (1.46 ± 0.17) × 10−18 cm3 molecule−1 s−1, in good agreement with the theoretical rate. The two primary carbonyls formation yields, glyoxal and propanal, were 57 ± 10% and 42 ± 12%, respectively, with OH scavenger compared to 38 ± 8% for glyoxal and 26 ± 5% for propanal without OH scavenger. Acetaldehyde and 2-hydroxypropanal were also identified and quantified with yields of 9 ± 3% and 5 ± 2%, respectively, in the presence of OH scavenger. For the OH production, an upper limit of 24% was estimated using mesitylene as OH tracer. Combining experimental and theoretical findings enabled the establishment of a chemical mechanism. Finally, the SOA formation was observed with mass yields of about 1.5%. This work provides additional information on the effect of the aldehyde functional group on the fragmentation of the primary ozonide.

Reaction kinetics of a series of alkanes with ClO and BrO radicals: A theoretical study

AbstractReactive halogen species (ClO and BrO) play a critical role in the stratospheric ozone depletion. However, the reactivity of these radical moieties with volatile organic compounds is also important in determining the composition of the Earth's atmosphere. In this study, the reactions of BrO and ClO radicals with a series of smaller linear alkanes (CnH2n+2, n = 1,2,3, and 4) were studied using the CCSD(T)/CBS//B3LYP/cc‐pVTZ level of theory. The kinetics of the title reactions were evaluated using the Canonical Variational Transition (CVT)/Small Curvature Tunneling (SCT)/Interpolated Single Point Energy (ISPE) methodology over the studied temperature range of 200‐3000 K. H‐abstraction from the nonterminal carbon was found to be thermodynamically and kinetically favorable in comparison to abstraction of H atom from the primary carbon. On a per‐hydrogen basis at 298 K, BrO and ClO are focused to abstract the secondary hydrogens of C3H8 faster than the primary hydrogens by factors of 40 and 110, respectively. On the other hand, for n‐C4H10 at the same temperature, the per‐hydrogen preferences for secondary H atoms are found to be factors of 220 and 270 for BrO and ClO, respectively. To aid in modeling the stratospheric environment and combustion regime, three parameter fits to the computed rate constants between 200 and 3000 K are provided. The branching ratios and thermochemical parameters for the various reaction channels are also discussed in this paper.

Cl-Initiated Photo-oxidation Studies of Methyl Valerate and Methyl Isovalerate under Tropospherically Relevant Conditions.

Rate coefficients for the reactions of methyl valerate (MV) and methyl isovalerate (MIV) were investigated using the relative rate (RR) technique in the temperature range of 268-363 K and at 760 Torr of N2. The rate coefficients at 298 ± 2 K were measured to be kMV+Cl = (1.69 ± 0.40) × 10-10 cm3 molecule-1 s-1 and kMIV+Cl = (1.06 ± 0.23) × 10-10 cm3 molecule-1 s-1. To comprehend the experimentally measured rate coefficients, kinetic parameters for the title reactions were computed at the CCSD(T)/cc-pVTZ//BHandHLYP/6-311+G(d,p) level of theory over the temperature range of 200-400 K using canonical variational transition (CVT) state theory in conjunction with small curvature tunneling (SCT) corrections and the interpolated single-point energy (ISPE) method. Additionally, the degradation mechanisms were proposed based on the analyzed products. Moreover, to further understand the atmospheric fate of these molecules in the troposphere, atmospheric lifetimes, radiative forcings (RFs), and global warming potentials (GWPs) were calculated.

Cl Atom-Initiated Photo-Oxidation Reactions of Vinyl Trifluoroacetate and Allyl Trifluoroacetate in Tropospheric Conditions.

Relative rate (RR) technique coupled with gas chromatography (GC) was used to investigate the kinetics of the reactions of two hydrofluoroesters (allyl trifluoroacetate (CF3C(O)OCH2CH═CH2, ATFA) and vinyl trifluoroacetate (CF3C(O)OCH═CH2, VTFA)) with Cl atoms between 268 and 363 K and at 760 Torr of N2 (or air). The temperature-dependent Arrhenius expressions for the title reactions were obtained to be k268-363KVTFA+Cl = [(7.83 ± 2.26) × 10-12 exp((974 ± 89)/T)] cm3 molecule-1 s-1 and k268-363KATFA+Cl = [(9.03 ± 1.92) × 10-12 exp((883 ± 65)/T)] cm3 molecule-1 s-1, respectively. A negative temperature dependency was observed for both the reactions. In addition to this, the kinetics of the studied reactions was evaluated computationally at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level of theory in the temperature range of 200-400 K using canonical variational transition (CVT) state theory in conjunction with small curvature tunneling (SCT) corrections and interpolated single point energy (ISPE) methods. The product analysis of the reactions of VTFA and ATFA with Cl atoms in the presence of O2 was also investigated using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR). The plausible oxidation mechanism of the title reactions was proposed based on the product analyses. Further, to comprehend the impact of these molecules on the troposphere, atmospheric lifetimes, lifetime-corrected radiative forcing (RF), and global warming potential (GWP) were estimated and are presented in this manuscript.

Theoretical study of mechanism and kinetics for the reaction of hydroxyl radical with 2′-deoxycytidine

The reaction mechanism and kinetics for the abstraction of hydrogen and addition of hydroxyl radical (OH) to 2′-deoxycytidine have been studied using density functional theory at MX06-2X/6-311+G(d,p) level in aqueous solution. The optimized geometries, energies, and thermodynamic properties of all stationary points along the hydrogen abstraction reaction and the addition reaction pathways are calculated. The single-point energy calculations of the main pathways at CCSD(T)/6-31+G(d,p)//MX06-2X/6-311+G(d,p) level are performed. The rate constants and the branching ratios of different channels are evaluated using the canonical variational transition (CVT) state theory with small-curvature tunneling (SCT) correction in aqueous solution to simulate the biological system. The branching ratios of hydrogen abstraction from the C1′ site and the C5′ site and OH radical addition to the C5 site and the C6 site are 57.27% and 12.26% and 23.85% and 5.69%, respectively. The overall calculated rate constant is 4.47 × 109 dm3 mol−1 s−1 at 298 K which is in good agreement with experiments. The study could help better understand reactive oxygen species causing DNA oxidative damage.

Effects of Reagent Rotation and Vibration on H + OH (υ, j)→ O + H2

The dynamics of the reaction H + OH → O ((3)P) + H2 have been studied in a series of quasi-classical trajectory (QCT) calculations and transition state theory (TST) methods using high quality (3)A' and (3)A″ potential energy surfaces (PESs). Accurate OH (υ, j) state resolved cross sections and rate constants on both potential energy surfaces are presented and fitted for OH at (υ = 0, j = 0-16) and (υ = 1, j = 0-6). The cross sections were calculated for different collisional energies (Ec), ranging from the threshold energy at each specific rovibrational state up to 1.0 eV with step sizes of 0.1 eV or less. They increase steeply with collision energy when the barrier to reaction can be overcome, after which the cross sections stay nearly constant with energy. State resolved rate constants in the temperature range 200-2500 K are presented based on the cross sections. Total thermal rate constants were calculated by summing the rates for reaction on the (3)A' and (3)A″ potential energy surfaces weighted by 1/3 and taking into account the thermal populations of the rovibrational states of the OH molecules. The currently calculated thermal rate constants generally agree well with previous indirectly obtained rate constants by Tsang et al. (Tsang, W.; Hampson, R. F. Chemical Kinetic Data Base for Combustion Chemistry. Part I. Methane and Related Compounds. J. Phys. Chem. Ref. Data 1986, 15, 1087-1279). It is shown that the improved canonical variational transition (CVT) treatments with the approximation of zero-curvature tunneling (ZCT) or small-curvature tunneling (SCT) produce results more in accord with the QCT results than the TST and CVT methods. The reactions are governed by the direct reaction mechanism. The rate constants for OH in excited vibrational and rotational states are orders of magnitude larger than the thermal rate constants, which needs to be taken into account in astrochemical models.

The atmospheric chemical reaction of 4-tert-butylphenol initiated by OH radicals

Environmental context 4-tert-Butylphenol, an environmental endocrine disruptor, can be taken in by humans and animals resulting in reproductive and developmental problems. We report a theoretical study on the degradation mechanism of 4-tert-butylphenol in the atmosphere, and calculate the atmospheric lifetime of this chemical. The data will help our understanding of the behaviour of 4-tert-butylphenol in the environment and thereby provide valuable information about its possible effect on human health. Abstract 4-tert-Butylphenol (TBP) is a typical environmental endocrine. In this paper, the OH-initiated degradation mechanism of TBP in the atmosphere is studied at the MPWB1K/6-31+G(d,p)//MPWB1K/6-311+G(3df,2p) level of computational theory. A profile of the potential energy surface is constructed and reaction pathways are analysed. The addition reactions of TBP with OH radicals are more important than abstraction reactions in the atmosphere. In subsequent reactions, O2 and NO may play an important role in the degradation process of TBP. The rate constants are calculated using the transition state theory and a canonical variational transition with small-curvature tunnelling correction. The Arrhenius equations of rate constants in the temperature range of 200–500K are fitted. The rate constant of the degradation of the TBP at 298.15K is 3.56×10–14cm3 molecule–1s–1 and the atmospheric lifetime is 10.8 months according to the pseudo-first-order kinetics.

Theoretical studies on atmospheric reactions of CH2FO2 with HO2 and HO2⋅H2O complex

The formations of CHFO, 1O2, O3 and 3O2 in CH2FO2+HO2 and CH2FO2+HO2⋅H2O reaction are studied by employing the quantum chemical calculations with B3LYP and CCSD(T) theoretical methods, canonical variational transition (CVT) state theory with small curvature tunneling (SCT) correction. The calculated results show that the formations of CHFO and 3O2 are main products in the naked reaction of CH2FO2+HO2. When water is added, the formations of CHFO and O3 are main products in CH2FO2+HO2⋅H2O reaction. Moreover, the calculated rate constants for the title reaction without and with a water molecule show that, although the single water molecule plays a positive catalytic effect on enhancing the rate for CHFO and O3 formation, in humid conditions the effective rate of CH2FO2+HO2 reaction will changes little with respect to dry conditions.

Ab initio and kinetic calculations for the reactions of Cl with SiH(n)Cl(4-n) (n=1,2,3,4).

The direct hydrogen abstraction reactions of Cl atom with SiH(n)Cl(4-n) (n=1,2,3,4) have been studied systematically using ab initio molecular orbital theory. Geometries have been optimized at the MP2 level with 6-311+G(d) basis set, QCISD(T)/6-311+G(d,p) has been used in the final single point energy calculation. The kinetic calculations of these reactions have been explored using the canonical variational transition (CVT) state theory method with small-curvature tunneling (SCT) effect correction over the temperature range of 200-2000 K. The CVT/SCT rate constants exhibit typical non-Arrhenius behavior and three-parameter rate-temperature formulas have been fitted for the reactions of Cl with SiH4, SiH3Cl, SiH2Cl2, and SiHCl3, respectively (in unit of cm(3) molecule(-1) s(-1)). The calculated CVT/SCT rate constants are in agreement with the available experimental values.

A theoretical study on the β-C–H fission of ethoxy radical

Abstract We present a direct ab initio dynamics study of the thermal gas-phase rate constants of the unimolecular decomposition reaction of C 2 H 5 O → CH 3 CHO + H. Both MPW1K/6-31+G(d,p) and QCISD/6-31+G(d,p) methods were employed to optimize the geometries of all stationary points and to calculate the minimum energy path (MEP). The energies of all the stationary points were refined at a series of multi-coefficient and multi-level methods. Among all methods, the QCISD(T)/aug-cc-pVTZ energies are in good agreement with the available experimental data. The rate constants were evaluated based on the energetics from the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d,p) level of theory using both microcanonical variational transition (mVT) state theory and canonical variational transition (CVT) state theory with the Eckart tunneling correction in the temperature range of 200–2500 K. The calculated rate constants at the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d,p) level of theory are in good consistent with the theoretical data from Caralp et al. The fitted Arrhenius expression in the temperature range 200–2500 K is k=1.30×10 9 T 1.42 e (−1.03×10 4 /T) s −1 . The falloff curves of pressure-dependent rate constants are performed using master-equation method within the temperature range of 391–471 K.