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Journal ArticleDOI

A theoretical study of the Ḣ- and HOȮ-assisted propen-2-ol tautomerizations: Reactive systems to evaluate collision efficiency definitions on chemically activated reactions using SS-QRRK theory

TL;DR: In this paper, the authors performed a theoretical kinetic study of the step-wise propen-2-ol tautomerization catalyzed by hydrogen and hydroperoxyl radicals.
About: This article is published in Combustion and Flame.The article was published on 2021-03-01 and is currently open access. It has received 8 citations till now. The article focuses on the topics: Potential energy surface & Hydroperoxyl.

Summary (2 min read)

1. Introduction

  • Bio-alcohols such as bio-ethanol and bio-butanol are promising alternatives to petroleum-based transportation fuels because of their liquid nature, potential for climate change mitigation [1], and broad range of applications [2].
  • Therefore, efforts have been conducted to unravel dominant routes in alcohol combustion chemistry.
  • Its original formulation underpredicts the values of pressure-dependent rate constants for reactive systems involving large molecules and at high temperatures.
  • The authors conducted a theoretical kinetic study of the propen-2-ol tautomerization to acetone in gas-phase assisted by hydrogen and hydroperoxyl radicals (hereafter Ḣ-assisted and HOȮ-assisted, respectively) in stepwise mechanisms.

2. Theoretical methodology

  • 1. Potential energy surface and high-pressure limit rate constants Electronic structure calculations were performed at the CCSD(T)/aug-cc-pVTZ//M06-2X/ccpVTZ level of theory using the Gaussian09 package [25] to explore the PESs of the following overall reactions: CH2=C(OH)CH3 + Ḣ ⟺ CH3COCH3 + Ḣ R1 CH2=C(OH)CH3 + HOȮ ⟺ CH3COCH3 + HOȮ R2 where CH2=C(OH)CH3 and CH3COCH3 are, respectively, propen-2-ol and acetone.
  • Therefore, their low pressure rate constants were computed following the indications given in their previous work [21], that is, with the implementation of a modified version of the original SS-QRRK/MSC approach, referred to as SS-QRRK/MSC-Dean, where the collision efficiency definition of Dean et al. [23,24] (Eq. (4)) is used.
  • For 𝑘𝑝, as shown in Fig. 8(a), one only distinguishes values between 100 atm and 10 atm in the 400 K – 800 K temperature range, and for all other conditions of temperature and pressure, the rate constants overlap with the curve of 10 atm.
  • 6. Stepwise against concerted HOȮ-assisted propen-2-ol tautomerization Fig. 10 compares the different rate constants of the HOȮ-assisted propen-2-ol tautomerization: overall stepwise (sum of the first and second stepwise routes), first stepwise route, second stepwise route, and those of the concerted reaction previously reported by us [15].

4. Conclusions

  • A theoretical kinetic study of the Ḣ- and HOȮ-assisted stepwise tautomerizations of propen-2ol into acetone was performed based on electronic structure calculations at the CCSD(T)/aug-ccpVTZ//M06-2X/cc-pVTZ.
  • It only has an appreciable influence (of one order of magnitude) when using the original SS-QRRK/MSC approach and diminishes to a factor of 1.7 after implementing the collision efficiency of Gilbert et al. [22].
  • In the chemically activated HOȮ-assisted tautomerization (green dashed line in Fig. 1(b)), the energized adduct does not stabilize (H* + M → H + M), and the population of final products (acetone + HOȮ) dominates during most of the stages of the simulation.
  • A subtle maximum in the rate constants of its first step, especially at low pressures, which coincides with the peak of multistructural anharmonicity at around 800 K.
  • Besides, the SS-QRRK/MSC-GCA approach proposed in this work and implemented in their bespoke Python [35] code extends the scope of the SS-QRRK theory [18,19] to chemically activated reactions, which easily allows the inclusion of variational effects, multidimensional tunneling, and multistructural anharmonicity in the falloff region of chemical reactions.

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Journal ArticleDOI
TL;DR: This paper performed a theoretical study to describe the early chemistry of nicotine degradation by investigating two important reactions that nicotine can undergo: hydrogen abstraction by hydroxyl radicals and unimolecular dissociation.
Abstract: Nicotine exposure results in health risks not only for smokers but also for second- and third-hand smokers. Unraveling nicotine's degradation mechanism and the harmful chemicals that are produced under different conditions is vital to assess exposure risks. We performed a theoretical study to describe the early chemistry of nicotine degradation by investigating two important reactions that nicotine can undergo: hydrogen abstraction by hydroxyl radicals and unimolecular dissociation. The former contributes to the control of the degradation mechanism below 800 K due to a non-Arrhenius kinetics, which implies an enhancement of reactivity as temperature decreases. The latter becomes important at higher temperatures due to its larger activation energy. This change in the degradation mechanism is expected to affect the composition of vapors inhaled by smokers and room occupants. Conventional cigarettes, which operate at temperatures higher than 1000 K, are more prone to yield harmful pyridinyl radicals via nicotine dissociation, while nicotine in electronic cigarettes and vaporizers, with operating temperatures below 600 K, will be more likely degraded by hydroxyl radicals, resulting in a vapor with a different composition. Although low-temperature nicotine delivery devices have been claimed to be less harmful due to their nonburning operating conditions, the non-Arrhenius kinetics that we observed for the degradation mechanism below 873 K suggests that nicotine degradation may be more rapidly initiated as temperature is reduced, indicating that these devices may be more harmful than it is commonly assumed.

11 citations

01 Jan 2007
TL;DR: Calculated rate constants of enol formation were included in the Utah Surrogate Mechanism to model the enol profile in a cyclohexane premixed flame and shows consistency with experimental data and gives 5% contribution of ethenol formation from OH + propene reaction, the rest coming from ethene + OH.
Abstract: Kinetics of enol generation from propene has been predicted in an effort to understand the presence of enols in flames. A potential energy surface for reaction of OH with propene was computed by CCSD(T)/cc-pVDZ//B3LYP/cc-pVTZ calculations. Rate constants of different product channels and branching ratios were then calculated using the Master Equation formulation (J. Phys. Chem. A 2006, 110, 10528). Of the two enol products, ethenol is dominant over propenol, and its pathway is also the dominant pathway for the OH + propene addition reactions to form bimolecular products. In the temperature range considered, hydrogen abstraction dominated propene + OH consumption by a branching ratio of more than 90%. Calculated rate constants of enol formation were included in the Utah Surrogate Mechanism to model the enol profile in a cyclohexane premixed flame. The extended model shows consistency with experimental data and gives 5% contribution of ethenol formation from OH + propene reaction, the rest coming from ethene + OH.

6 citations

Journal ArticleDOI
TL;DR: In this paper , the reaction kinetics of prenol and isoprenol with OH radicals were investigated over the temperature range of 900-1290 K and pressure of 1-5 atm by utilizing a shock tube and OH laser diagnostic.
Abstract: The presence of two functional groups (OH and double bond) in C5 methyl-substituted enols (i.e., isopentenols), such as 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), makes them excellent biofuel candidates as fuel additives. As OH radicals are abundant in both combustion and atmospheric environments, OH-initiated oxidation of these isopentenols over wide ranges of temperatures and pressures needs to be investigated. In alkenes, OH addition to the double bond is prominent at low temperatures (i.e., below ∼700 K), and H-atom abstraction dominates at higher temperatures. However, we find that the OH-initiated oxidation of prenol and isoprenol displays a larger role for OH addition at higher temperatures. In this work, the reaction kinetics of prenol and isoprenol with OH radicals was investigated over the temperature range of 900-1290 K and pressure of 1-5 atm by utilizing a shock tube and OH laser diagnostic. To rationalize these chemical systems, variational transition state theory calculations with multi-structural torsional anharmonicity and small curvature tunneling corrections were run using a potential energy surface characterized at the UCCSD(T)/jun-cc-pVQZ//M06-2X/6-311++G(2df,2pd) level of theory. A good agreement was observed between the experiment and theory, with both predicting a non-Arrhenius behavior and negligible pressure effects. OH additions to the double bond of prenol and isoprenol were found to be important, with at least 50% contribution to the total rate constants even at temperatures as high as 700 and 2000 K, respectively. This behavior was attributed to the stabilizing effect induced by hydrogen bonding between the reacting OH radical and the OH functional group of isopentenols at the saddle points. These stabilizing intermolecular interactions help mitigate the entropic effects that hinder association reactions as temperature increases, thus extending the prominent role of addition pathways to high temperatures. The site-specific rate constants were also found to be slower than their analogous reactions of OH + n-alkenes.

4 citations

Journal ArticleDOI
TL;DR: In this article , a chemical kinetic model for isoprene pyrolysis was proposed and validated against single-pulse shock tube (SPST) and jet-stirred reactor (JSR) experimental data in the temperature range of 850-1690 K.

3 citations

Journal ArticleDOI
01 Feb 2022-Fuel
TL;DR: In this article, a quantum chemical (QC) model of diesel surrogate fuel (DSF) is proposed, which takes into account the CO2 pyrolysis effect (C O 2 ⇆ C O + 1 2 O 2 ).

3 citations

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Abstract: In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlof, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects i f the exponents of the functions are optimized in atomic correlated calculations, although the primitive (s p) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations i f the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlof and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of c o r r e l a t i o n c o n s i s t e n t b a s i s s e t s, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.

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TL;DR: The M06-2X meta-exchange correlation function is proposed in this paper, which is parametrized including both transition metals and nonmetals, and is a high-non-locality functional with double the amount of nonlocal exchange.
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TL;DR: In this paper, the authors show that the cause of these errors is a result of two inter-related problems: near duplication of the exponents in two of the d sets and a lack of high-exponent functions in the early members of the sets.
Abstract: For molecules containing second row atoms, unacceptable errors have been found in extrapolating dissociation energies calculated with the standard correlation consistent basis sets to the complete basis set limit. By carefully comparing the convergence behavior of De(O2) and De(SO), we show that the cause of these errors is a result of two inter-related problems: near duplication of the exponents in two of the d sets and a lack of high-exponent functions in the early members of the sets. Similar problems exist for the f sets (and probably in higher angular momentum sets), but have only a minor effect on the calculated dissociation energies. A number of approaches to address the problems in the d sets were investigated. Well behaved convergence was obtained by augmenting the (1d) and (2d) sets with a high-exponent function and by replacing the (3d) set by the (4d) set and the (4d) set by the (5d) set and so on. To ensure satisfactory coverage of both the L and M shell regions, the exponents of the new d se...

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TL;DR: Etude de l'une des limitations dans la solution of l'equation de Schrodinger a savoir, le probleme de la correlation electronique et d'une nouvelle approche de sa resolution, la theorie des amas couples as mentioned in this paper.
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"A theoretical study of the Ḣ- and H..." refers methods in this paper

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TL;DR: In this paper, the authors described the rate constants for thermal unimolecular reactions and recombinations at the low pressure limit, at the high pressure limit and in the intermediate falloff range.
Abstract: This paper describes the calculation of rate constants for thermal unimolecular reactions and recombinations at the low pressure limit, at the high pressure limit, and in the intermediate falloff range, as well as the calculation of specific rate constants for unimolecular rearrangements. The most uncertain factors of the theory are identified by comparison with the NO/sub 2/, ClNO, H/sub 2/O, and O/sub 3/ systems. Weak collision and centrifugal barrier effects are discussed for low pressure rate constants. Simplified adiabatic channel calculations are proposed for specific rate constants and high pressure rate constants. Reduced falloff curves are presented in factorized form with weak collision and strong collision broadening factors. Simple falloff expressions are derived. 5 figures, 44 references, 3 tables.

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Frequently Asked Questions (1)
Q1. What have the authors contributed in "A theoretical study of the ￿- and ho￿-assisted propen-2-ol tautomerizations: reactive systems to evaluate collision efficiency definitions on chemically activated reactions using ss-qrrk theory" ?

In this work, the authors performed a theoretical kinetic study of the step-wise propen-2-ol tautomerization catalyzed by hydrogen and hydroperoxyl radicals.