Theoretical Studies on the Kinetics of Multi-Channel Gas-Phase

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Theoretical Studies on the Kinetics of Multi-Channel GasPhase Unimolecular Decomposition of Acetaldehyde Vahid Saheb, S. Rasoul Hashemi, and S. Mohammad Ali Hosseini J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04771 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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The Journal of Physical Chemistry

Theoretical Studies on the Kinetics of Multi-Channel Gas-Phase Unimolecular Decomposition of Acetaldehyde Vahid Saheb* , S. Rasoul Hashemi, S. Mohammad Ali Hosseini Department of Chemistry, Shahid Bahonar University of Kerman, Kerman, Iran

Abstract Theoretical kinetic studies are performed on the multi-channel thermal decomposition of acetaldehyde. The geometries of the stationary points on the potential energy surface of the reaction are optimized at the MP2(full)/6-311++G(2d,2p) level of theory. More accurate energies are obtained by single point energy calculations at the CCSD(T,full)/augh-cc-pVTZ+2df, CBS-Q and G4 levels of theory. Here, by applying steady-state approximation to the thermally-activated species CH3CHO* and CH2CHOH* and performing statistical mechanical manipulations, expressions for the rate constants for different product channels are derived. Special attempts are made to compute accurate energy-specific rate coefficients for different channels by using semi-classical transition state theory. It is found that the isomerization of CH3CHO to the enol-form CH2CHOH plays a significant role in the unimolecular reaction of CH3CHO. The possible products of the reaction are formed via unimolecular decomposition of CH3CHO and CH2CHOH. The computed rate coefficients reveal that the dominant channel at low temperatures and high pressures is the formation of CH2CHOH due to the low barrier height for CH3CHO → CH2CHOH isomerization process. However, at high temperatures, the product channel CH3 + CHO becomes dominant.

*

Corresponding author. Tel.: +98 341 3222033; fax: +98 341 3222033. E-mail address: [email protected]

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1. Introduction Acetaldehyde unimolecular decomposition has received considerable scrutiny due to its importance in combustion chemistry of new generation oxygenated fuels.1-20 All early experimental and theoretical kinetic studies have considered the C-C bond scission of CH3CHO to CH3 + CHO (R1) as the only active initiation channel.1-15 However, recent experimental studies by more sensitive apparatus17-19 and sophisticated theoretical investigation on the potential energy surface (PES) of CH3CHO17,20 have suggested that other reaction channels may be of importance especially at higher temperatures. Photodissociation studies of CH3CHO have revealed the possibility of an active “roaming” mechanism leading to considerable amount of the molecular product CH4 + CO.16 Sivaramakrishnan et al. have used a high-temperature shock-tube to study the unimolecular decomposition of CH3CHO at very low concentrations and determined the H-atom forming channels (originating from C-C bond dissociation) to be about 0.8. They assigned the non Hatom yield to the formation of CH4 and CO primarily due to the “roaming” mechanism.17 However, CH4 was not detected in the pyrolysis studies of CH3CHO and its deuterated isotopologues by Vasiliou and coworkers.18-19 Their observations question the validity of the “roaming” mechanism for CH4 formation. They observed CH3, CO, H, H2, CH2CHOH (vinyl alcohol), CH2CO (ketene), C2H2 and H2O in their microtubular reactor equipped with sensitive detectors. On the basis of the latter detected products, they proposed that vinyl alcohol is a dominant intermediate and CH2CHOH, CH2CO, C2H2 and H2O are originated from the decomposition of acetaldehyde and vinyl alcohol. Although CH2CO and C2H2 have been observed in some previous experimental studies, but they were attributed to secondary reactions.10-11 CH2CHOH and H2O were observed by Vasiliou et al. for the first time.18-19


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Some theoretical studies have been performed to locate stationary points on the PES of CH3CHO and calculate its unimolecular decomposition rate coefficients. Early theoretical studies have been concentrated on the rate constant of C-C bond dissociation process, and the product energy and state distributions.13-15 The recent photo-dissociation and shock-tube experimental studies16-17 have prompted theoretical studies17,20 on roaming radical kinetics in the decomposition of acetaldehyde. Although trajectory calculations are often used to investigate the kinetics of roaming processes,21-23 Klippenstein et al. have shown statistical rate theories could be used if more accurate state counting procedures for the low frequency motions are employed.24 To resolve some paradoxes in the thermal unimolecular reaction mechanism of CH3CHO, Sivaramakrishnan et al. have explored the PES of the unimolecular decomposition of CH3CHO and its subsequent reaction with H atom, and have modeled the reaction by CHEKIN package.25 They highlighted that keto-enol tautomerization of CH3CHO to CH2CHOH was an important process at high temperature. In this research, theoretical studies are carried out on the multi-channel elementary unimolecular decomposition reaction of acetaldehyde. All important reaction paths, especially CH3CHO ⇄ CH2CHOH process, are considered in statistical rate calculations. Here, on the base of energies and geometrical parameters obtained by ab initio methods, statistical rate theories are used to compute the thermal rate constants for formation all possible products of the title reaction.

Computational details Electronic-Structure Calculations: In the present study, first, second-order Møller-Plesset perturbation theory (MP2)26 method along with the 6-311+G(d,p) standard basis set is used to locate the geometries of all stationary points, i.e., minimum energy structures and saddle 3 ACS Paragon Plus Environment


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points. Next, the geometries are re-optimized by MP2 method employing the larger standard basis set 6-311++G(2d,2p). In order to obtain accurate energies, single-point energy calculations are performed on the MP2/6-311++G(2d,2p) optimized geometries, with the unrestricted coupled cluster method with single, double, and noniterative triple excitations uCCSD(T)27 with the standard augh-cc-pVTZ+2df basis set.28 The latter basis set, developed by Martin and de Oliveira, is aug-cc-pVTZ29 basis set in which high exponent d- and f-type basis functions is added to describe inner-shell correlation effects correctly. In all above quantum-chemical calculations, all electrons are included in the correlation calculations. Single-point energy calculations are performed by CBS-Q30 and G431 combination methods for the purpose of comparison. Harmonic vibrational frequencies and xij vibrational anharmonicity coefficients are calculated at the MP2(full)/6-311++G(2d,2p) level of theory. The Gaussian 09 package of programs32 is used to carry out all of the quantum chemical calculations. Kinetics calculations: RRKM statistical rate theory has been successfully used during last decades for computing the rate constants for unimolecular reactions as a function of temperature and pressure.33,34 The rate constant for the unimolecular reaction of a molecule with a given total energy E and determined total angular momentum quantum number is given by

=

ℎ

(1)

where is the number of quantum states of the TS bearing an energy equal to or less than

and at the given ; is the density of states for the decomposing molecule at the specified

J and E. The quantities and ρ(E) are usually estimated by semi-empirical or direct-count


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methods. In conventional use of the RRKM theory, vibrational anharmonicities and tunneling effects are not satisfactorily treated. In SCTST approach, developed by Miller and colleagues35-38, both latter drawbacks are properly accounted for. In this theory, is

replaced by G(E), the cumulative reaction probability (CRP). The cumulative reaction probability, which is the sum of probabilistic quantum pathways leading to the product, is defined as ( ) = … ( ) (2) ‡

The semi-classical tunneling probability, Pn, in the above equation is expressed as () =

1 (3) 1 + exp [2 (!, )]

where θ(n,T) is the barrier penetration integral and is given by (!, ) =

% Δ 2 (4) Ω( 1 + )1 + 4+(( Δ ⁄Ω,(

Considering a second-order expansion of potential energy surface about transition state, ∆E and .( , needed for computing θ(n,T), are given by the following expressions: (78

(78 (78

498

498 :94

1 1 1 Δ = Δ/0 + 10 − + 34 5!4 + 6 + +4: 5!4 + 6 5!: + 6 (5) 2 2 2 (78

1 Ω( = 3

Theoretical Studies on the Kinetics of Multi-Channel Gas-Phase

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