Thermochemistry and Kinetic Analysis of the Unimolecular Oxiranyl Radical Dissociation Reaction: A Theoretical Study

2018

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Sulfide alkoxy radicals are important intermediates during the partial oxidation of alkyl sulfides in atmospheric chemistry and in combustion. The atmospheric reaction sequence to formation of the alkoxy radicals includes (1) initial reaction with OH to create a radical on a carbon site, (2) the carbon radical then associates with 3 O 2 to form a peroxy radical, and (3) an NO radical reacts with the peroxy radical to form an alkoxy radical (RO•) plus NO 2 . This study determines structural parameters, internal rotor potentials, bond dissociation energies, and thermochemical properties (Δ f H°, S°, and C p (T)) of 3 corresponding alcohols HOCH 2 SCH 2 CH 3 , CH 3 SCH(OH)CH 3 , and CH 3 SCH 2 CH 2 OH of methyl ethyl sulfides studied in order to characterize the thermochemistry of the respective alkoxy radicals. The lowest energy molecular structures were calculated using the B3LYP density functional level of theory with the 6-311G(2d,d,p) basis set. Standard enthalpies of formation (Δ f H°2 98 ) for the radicals and their parent molecules were calculated using B3LYP/6-31 + G(2d,p), CBS-QB3, M062x/6-311 + g(2d,p), and G3MP2B3 methods. Isodesmic reactions were used to determine Δ f H°values. Internal rotation potential energy diagrams and rotation barriers were investigated using the B3LYP/6-31 + G(d,p) level theory. The contributions for S°2 98 and C p (T) were calculated using the rigid rotor harmonic oscillator approximation based on the structures and vibrational frequencies obtained by CBS-QB3 calculations, with contributions from torsion frequencies replaced by internal rotor contributions. Group additivity and hydrogen bond increment values were developed for estimating properties of structurally similar and larger sulfur-containing peroxide molecules and their radicals.

Section: Resultsmentioning

confidence: 99%

Structural and thermochemical properties of methyl ethyl sulfide alcohols: HOCH₂SCH₂CH₃, CH₃SCH(OH)CH₃, CH₃SCH₂CH₂OH, and radicals corresponding to loss of H atom

2018

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“…TVR represents the sum of the contributions from translations, vibrations, and external rotations . Internal rotor indicates the contribution from hindered internal rotation, which replaces the torsion frequency contributions in the TVR heat capacity and entropy data . Corrections for the standard entropy based on the symmetry numbers of the targeted radical or molecule were included into the calculation of the standard entropy values .…”

Section: Resultsmentioning

confidence: 99%

Structural and thermochemical studies on CH₃SCH₂CHO, CH₃CH₂SCHO, CH₃SC(═O)CH_3, and radicals corresponding to loss of H atom

2017

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CH 3 SCH 2 CHO, CH 3 CH 2 SCHO, and CH 3 SC(═O)CH 3 are intermediates during the partial oxidation of CH 3 SCH 2 CH 3 in the atmosphere and in combustion processes. Thermochemical properties (ΔH f Investigations of the MES partial oxidation reaction mechanism and kinetics in the atmosphere are helpful in understanding the combustion of sulfur mustard gases. Mansilla et al [7] and Zheng et al [6] experimentally investigated the degradation of MES in the atmosphere. Cao et al [1] have recently done some fundamental computational studies on reactions of MES with OH radical in the atmosphere. To better understand the oxidation process and analyze the kinetics, structural and thermochemical properties of each proposed intermediate and product are required.Vandeputte et al [10] and Zheng et al [8] have developed the thermochemistry and reaction kinetics for the oxidations of dimethyl sulfide [10] and diethyl sulfide, [8] respectively. On the basis of their work, the partial oxidation of MES in the atmosphere would generate intermediates of CH 3 SCH 2 CHO, CH 3 CH 2 SCHO, and CH 3 SC(═O)CH 3 . [1,7] These aldehydes and ketones are also likely intermediates during the combustion of CH 3 SCH 2 CH 3 . [8,9,11] There is limited available information about the thermochemical properties for these molecules.Besides the importance of intermediate products during MES combustion, CH 3 SCH 2 CHO, CH 3 CH 2 SCHO, and CH 3 SC(═O)CH 3 play important roles in troposphere chemistry. According to Mellouki et al, [12] they are precursors of free radicals and important intermediates of nitrogen oxide transport in the atmosphere. According to Jang and Kamens, these aldehydes and ketones also contribute to the formation of aerosols in the atmosphere through heterogeneous catalytic reactions. [13] Oxidation of compounds containing sulfur, carbon, and hydrogen in the atmosphere results in SO x formation, which is considered an important component of aerosol formations. [14] In this paper, the symbol ("•") represents the radical site located on the preceding carbon or oxygen atom. The molecules and radicals investigated in this study include the following: CH 3 SCH 2 CHO (methylthioacetaldehyde, CAS number: 23328-62-3) with the radicals of CH 2 •SCH 2 CHO, CH 3 SCH•CHO, and CH 3 SCH 2 C•═O; CH 3 CH 2 SCHO (ethyl thioformate, CAS number: 21071-39-6) with the radicals of CH 2 •CH 2 SCHO, CH3CH•SCHO, and CH 3 CH 2 SC•═O; and CH 3 SC(═O)CH 3 (methyl thiolacetate, CAS number: 1534-08-3) with the radicals of CH 2 •SC(═O)CH 3 and CH 3 SC(═O)CH 2 .Geometric structures, standard enthalpies of formation, H─R bond energies, frequencies, moments of inertia, internal rotor potentials, and the entropy and heat capacity values at 298 K and 1 atm are determined in this study. FIGURE 1 Optimized structures of CH 3 SCH 2 CHO, CH 2 •SCH 2 CHO, CH 3 SCH•CHO, and CH 3 SCH 2 C•O (Bond distances in Å; Bond Angles in degrees [°]). A, CH 3 SCH 2 CHO dihedrals: C─S─C─C = −70.3°and S─C─C ═ O = 102.3°. B, CH 2 •SCH 2 CHO dihedrals: C─S─C─C = −66.9°a nd S─C─C═O = 109.0°...

“…TVR represents the sum of the contributions from translations, vibrations, and external rotations . Internal rotor indicates the contribution from hindered internal rotation, which replaces the torsion frequency contribution in the TVR heat capacity and entropy data . Corrections for the standard entropy based on the symmetry number of the targeted radical or molecule were included into the calculation of the standard entropy values .…”

Section: Resultsmentioning

confidence: 99%

Structures and thermochemistry of methyl ethyl sulfide and its hydroperoxides: HOOCH₂SCH₂CH₃, CH₃SCH(OOH)CH₃, CH₃SCH₂CH₂OOH, and radicals

2017

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Hydroperoxides and the corresponding peroxy radicals are important intermediates during the partial oxidation of methyl ethyl sulfide (CH3SCH2CH3) in both atmospheric chemistry and in combustion. Structural parameters, internal rotor potentials, bond dissociation energies, and thermochemical properties (ΔHfo, So and Cp(T)) of 3 corresponding hydroperoxides CH2(OOH)SCH2CH3, CH3SCH(OOH)CH3, CH3SCH2CH2OOH of methyl ethyl sulfides, and the radicals formed via loss of a hydrogen atom are important to understanding the oxidation reactions of MES. The lowest energy molecular structures were identified using the density functional B3LYP/6‐311G(2d,d,p) level of theory. Standard enthalpies of formation (ΔHfo298) for the radicals and their parent molecules were calculated using the density functional B3LYP/6‐31G(d,p), B3LYP/6‐31 + G(2d,p), and the composite CBS‐QB3 ab initio methods. Isodesmic reactions were used to determine ∆Hfo values. Internal rotation potential energy diagrams and rotation barriers were investigated using the B3LYP/6‐31G(d,p) level theory. Contributions for So298 and Cp(T) were calculated using the rigid rotor harmonic oscillator approximation based on the structures and vibrational frequencies obtained by the density functional calculations, with contributions from torsion frequencies replaced by internal rotor contributions. The recommended values for enthalpies of formation of the most stable conformers of CH3SCH2CH2, CH2(OOH)SCH2CH3, CH3SCH(OOH)CH3, and CH3SCH2CH2OOH are −14.0, −33.0, −37.2, and −32.7 kcal/mol, respectively. Group additivity values were developed for estimating properties of structurally similar and larger sulfur‐containing peroxides. Groups for use in group additivity estimation of sulfur peroxide thermochemical properties were developed.