A Theoretical Study of the Reaction between CH3S(OH)CH3 and O2

New mechanism for the participation of aromatic oxidation products in atmospheric nucleation

Oxygenated organic molecules (OOMs) are recognized as important precursors for new particle formation (NPF) in the urban atmosphere. The paper theoretically studied the formation of OOMs by styrene oxidation processes initiated by OH radicals, focusing on the OOMs nucleation mechanism. The results found that in the presence of an H2SO4 molecule, lowly oxygenated organic molecules containing a benzene ring (LOMBs) can form stable clusters and grow to the scale of a critical nucleus through pi-pi stacking and OH hydrogen bonding. In addition, LOMBs are more readily generated in a styrene-oxidized system in the presence/absence of NOx than highly oxygenated organic molecules (HOMs). The reaction of OH radicals with other aromatics containing a branched chain on the benzene ring produces LOMBs to varying degrees, with pi-pi stacking playing an essential role. This result suggests that, in the presence of H2SO4 molecules, LOMBs may play a more significant role in promoting nucleation than HOMs. Our findings serve as a pivotal foundation for future investigations into the oxidation and nucleation processes of diverse aromatics in urban environments.

Understanding the oxidation mechanism of methanesulfinic acid by ozone in the atmosphere

Methanesulfinic acid (MSIA) is an important intermediate in the oxidation of dimethyl sulfide (DMS) in the marine boundary layer. The oxidation of MSIA by ozone in the gas phase to form methanesulfonic acid (MSA) was investigated using theoretical calculations in this paper. Three pathways can be found for the reaction of MSIA with ozone. The highest energy barrier is 13.02 kcal mol⁻¹ in the most favorable pathway. By comparing the reaction rate of MSIA + O₃ with that of MSIA + OH, it can be concluded that the oxidation of MSIA by O₃ to form MSA is of minor significance relative to its oxidation by OH radical in the gas phase. This study can provide some information for the theoretical and experimental studies in the significantly heterogeneous and aqueous-phase oxidation of MSIA by O₃.

First principles study of photo-oxidation degradation mechanisms in P3HT for organic solar cells

We present a theoretical study of degradation mechanisms for photoinduced oxidation in organic polymers in the condensed phase, using poly(3-hexylthiophene) (P3HT) as an example.

On the Mechanism of Gas-Phase Reaction of C1−C3 Aliphatic Thiols + OH Radicals

A theoretical study on the mechanism of the OH + aliphatic thiols reactions is presented. Optimum geometries and frequencies have been computed at the BHandHLYP/6-311++G(2d,2p) level of theory for all stationary points. Energy values have been improved by single-point calculations at the above geometries using CCSD(T)/6-311++G(d,p). Twelve possible channels have been modeled, three of them including the possible influence of molecular oxygen, and three of them involving excess of OH. The only channels that have been found to significantly contribute to the overall reaction in the troposphere are the hydrogen abstractions from the -SH group and from the alkyl groups. Our analysis supports a stepwise mechanism involving the formation of a short-lived, weakly bonded adduct in the entrance channel, for the abstraction paths. The results proposed in the present work seem to provide a viable explanation for diverse findings previously reported from experimental investigations.

Experimental and Theoretical Studies of the Reaction of the OH Radical with Alkyl Sulfides: 1. Direct Observations of the Formation of the OH−DMS Adduct−Pressure Dependence of the Forward Rate of Addition and Development of a Predictive Expression at Low Temperature

A pulsed laser photolysis-pulsed laser-induced fluorescence (PLP-PLIF) system was employed to study the kinetics and mechanisms of reactions (1) OH + h6-DMS --> products and (2) OH + d6-DMS --> products. We report direct observations of the rate coefficients for the formation and dissociation of the h6-OHDMS and d6-OHDMS adducts over the pressure range 50-650 Torr and between 240 and 245 K, together with measurements of the oxygen dependence of the effective rate coefficients for reactions 1 and 2 under similar conditions. The effective rate coefficients increased as a function of O2 concentration reaching their limiting values in each case. The values of the adduct formation rate, obtained from the O2 dependencies, were in excellent agreement with values determined from direct observation of adduct equilibration in N2. OH regeneration is insignificant. The rate coefficients for the formation of the adduct isotopomers showed slight differences in their falloff behavior and do not approach the high-pressure limit in either case. The equilibrium constants obtained show no dependence on isotopomer and are in good agreement with previous work. A "second-law" analysis of the temperature dependence of the equilibrium constant gives an adduct bond strength (DeltaH degrees =-10.9 +/- 1.0 kcal mol(-1)), also in good agreement with previously reported values. Using the entropy calculated from the ab initio vibrational frequencies, we obtain a "third-law" value for the reaction enthalpy at 240 K, DeltaH(240K) degrees = -10.5 kcal mol(-1) in good agreement with the other approach. The rate coefficient for the reactions of the adducts with O2 was obtained from an analysis of the O2 dependence and was determined to be 6.3 +/- 1.2 x 10(-13) cm3 molecule(-1) s(-1), with no dependence on pressure or isotopomer. The pressure and temperature dependence for all of the elementary processes in the initial steps of the dimethylsulfide (DMS) oxidation mechanism have been characterized in the range 238-245 K, allowing the formulation of an expression which can be used to calculate the effective rate coefficient for reaction 1 at any pressure and oxygen concentration. The expression can calculate the effective rate coefficient for reaction 1 to +/- 40% over the range 220-260 K, with the largest errors at the extremes of this range. Gaussian 03 has been used to calculate the structure of the OH-DMS adduct and its deuterated isotopomer. We find similar bound structures for both isotopomers. The calculated enthalpies of formation of the adducts are lower than the experimentally determined values.

Electronic structure study of the initiation routes of the dimethyl sulfide oxidation by OH

In the present work the potential energy surface (PES) corresponding to the different initiation routes of the oxidation mechanism of DMS by hydroxyl radical in the absence of O(2) has been studied, and connections among the different stationary points have been established. Single-point high level electronic structure calculations at lower level optimized geometries have been shown to be necessary to assure convergence of energy barriers and reaction energies. Our results demonstrate that the oxidation of DMS by OH turns out to be initiated via three channels: a hydrogen abstraction channel that through a saddle point structure finally leads to CH(3)SCH(2) + H(2)O, an addition-elimination channel that firstly leads to an adduct complex (AD) and then via an elimination saddle point structure finally gives CH(3)SOH and CH(3) products, and a third channel that through a concerted pathway leads to CH(3)OH and CH(3)S. The H-abstraction and the addition-elimination channels initiate by a common pathway that goes through the same reactant complex (RC). Our theoretical results agree quite well with the branching ratios experimentally assigned to the formation of the different products. Finally, the calculated equilibrium constants of the formation of the complex AD and the hexadeuterated complex AD from the corresponding reactants, as a function of the temperature, are in good accordance with the experimental values.