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

Abstract

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.

Kinetic Study on the Reaction of OH Radical with Dimethyl Sulfide in the Absence of Oxygen

“…The S N 2-type pathway (R1d) takes place via a unique stationary point that corresponds to the nucleophilic-substitution structure (SP met ). Due to its high adiabatic potential energy barrier, [27] this channel turns out to be totally irrelevant in DMS oxidation by OH and will be neglected in the discussion on kinetics.…”

“…The details of the electronic-structure calculations for this reaction were widely analyzed in our first work, [27] so here we briefly describe the main features. The dynamical calculations are described in detail.…”

“…Therefore, some small differencies could be observed if one compares values from this work with those published before. [27] A short description of the potential-energy surface of reaction (R1) at the MPW1K/MG3S electronic-structure level (see below) follows. The formation of the adduct (AD) proceeds via the complex RC (the common point with the indirect H-abstraction route), which is connected to the AD via a saddle-point (SP rot ) that represents rotation of the OH group from an SÀH to an SÀO interaction.…”

“…All the geometries and energies already published [27] were recalculated with the new version of Gaussian-the Gaussian 03 package of programs [28] -in order to avoid discrepancies between the two versions. Therefore, some small differencies could be observed if one compares values from this work with those published before.…”

Chemical evolution: The mechanism of the formation of adenine under prebiotic conditions

“…The key rate-determining step was recomputed at the MP2/6-311ϩG** and the gas-phase reaction barrier with zeropoint vibrational energy (ZPVE) correction is 43.9 kcal/mol, which is higher than that at B3LYP/6-311ϩG**. It has been reported that although MP2 in general gives accurate geometries for reaction complexes, it also overestimates barrier heights (28,29). Thus for calibration, we performed higher-level CCSD(T)/ cc-pVTZ single-point computations on the simpler HCN ϩ NH 3 ϩ H 2 O system on MP2 geometries optimized at 6-311ϩG** level.…”