A study of the atmospherically important reactions of dimethylsulfide (DMS) with I2 and ICl using infrared matrix isolation spectroscopy and electronic structure calculations

Abstract

The reactions of dimethylsulfide (DMS) with molecular iodine (I(2)) and iodine monochloride (ICl) have been studied by infrared matrix isolation spectroscopy by co-condensation of the reagents in an inert gas matrix. Molecular adducts of DMS + I(2) and DMS + ICl have also been prepared using standard synthetic methods. The vapour above each of these adducts trapped in an inert gas matrix gave the same infrared spectrum as that recorded for the corresponding co-condensation reaction. In each case, the infrared spectrum has been interpreted in terms of a van der Waals adduct, DMS : I(2) and DMS : ICl, with the aid of infrared spectra computed for their minimum energy structures at the MP2 level. Computed relative energies of minima and transition states on the potential energy surfaces of these reactions were used to understand why they do not proceed further than the reactant complexes DMS : I(2) and DMS : ICl. The main findings of this research are compared with results obtained earlier for the DMS + Cl(2) and DMS + Br(2) reactions, and the atmospheric implications of the conclusions are also considered.

A Study of the Atmospherically Important Reactions between Dimethyl Selenide (DMSe) and Molecular Halogens (X2 = Cl2, Br2, and I2) with ab initio Calculations

“…This is the region where CH 3 rocking and deformation modes in DMSe:X 2 and DMSeX 2 occur (see Tables –). It is expected that when matrix isolation infrared spectra are recorded for the intermediates obtained from the DMSe + X 2 reactions that the relative band intensities obtained in this spectral region, combined with the results of the electronic structure calculations shown in Figures –, will be able to determine whether the intermediate observed is the van der Waals adduct or the covalent complex, as was achieved previously for the DMS + X 2 reactions. ,− As stated earlier, based on the schematic potential energy diagrams shown in Figures –, it is expected that the covalent complex will be observed from the DMSe + Cl 2 and DMSe + Br 2 reactions and the van der Waals complex will be observed from the DMSe + I 2 reaction.…”

“…For complexes containing I 2 , two sets of calculations were performed: one with the effective core potential (ECP) I-atom basis set, LANL2DZ, 39−41 and the other with the more flexible atomic iodine ECP basis set, aug-cc-VDZ-PP, that we used for DMS + I 2 and DMS + ICl calculations. 19 Harmonic vibrational analysis was performed to verify each minimum on the potential energy surface. Every stationary point identified was characterized by the number of negative eigenvalues of its Hessian matrix: 0 for a minimum and 1 for a transition state.…”

“…Full geometry optimization of reactants, intermediates, and transition states was performed with the Gaussian 09 suite of programs . The computational approach used to study the reactions of DMSe with molecular halogens, X 2 , was based on that used for the analogous reactions of DMS with halogens. ,− The computations were carried out at the Möller–Plesset (MP2) level using the aug-cc-pVDZ basis set , for all atoms except for iodine. For complexes containing I 2 , two sets of calculations were performed: one with the effective core potential (ECP) I-atom basis set, LANL2DZ, − and the other with the more flexible atomic iodine ECP basis set, aug-cc-VDZ-PP, that we used for DMS + I 2 and DMS + ICl calculations .…”

“…DMS results from the decay of ocean phytoplankton in the marine boundary layer and is known to be the major natural source of sulfur in the atmosphere. , It plays an important role in climate regulation, as its oxidation leads to aerosol production and cloud formation . The main oxidants of DMS in the atmosphere are the hydroxyl radical (OH), the nitrate radical (NO 3 ), atomic and molecular halogens, and other halogen-containing reactive species. − Recently, using a flow-tube interfaced to a photoelectron spectrometer, we measured the rate coefficient at room temperature for the DMS + Cl 2 reaction and studied the DMS + Cl 2 , DMS + Br 2 , DMS + I 2 , and DMS + ICl reactions using photoelectron spectroscopy, infrared matrix isolation spectroscopy, and electronic structure calculations. ,− …”

“…In our recent work on the reactions of DMS with Cl 2 , Br 2 , and I 2 , ,− for each reaction, key minima on the potential energy surface were shown to be the reagents, a van der Waals complex DMS:X 2 , a covalent intermediate DMSX 2 , and the products CH 3 SCH 2 X + HX. The relative energies of these minima and transition states which interconnect them were determined by electronic structure calculations.…”