Kinetics and mechanism of the reaction between atomic chlorine and dimethyl selenide; comparison with the reaction between atomic chlorine and dimethyl sulfide

Abstract

Dimethyl selenide is the most abundant gaseous selenium species in marine environments. In this work, the value of the rate coefficient for the gas-phase reaction between dimethyl selenide and Cl atoms has been determined for the first time. The value of the second-order rate coefficient obtained was (5.0 ± 1.4) × 10 −10 cm 3 molecule −1 s −1 . The very fast nature of the reaction means that, when estimating the lifetime of dimethyl selenide in the atmosphere, loss due to reaction with Cl atoms should be considered along with loss due to reaction with O 3 and with OH and NO 3 radicals.Analysis of the available kinetic data suggests that at 760 Torr the dominant reaction pathway for the reaction of Cl atoms with dimethyl selenide will be the addition of Cl to the Se atom forming an adduct of the type CH 3 Se(Cl)CH 3 . Theoretical calculations, at the B3LYP/6-311++G(2df,p)//B3LYP/6-311++G(d,p) level of theory, show that at 298 K the value of Δ r H o for the formation of the adduct is −111.4 kJ mol −1 . This value may be compared to −97.0 kJ mol −1 , the value calculated for Δ r H o for the formation of the analogous sulphur adduct, CH 3 S(Cl)CH 3 , following the reaction between Cl atoms and dimethyl sulphide. Variational RRKM theory was used to predict the thermal decomposition rates of the two adducts back to starting materials. The estimated rate constant for the decomposition of the selenium adduct to the reactants is 5 × 10 −5 s −1 , compared to 0.02 s −1 in the case of the sulphur adduct.However, our calculations suggest that the CH 3 Se(Cl)CH 3 adduct, which initially is formed highly excited, will not be stabilised under atmospheric conditions, but rather that it will decompose to yield CH 3 SeCl and CH 3 , a process that is calculated to be exothermic with respect to the initial reactants by 5.8 kJ mol −1 . The formation of CH 3 SCl and CH 3 from the sulphur adduct, on the other hand, is endothermic by 20.8 kJ mol −1 with respect to the initial reactants, and is thus not expected to occur.

Dimethyl Sulfide and Dimethyl Sulfoxide and Their Oxidation in the Atmosphere

“…The most recent value of 17.7 kcal mol -1 , reported by Enami et al, at the QCISD(T)/MP/6-311++G(2df.2p) level lies between the previous determinations. Thompson et al reported values of Δ r H ° for formation of the (CH 3 ) 2 S−Cl adduct which are in good agreement with the value of Wilson and Hirst 167 but not with the calculated values of by McKee 106 or Resende and De Almeida …”

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

“…It is also interesting to compare the reaction of DMSe with atomic and molecular chlorine, as both reactions involve the formation of a covalent intermediate: DMSeCl and DMSeCl 2 . The DMSeCl intermediate forms on addition of Cl to the Se atom . The Se–Cl bond is longer in DMSeCl (2.73 Å at the B3LYP/6-311++G(d,p)level) than in DMSeCl 2 (2.562 Å, this work), while the C–Se–C bond angle is smaller in DMSeCl (98.1° at the B3LYP/6-311++G(d,p) level) than in DMSeCl 2 (98.5°, this work).…”

“…The DMSeCl intermediate forms on addition of Cl to the Se atom . The Se–Cl bond is longer in DMSeCl (2.73 Å at the B3LYP/6-311++G(d,p)level) than in DMSeCl 2 (2.562 Å, this work), while the C–Se–C bond angle is smaller in DMSeCl (98.1° at the B3LYP/6-311++G(d,p) level) than in DMSeCl 2 (98.5°, this work). The same trends are observed on comparing the structures of the covalent intermediates of DMS with atomic and molecular chlorine computed at the UMP2/DZP and MP2/aug-cc-PVDZ levels, respectively. , …”

Environmental effects of large igneous province magmatism: a Siberian perspective

“…Langer et al [768] coupled cw photolysis with gas chromatographic detection of products to show that the yield of CH 3 Cl is (1.34 ± 0.07) x 10 . Theoretical studies by Wilson and Hirst [1459], Resende and De Almeida [1114] and Thompson et al [1299] support the experimentally-observed dominance of the H-abstraction pathway at low pressure. Diaz-de-Mera et al [390] have employed a discharge flow − mass spectrometry technique to measure rate constants at pressures of 0.5 − 1.0 Torr He over the temperature range 259 − 364 K. The 298 K rate constant reported by Diaz-de-Mera et al is nearly a factor of 3 slower than the low-temperature limit value reported by Stickel et al Furthermore, Diazde-Mera et al report a small positive activation energy whereas Stickel et al report a small negative activation energy.…”