Time-resolved gas-phase kinetic and quantum chemical studies of reactions of silylene with chlorine-containing species. 2. CH3Cl

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of both silacyclopent-3-ene and phenylsilane, have been carried out to obtain second-order rate constants for its reaction with CH3Cl. The reaction was studied in the gas phase at six temperatures in the range 294-606 K. The second-order rate constants gave a curved Arrhenius plot with a minimum value at T approximate to 370 K. The reaction showed no pressure dependence in the presence of up to 100 Torr SF6. The rate constants, however, showed a weak dependence on laser pulse energy. This suggests an interpretation requiring more than one contributing reaction pathway to SiH2 removal. Apart from a direct reaction of SiH2 with CH3Cl, reaction of SiH2 with CH3 (formed by photodissociation of CH3Cl) seems probable, with contributions of up to 30% to the rates. Ab initio calculations (G3 level) show that the initial step of reaction of SiH2 with CH3Cl is formation of a zwitterionic complex (ylid), but a high-energy barrier rules out the subsequent insertion step. On the other hand, the Cl-abstraction reaction leading to CH3 + ClSiH2 has a low barrier, and therefore, this seems the most likely candidate for the main reaction pathway of SiH2 with CH3Cl. RRKM calculations on the abstraction pathway show that this process alone cannot account for the observed temperature dependence of the rate constants. The data are discussed in light of studies of other silylene reactions with haloalkanes.

Linking carbon and sulphur cycling during simulated drought cycles in peat from six sites across the UK

Water table draw-down is thought to increase peat decomposition and, therefore, DOC release. However, several studies have shown lower DOC concentrations during droughts relative to ‘normal’ periods with high water table. We carried out controlled incubation experiments at 10°C on 10x10 cm peat soil cores collected from six UK sites across a sulphur deposition gradient. Our aim was to quantify the balance between microbial consumption and chemical precipitation of DOC due to episodic acidification driven by sulphur redox reactions by comparing changes in soil water chemistry to microbial activity (i.e. soil respiration and trace gas fluxes). During dry periods, all sites showed a concurrent increase in SO4 and soil respiration and a decline in DOC. However, the magnitude of change in both DOC and SO4 varied considerably between sites according to historical sulphur deposition loads and the variation in acid/base chemistry.

Processes controlling DOC in pore water during simulated drought cycles in six different UK peats

The effect of episodic drought on dissolved organic carbon (DOC) dynamics in peatlands has been the subject of considerable debate, as decomposition and DOC production is thought to increase under aerobic conditions, yet decreased DOC concentrations have been observed during drought periods. Decreased DOC solubility due to drought-induced acidification driven by sulphur (S) redox reactions has been proposed as a causal mechanism; however evidence is based on a limited number of studies carried out at a few sites. To test this hypothesis on a range of different peats, we carried out controlled drought simulation experiments on peat cores collected from six sites across Great Britain. Our data show a concurrent increase in sulphate (SO4) and a decrease in DOC across all sites during simulated water table draw-down, although the magnitude of the relationship between SO4 and DOC differed between sites. Instead, we found a consistent relationship across all sites between DOC decrease and acidification measured by the pore water acid neutralising capacity (ANC). ANC provided a more consistent measure of drought-induced acidification than SO4 alone because it accounts for differences in base cation and acid anions concentrations between sites. Rewetting resulted in rapid DOC increases without a concurrent increase in soil respiration, suggesting DOC changes were primarily controlled by soil acidity not soil biota. These results highlight the need for an integrated analysis of hydrologically driven chemical and biological processes in peatlands to improve our understanding and ability to predict the interaction between atmospheric pollution and changing climatic conditions from plot to regional and global scales.

Investigation of Petasis and Ugi reactions in series in an automated microreactor system

Petasis and Ugi reactions are used successively without intermediate purification, effectively accomplishing a six-component reaction. The examined reactions are transferred from traditional batch reactors to an automated continuous flow microreactor setup, where optimization and kinetic analyses are performed, proposed mechanisms evaluated, and rate-limiting steps determined.