The effects of wine composition and postbottling oxygen exposure on 3-mercaptohexanol (3-MH), hydrogen sulfide (H2S), and methyl mercaptan (MeSH) were investigated. A Sauvignon blanc wine with initial copper concentration of 0.1 mg/L was treated with copper sulfate and/or glutathione (GSH) prior to bottling to give final concentrations of 0.3 and 20 mg/L, respectively. The wines were bottled with a synthetic closure previously stored in either ambient air or nitrogen to study the effect of the oxygen normally present in the closure. Bottled wines were stored for 6 months in either air or nitrogen to study the effect of oxygen ingress through the closure. Copper addition resulted in a rapid initial decrease in 3-MH. During storage, a further decrease of 3-MH was observed, which was lower with GSH addition and lowered oxygen exposure. H2S accumulated largely during the second 3 months of bottle storage, with the highest concentrations attained in the wines treated with GSH and copper. Lower oxygen from and through the closure promoted H2S accumulation. The concentration of MeSH was virtually not affected by the experimental variables at 6 months, although differences were observed after 3 months of storage. The implications for wine quality are discussed.

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Chemical reactions are usually performed in solvents. The course of the reaction varies with solvent, temperature, etc. Solvent molecules are usually not taken into account in quantum simulations due to the rising computational effort. However, their ability to transfer heat (kinetic energy) to and from the reacting species can be incorporated in the simulations. We demonstrate that different chemistry follows by varying absolute simulation temperature and the range for temperature fluctuations in Car−Parrinello dynamics simulations applied to ruthenium catalyzed metathesis reactions. The bisphosphine Cl2(PH3)2RuCH2 complex and its monophosphine equivalent were taken as model compounds for the Grubbs catalyst. Depending on temperature conditions set in the molecular dynamics simulations, the interaction of these ruthenium−phosphine catalysts with ethylene revealed insertion, the formation of a metallacyclobutane, or a pure metathesis reaction. It was found that selective bond vibrational activation is required to induce metathesis activity. A comparison with recent experimental results from Grubbs et al. (J. Am. Chem. Soc. 1997, 119, 3887) shows that the results of our simulations reveal and lend support to the mechanism proposed by Grubbs. These include, in the order of appearance in the reaction scheme, the relative easiness of Cl−Ru−Cl cis−trans configurational exchange, carbene rotation and the loss of one phosphine, formation of a metallacyclobutane intermediate, and finally completion of the metathesis reaction. The presence of a small fraction of the monophosphine complex (the reaction proposed in Scheme , which was adopted from Grubbs et al.) is confirmed by the present simulations. Starting from the observation that selective bond excitation is necessary to exhibit reactivity, the higher activity of the monophosphine system compared to bisphosphine complexes reported by Grubbs et al. can be understood. We stress that we have also found that the bisphosphine is active, which also agrees with the experimental data. Finally, the fact that ruthenium−carbene complexes based on sterically crowded phosphines give higher metathesis activity, can be rationalized by their intrinsically more labile Ru−P bonds as indicated by longer Ru−P bond lengths.

The evolution of different volatile sulfur compounds (VSCs) during bottle maturation of two Shiraz wines submitted to controlled oxygen exposure prior to bottling (through micro-oxygenation, MOX) and postbottling (through the closure) was investigated. H(2)S, methyl mercaptan (MeSH), and dimethyl sulfide (DMS) were found to increase during aging. Lower postbottling oxygen exposure, as obtained by different degrees of oxygen ingress through the closure, resulted in increased H(2)S and methyl mercaptan. In one wine MOX increased the concentration of H(2)S and methyl mercaptan during maturation. Dimethyl disulfide and DMS were not affected by any form of oxygen exposure. Overall, postbottling oxygen had a stronger influence than MOX on the evolution of VSCs. Data suggest that dimethyl disulfide was not a precursor to methyl mercaptan during bottle maturation. For the two wines studied, a consumption of oxygen of 5 mg/L over 12 months was the most effective oxygen exposure regimen to decrease accumulation of MeSH and H(2)S during bottle aging.

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