Factors Controlling Regioselectivity in the Reduction of Polynitroaromatics in Aqueous Solution

Regioselectivities in the bisulfide reduction of 10 polynitroaromatics (PNAs) to monoamine products have been determined; four of these compounds have also been reduced by anoxic sediments in heterogeneous aqueous solution, and the same regioselectivities are observed. Analyses of Austin Model 1−Solvation Model 2 electrostatic potential surfaces for the radical anions of these polynitroaromatic compounds provides a reliable method of predicting the regioselectivity of their reduction. In particular, at their minimum-energy geometries in aqueous solution, it is the more negative nitro group that is selectively reduced. This is consistent with a mechanism where regioselection occurs upon kinetic protonation at the site of maximum negative charge in the radical anion formed after the first electron transfer to the neutral PNA. Inclusion of solvation effects is critical in order to confidently predict the electrostatic prefer ence for the reduction of one nitro group over the others. Sterically uncongested nitroaromatic radical anions have gas-phase geometries in which the nitro group is coplanar with the aromatic ring. However, ortho substituents and solvation effects both oppose this tendency and can lead to nitro groups that are rotated out of the ring plane and pyramidalized

High Methane Emissions from a Midlatitude Reservoir Draining an Agricultural Watershed

Reservoirs are a globally significant source of methane (CH₄), although most measurements have been made in tropical and boreal systems draining undeveloped watersheds. To assess the magnitude of CH₄ emissions from reservoirs in midlatitude agricultural regions, we measured CH₄ and carbon dioxide (CO₂) emission rates from William H. Harsha Lake (Ohio, U.S.A.), an agricultural impacted reservoir, over a 13 month period. The reservoir was a strong source of CH₄ throughout the year, emitting on average 176 ± 36 mg C m^–2 d^–1, the highest reservoir CH₄ emissions profile documented in the United States to date. Contrary to our initial hypothesis, the largest CH₄ emissions were during summer stratified conditions, not during fall turnover. The river–reservoir transition zone emitted CH₄ at rates an order of magnitude higher than the rest of the reservoir, and total carbon emissions (i.e., CH₄ + CO₂) were also greater at the transition zone, indicating that the river delta supported greater carbon mineralization rates than elsewhere. Midlatitude agricultural impacted reservoirs may be a larger source of CH₄ to the atmosphere than currently recognized, particularly if river deltas are consistent CH₄ hot spots. We estimate that CH₄ emissions from agricultural reservoirs could be a significant component of anthropogenic CH₄ emissions in the U.S.A