Volatile Methyl Siloxane Atmospheric Oxidation Mechanism from a Theoretical Perspective─How is the Siloxanol Formed?

Despite several investigations on the atmospheric fate of cyclic volatile methyl siloxanes (VMS), the oxidation chemistry of these purely anthropogenic, high production volume compounds is poorly understood. This led to uncertainties in the environmental impact and fate of the oxidation products. According to laboratory measurements, the main VMS oxidation product is the siloxanol (a -CH3 replaced with an -OH); however, none of the mechanisms proposed to date satisfactorily explain its formation. Motivated by our previous experimental observations of VMS oxidation products, we use theoretical quantum chemical calculations to (1) explore a previously unconsidered reaction pathway to form the siloxanol from a reaction of a siloxy radical with gas-phase water, (2) investigate differences in reaction rates of radical intermediates in hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) oxidation, and (3) attempt to explain the experimentally observed products. Our results suggest that while the proposed reaction of the siloxy radical with water to form the siloxanol can occur, it is too slow to compete with other unimolecular reactions and thus cannot explain the observed siloxanol formation. We also find that the reaction between the initial D3 peroxy radical (RO2•) with HO2• is slower than previously anticipated (calculated as 3 × 10-13 cm3 molecule-1 s-1 for D3 and 2 × 10-11 cm3 molecule-1 s-1 for D4 compared to the general rate of ∼1 × 10-11 cm3 molecule-1 s-1). Finally, we compare the anticipated fates of the RO2• under a variety of conditions and find that a reaction with NO (assuming a general RO2• + NO bimolecular rate constant of 9 × 10-12 cm3 molecule-1 s-1) will likely be the dominant fate in urban conditions, while isomerization can be important in cleaner environments.

Mn-intercalated MoSe2 under pressure: Electronic structure and vibrational characterization of a dilute magnetic semiconductor.

Intercalation offers a promising way to alter the physical properties of two-dimensional (2D) layered materials. Here, we investigate the electronic and vibrational properties of 2D layered MoSe2 intercalated with atomic manganese at ambient and high pressure up to 7 GPa by Raman scattering and electronic structure calculations. The behavior of optical phonons is studied experimentally with a diamond anvil cell and computationally through density functional theory calculations. Experiment and theory show excellent agreement in optical phonon behavior. The previously Raman inactive A2u mode is activated and enhanced with intercalation and pressure, and a new Raman mode appears upon decompression, indicating a possible onset of a localized structural transition, involving the bonding or trapping of the intercalant in 2D layered materials. Density functional theory calculations reveal a shift of the Fermi level into the conduction band and spin polarization in MnxMoSe2 that increases at low Mn concentrations and low pressure. Our results suggest that intercalation and pressurization of van der Waals materials may allow one to obtain dilute magnetic semiconductors with controllable properties, providing a viable route for the development of new materials for spintronic applications.

Less NMDA Receptor Binding in Dorsolateral Prefrontal Cortex and Anterior Cingulate Cortex Associated With Reported Early-Life Adversity but Not Suicide.

BACKGROUND: Glutamate is an excitatory neurotransmitter binding to 3 classes of receptors, including the N-methyl, D-aspartate (NMDA) receptor. NMDA receptor binding is lower in major depression disorder and suicide. NMDA receptor blocking with ketamine can have antidepressant and anti-suicide effects. Early-life adversity (ELA) may cause glutamate-mediated excitotoxicity and is more common with major depression disorder and in suicide decedents. We sought to determine whether NMDA-receptor binding is altered with suicide and ELA. METHODS: A total 52 postmortem cases were organized as 13 quadruplets of suicide and non-suicide decedents matched for age, sex, and postmortem interval, with or without reported ELA (≤16 years). Tissue blocks containing dorsal prefrontal (BA8), dorsolateral prefrontal (BA9), or anterior cingulate (BA24) cortex were collected at autopsy. Psychiatrically healthy controls and suicide decedents underwent psychological autopsy to determine psychiatric diagnoses and details of childhood adversity. NMDA receptor binding was determined by quantitative autoradiography of [3H]MK-801 binding (displaced by unlabeled MK-801) in 20-µm-thick sections. RESULTS: [3H]MK-801 binding was not associated with suicide in BA8, BA9, or BA24. However, [3H]MK-801 binding with ELA was less in BA8, BA9, and BA24 independent of suicide (P < .05). [3H]MK-801 binding was not associated with age or postmortem interval in any brain region or group. CONCLUSIONS: Less NMDA receptor binding with ELA is consistent with the hypothesis that stress can cause excitotoxicity via excessive glutamate, causing either NMDA receptor downregulation or less receptor binding due to neuron loss consequent to the excitotoxicity.

Comparative impacts of stormwater runoff on water quality of an urban, a suburban, and a rural stream.

Water quality data at 12 sites within an urban, a suburban, and a rural stream were collected contemporaneously during four wet and eight dry periods. The urban stream yielded the highest biochemical oxygen demand (BOD), orthophosphate, total suspended sediment (TSS), and surfactant concentrations, while the most rural stream yielded the highest total organic carbon concentrations. Percent watershed development and percent impervious surface coverage were strongly correlated with BOD (biochemical oxygen demand), orthophosphate, and surfactant concentrations but negatively with total organic carbon. Excessive fecal coliform abundance most frequently occurred in the most urbanized catchments. Fecal coliform bacteria, TSS, turbidity, orthophosphate, total phosphorus, and BOD were significantly higher during rain events compared to nonrain periods. Total rainfall preceding sampling was positively correlated with turbidity, TSS, BOD, total phosphorus, and fecal coliform bacteria concentrations. Turbidity and TSS were positively correlated with phosphorus, fecal coliform bacteria, BOD, and chlorophyll a, which argues for better sedimentation controls under all landscape types.