Water Vapor Vertical Profiles on Mars in Dust Storms Observed by TGO/NOMAD

It has been suggested that dust storms efficiently transport water vapor from the near‐surface to the middle atmosphere on Mars. Knowledge of the water vapor vertical profile during dust storms is important to understand water escape. During Martian Year 34, two dust storms occurred on Mars: a global dust storm (June to mid‐September 2018) and a regional storm (January 2019). Here we present water vapor vertical profiles in the periods of the two dust storms (Ls = 162–260° and Ls = 298–345°) from the solar occultation measurements by Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). We show a significant increase of water vapor abundance in the middle atmosphere (40–100 km) during the global dust storm. The water enhancement rapidly occurs following the onset of the storm (Ls~190°) and has a peak at the most active period (Ls~200°). Water vapor reaches very high altitudes (up to 100 km) with a volume mixing ratio of ~50 ppm. The water vapor abundance in the middle atmosphere shows high values consistently at 60°S‐60°N at the growth phase of the dust storm (Ls = 195°–220°), and peaks at latitudes greater than 60°S at the decay phase (Ls = 220°–260°). This is explained by the seasonal change of meridional circulation: from equinoctial Hadley circulation (two cells) to the solstitial one (a single pole‐to‐pole cell). We also find a conspicuous increase of water vapor density in the middle atmosphere at the period of the regional dust storm (Ls = 322–327°), in particular at latitudes greater than 60°S

Integrated Economic and Climate Modeling

This survey examines the history and current practice in integrated assessment models (IAMs) of the economics of climate change. It begins with a review of the emerging problem of climate change. The next section provides a brief sketch of the rise of IAMs in the 1970s and beyond. The subsequent section is an extended exposition of one IAM, the DICE/RICE family of models. The purpose of this description is to provide readers an example of how such a model is developed and what the major components are. The final section discusses major important open questions that continue to occupy IAM modelers. These involve issues such as the discount rate, uncertainty, the social cost of carbon, the potential for catastrophic climate change, algorithms, and fat-tailed distributions. These issues are ones that pose both deep intellectual challenges as well as important policy implications for climate change and climate-change policy

Protein Control of Electron Transfer Rates via Polarization: Molecular Dynamics Studies of Rubredoxin

The protein matrix of an electron transfer protein creates an electrostatic environment for its redox site, which influences its electron transfer properties. Our studies of Fe-S proteins indicate that the protein is highly polarized around the redox site. Here, measures of deviations of the environmental electrostatic potential from a simple linear dielectric polarization response to the magnitude of the charge are proposed. In addition, a decomposition of the potential is proposed here to describe the apparent deviations from linearity, in which it is divided into a “permanent” component that is independent of the redox site charge and a dielectric component that linearly responds or polarizes to the charge. The nonlinearity measures and the decomposition were calculated for Clostridium pasteurianum rubredoxin from molecular dynamics simulations. The potential in rubredoxin is greater than expected from linear response theory, which implies it is a better electron acceptor than a redox site analog in a solvent with a dielectric constant equivalent to that of the protein. In addition, the potential in rubredoxin is described well by a permanent potential plus a linear response component. This permanent potential allows the protein matrix to create a favorable driving force with a low activation barrier for accepting electrons. The results here also suggest that the reduction potential of rubredoxin is determined mainly by the backbone and not the side chains, and that the redox site charge of rubredoxin may help to direct its folding

Catalytic Nitrile Hydration with [Ru(η⁶‑<i>p</i>‑cymene)Cl₂(PR₂R′)] Complexes: Secondary Coordination Sphere Effects with Phosphine Oxide and Phosphinite Ligands

The rates of nitrile hydration reactions were investigated using [Ru­(η⁶-<i>p</i>-cymene)­Cl₂(PR₂R′)] complexes as homogeneous catalysts, where PR₂R′ = PMe₂(CH₂P­(O)­Me₂), PMe₂(CH₂CH₂P­(O)­Me₂), PPh₂(CH₂P­(O)­Ph₂), PPh₂(CH₂CH₂P­(O)­Ph₂), PMe₂OH, P­(OEt)₂OH. These catalysts were studied because the rate of the nitrile-to-amide hydration reaction was hypothesized to be affected by the position of the hydrogen bond accepting group in the secondary coordination sphere of the catalyst. Experiments showed that the rate of nitrile hydration was fastest when using [Ru­(η⁶-<i>p</i>-cymene)­Cl₂PMe₂OH]: i.e., the catalyst with the hydrogen bond accepting group capable of forming the most stable ring in the transition state of the rate-limiting step. This catalyst is also active at pH 3.5 and at low temperaturesconditions where α-hydroxynitriles (cyanohydrins) produce less cyanide, a known poison for organometallic nitrile hydration catalysts. The [Ru­(η⁶-<i>p</i>-cymene)­Cl₂PMe₂OH] catalyst completely converts the cyanohydrins glycolonitrile and lactonitrile to their corresponding α-hydroxyamides faster than previously investigated catalysts. [Ru­(η⁶-<i>p</i>-cymene)­Cl₂PMe₂OH] is not, however, a good catalyst for acetone cyanohydrin hydration, because it is susceptible to cyanide poisoning. Protecting the −OH group of acetone cyanohydrin was shown to be an effective way to prevent cyanide poisoning, resulting in quantitative hydration of acetone cyanohydrin acetate

Catalytic Nitrile Hydration with [Ru(η⁶‑<i>p</i>‑cymene)Cl₂(PR₂R′)] Complexes: Secondary Coordination Sphere Effects with Phosphine Oxide and Phosphinite Ligands

The rates of nitrile hydration reactions were investigated using [Ru­(η⁶-<i>p</i>-cymene)­Cl₂(PR₂R′)] complexes as homogeneous catalysts, where PR₂R′ = PMe₂(CH₂P­(O)­Me₂), PMe₂(CH₂CH₂P­(O)­Me₂), PPh₂(CH₂P­(O)­Ph₂), PPh₂(CH₂CH₂P­(O)­Ph₂), PMe₂OH, P­(OEt)₂OH. These catalysts were studied because the rate of the nitrile-to-amide hydration reaction was hypothesized to be affected by the position of the hydrogen bond accepting group in the secondary coordination sphere of the catalyst. Experiments showed that the rate of nitrile hydration was fastest when using [Ru­(η⁶-<i>p</i>-cymene)­Cl₂PMe₂OH]: i.e., the catalyst with the hydrogen bond accepting group capable of forming the most stable ring in the transition state of the rate-limiting step. This catalyst is also active at pH 3.5 and at low temperaturesconditions where α-hydroxynitriles (cyanohydrins) produce less cyanide, a known poison for organometallic nitrile hydration catalysts. The [Ru­(η⁶-<i>p</i>-cymene)­Cl₂PMe₂OH] catalyst completely converts the cyanohydrins glycolonitrile and lactonitrile to their corresponding α-hydroxyamides faster than previously investigated catalysts. [Ru­(η⁶-<i>p</i>-cymene)­Cl₂PMe₂OH] is not, however, a good catalyst for acetone cyanohydrin hydration, because it is susceptible to cyanide poisoning. Protecting the −OH group of acetone cyanohydrin was shown to be an effective way to prevent cyanide poisoning, resulting in quantitative hydration of acetone cyanohydrin acetate

Mechanistic Investigations and Secondary Coordination Sphere Effects in the Hydration of Nitriles with [Ru(η⁶‑arene)Cl₂PR₃] Complexes

The mechanism of the nitrile-to-amide hydration reaction using [Ru­(η⁶-arene)­Cl₂(PR₃)] complexes as catalysts was investigated (η⁶-arene = C₆H₆, <i>p</i>-cymene, C₆Me₆; R = NMe₂, OMe, OEt, Et, iPr). Experiments showed that the mechanism involves the following general sequence of reactions: substitution of a chloride ligand by the nitrile substrate, intermolecular nucleophilic attack by water to form an amidate intermediate, and dissociation of the resulting amide. The effects of secondary coordination sphere interactions on the rates and yields of the hydration reaction were investigated. Ligands that are capable of acting as hydrogen bond acceptors with the entering water molecule result in faster rates and higher yields than non-hydrogen-bonding ligands. The faster rates are attributable to the H-bonding-facilitated deprotonation of the water as the oxygen of the water bonds to the coordinated nitrile. DFT calculations on the proposed H-bonding intermediates support this interpretation. Most homogeneous catalysts will not hydrate cyanohydrins because of the equilibrium amounts of cyanide that are present in solutions of cyanohydrins; the cyanide poisons the catalyst. Because of its increased catalytic reactivity due to secondary coordination sphere effects, the [Ru­(η⁶-arene)­Cl₂(P­(NMe₂)₃)] catalyst gives significant yields of cyanohydrin hydration products with glycolonitrile, lactonitrile, acetone cyanohydrin, and mandelonitrile. A Taft plot showed that an increase in the steric bulk of the nitrile results in a decrease in the hydration rate, and a Hammett plot showed that electron-withdrawing groups facilitate nitrile hydration. The decrease in rate as the size of the cyanohydrin increases is likely due to both increased steric bulk and to the addition of electron-donating groups on the nitrile. The [Ru­(η⁶-arene)­Cl₂(PR₃)] catalysts are initially less susceptible to cyanide poisoning than other homogeneous nitrile hydration catalysts because [Ru­(η⁶-<i>p</i>-cymene)­(CN)­(Cl)­(P­(NMe₂)₃)] forms in the presence of cyanide. The electron-withdrawing cyanide ligand facilitates nucleophilic attack of water on a coordinated nitrile in this molecule

Mechanistic Investigations and Secondary Coordination Sphere Effects in the Hydration of Nitriles with [Ru(η⁶‑arene)Cl₂PR₃] Complexes

The mechanism of the nitrile-to-amide hydration reaction using [Ru­(η⁶-arene)­Cl₂(PR₃)] complexes as catalysts was investigated (η⁶-arene = C₆H₆, <i>p</i>-cymene, C₆Me₆; R = NMe₂, OMe, OEt, Et, iPr). Experiments showed that the mechanism involves the following general sequence of reactions: substitution of a chloride ligand by the nitrile substrate, intermolecular nucleophilic attack by water to form an amidate intermediate, and dissociation of the resulting amide. The effects of secondary coordination sphere interactions on the rates and yields of the hydration reaction were investigated. Ligands that are capable of acting as hydrogen bond acceptors with the entering water molecule result in faster rates and higher yields than non-hydrogen-bonding ligands. The faster rates are attributable to the H-bonding-facilitated deprotonation of the water as the oxygen of the water bonds to the coordinated nitrile. DFT calculations on the proposed H-bonding intermediates support this interpretation. Most homogeneous catalysts will not hydrate cyanohydrins because of the equilibrium amounts of cyanide that are present in solutions of cyanohydrins; the cyanide poisons the catalyst. Because of its increased catalytic reactivity due to secondary coordination sphere effects, the [Ru­(η⁶-arene)­Cl₂(P­(NMe₂)₃)] catalyst gives significant yields of cyanohydrin hydration products with glycolonitrile, lactonitrile, acetone cyanohydrin, and mandelonitrile. A Taft plot showed that an increase in the steric bulk of the nitrile results in a decrease in the hydration rate, and a Hammett plot showed that electron-withdrawing groups facilitate nitrile hydration. The decrease in rate as the size of the cyanohydrin increases is likely due to both increased steric bulk and to the addition of electron-donating groups on the nitrile. The [Ru­(η⁶-arene)­Cl₂(PR₃)] catalysts are initially less susceptible to cyanide poisoning than other homogeneous nitrile hydration catalysts because [Ru­(η⁶-<i>p</i>-cymene)­(CN)­(Cl)­(P­(NMe₂)₃)] forms in the presence of cyanide. The electron-withdrawing cyanide ligand facilitates nucleophilic attack of water on a coordinated nitrile in this molecule