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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 nearâsurface 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 midâSeptember 2018) and a regional storm (January 2019). Here we present water vapor vertical profiles in the periods of the two dust storms (Ls = 162â260° and Ls = 298â345°) 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 (40â100 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°Sâ60°N at the growth phase of the dust storm (Ls = 195°â220°), and peaks at latitudes greater than 60°S at the decay phase (Ls = 220°â260°). This is explained by the seasonal change of meridional circulation: from equinoctial Hadley circulation (two cells) to the solstitial one (a single poleâtoâpole cell). We also find a conspicuous increase of water vapor density in the middle atmosphere at the period of the regional dust storm (Ls = 322â327°), 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 ïŹnal 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
Solvation Free Energy Reaction Curves for Electron Transfer in Aqueous Solution:Â Theory and Simulation
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 âpermanentâ 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
Protein Control of Electron Transfer Rates via Polarization: Molecular Dynamics Studies of Rubredoxin
Catalytic Nitrile Hydration with [Ru(η<sup>6</sup>â<i>p</i>âcymene)Cl<sub>2</sub>(PR<sub>2</sub>RâČ)] Complexes: Secondary Coordination Sphere Effects with Phosphine Oxide and Phosphinite Ligands
The rates of nitrile hydration reactions
were investigated using
[RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>(PR<sub>2</sub>RâČ)] complexes as homogeneous catalysts, where PR<sub>2</sub>RâČ = PMe<sub>2</sub>(CH<sub>2</sub>PÂ(O)ÂMe<sub>2</sub>), PMe<sub>2</sub>(CH<sub>2</sub>CH<sub>2</sub>PÂ(O)ÂMe<sub>2</sub>), PPh<sub>2</sub>(CH<sub>2</sub>PÂ(O)ÂPh<sub>2</sub>), PPh<sub>2</sub>(CH<sub>2</sub>CH<sub>2</sub>PÂ(O)ÂPh<sub>2</sub>), PMe<sub>2</sub>OH, PÂ(OEt)<sub>2</sub>OH. 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 [RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>PMe<sub>2</sub>OH]:
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 temperaturesîžconditions
where α-hydroxynitriles (cyanohydrins) produce less cyanide,
a known poison for organometallic nitrile hydration catalysts. The
[RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>PMe<sub>2</sub>OH] catalyst completely converts the cyanohydrins glycolonitrile
and lactonitrile to their corresponding α-hydroxyamides faster
than previously investigated catalysts. [RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>PMe<sub>2</sub>OH] is not, however,
a good catalyst for acetone cyanohydrin hydration, because it is susceptible
to cyanide poisoning. Protecting the âOH 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(η<sup>6</sup>â<i>p</i>âcymene)Cl<sub>2</sub>(PR<sub>2</sub>RâČ)] Complexes: Secondary Coordination Sphere Effects with Phosphine Oxide and Phosphinite Ligands
The rates of nitrile hydration reactions
were investigated using
[RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>(PR<sub>2</sub>RâČ)] complexes as homogeneous catalysts, where PR<sub>2</sub>RâČ = PMe<sub>2</sub>(CH<sub>2</sub>PÂ(O)ÂMe<sub>2</sub>), PMe<sub>2</sub>(CH<sub>2</sub>CH<sub>2</sub>PÂ(O)ÂMe<sub>2</sub>), PPh<sub>2</sub>(CH<sub>2</sub>PÂ(O)ÂPh<sub>2</sub>), PPh<sub>2</sub>(CH<sub>2</sub>CH<sub>2</sub>PÂ(O)ÂPh<sub>2</sub>), PMe<sub>2</sub>OH, PÂ(OEt)<sub>2</sub>OH. 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 [RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>PMe<sub>2</sub>OH]:
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 temperaturesîžconditions
where α-hydroxynitriles (cyanohydrins) produce less cyanide,
a known poison for organometallic nitrile hydration catalysts. The
[RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>PMe<sub>2</sub>OH] catalyst completely converts the cyanohydrins glycolonitrile
and lactonitrile to their corresponding α-hydroxyamides faster
than previously investigated catalysts. [RuÂ(η<sup>6</sup>-<i>p</i>-cymene)ÂCl<sub>2</sub>PMe<sub>2</sub>OH] is not, however,
a good catalyst for acetone cyanohydrin hydration, because it is susceptible
to cyanide poisoning. Protecting the âOH 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(η<sup>6</sup>âarene)Cl<sub>2</sub>PR<sub>3</sub>] Complexes
The mechanism of the nitrile-to-amide hydration reaction
using
[RuÂ(η<sup>6</sup>-arene)ÂCl<sub>2</sub>(PR<sub>3</sub>)] complexes
as catalysts was investigated (η<sup>6</sup>-arene = C<sub>6</sub>H<sub>6</sub>, <i>p</i>-cymene, C<sub>6</sub>Me<sub>6</sub>; R = NMe<sub>2</sub>, 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 [RuÂ(η<sup>6</sup>-arene)ÂCl<sub>2</sub>(PÂ(NMe<sub>2</sub>)<sub>3</sub>)] 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 [RuÂ(η<sup>6</sup>-arene)ÂCl<sub>2</sub>(PR<sub>3</sub>)] catalysts are initially less
susceptible to cyanide poisoning than other homogeneous nitrile hydration
catalysts because [RuÂ(η<sup>6</sup>-<i>p</i>-cymene)Â(CN)Â(Cl)Â(PÂ(NMe<sub>2</sub>)<sub>3</sub>)] 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(η<sup>6</sup>âarene)Cl<sub>2</sub>PR<sub>3</sub>] Complexes
The mechanism of the nitrile-to-amide hydration reaction
using
[RuÂ(η<sup>6</sup>-arene)ÂCl<sub>2</sub>(PR<sub>3</sub>)] complexes
as catalysts was investigated (η<sup>6</sup>-arene = C<sub>6</sub>H<sub>6</sub>, <i>p</i>-cymene, C<sub>6</sub>Me<sub>6</sub>; R = NMe<sub>2</sub>, 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 [RuÂ(η<sup>6</sup>-arene)ÂCl<sub>2</sub>(PÂ(NMe<sub>2</sub>)<sub>3</sub>)] 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 [RuÂ(η<sup>6</sup>-arene)ÂCl<sub>2</sub>(PR<sub>3</sub>)] catalysts are initially less
susceptible to cyanide poisoning than other homogeneous nitrile hydration
catalysts because [RuÂ(η<sup>6</sup>-<i>p</i>-cymene)Â(CN)Â(Cl)Â(PÂ(NMe<sub>2</sub>)<sub>3</sub>)] forms in the presence of cyanide. The electron-withdrawing
cyanide ligand facilitates nucleophilic attack of water on a coordinated
nitrile in this molecule