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James T Hynes - One of the best experts on this subject based on the ideXlab platform.
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reactIon mechanism for direct proton transfer from carbonic acid to a strong base in aqueous solutIon i acid and base coordinate and charge dynamics
Journal of Physical Chemistry B, 2016Co-Authors: Snehasis Daschakraborty, Dina Pines, Yifa Mille, Philip M Kiefe, Ehud Pines, Yai Motro, James T HynesAbstract:ProtonatIon by carbonic acid H2CO3 of the strong base methylamine CH3NH2 in a neutral Contact Pair in aqueous solutIon is followed via Car–Parrinello molecular dynamics simulatIons. Proton transfer (PT) occurs to form an aqueous solvent-stabilized Contact Ion Pair within 100 fs, a fast time scale associated with the compressIon of the acid–base hydrogen-bond (H-bond), a key reactIon coordinate. This rapid barrierless PT is consistent with the carbonic acid-protonated base pKa difference that considerably favors the PT, and supports the view of intact carbonic acid as potentially important proton donor in assorted biological and environmental contexts. The charge redistributIon within the H-bonded complex during PT supports a Mulliken picture of charge transfer from the nitrogen base to carbonic acid without altering the transferring hydrogen’s charge from approximately midway between that of a hydrogen atom and that of a proton.
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reactIon mechanism for direct proton transfer from carbonic acid to a strong base in aqueous solutIon ii solvent coordinate dependent reactIon path
Journal of Physical Chemistry B, 2016Co-Authors: Snehasis Daschakraborty, Dina Pines, Ehud Pines, Philip M Kiefer, Yifat Miller, Yair Motro, James T HynesAbstract:The protonatIon of methylamine base CH3NH2 by carbonic acid H2CO3 within a hydrogen (H)-bonded complex in aqueous solutIon was studied via Car–Parrinello dynamics in the preceding paper (Daschakraborty, S.; Kiefer, P. M.; Miller, Y.; Motro, Y.; Pines, D.; Pines, E.; Hynes, J. T. J. Phys. Chem. B 2016, DOI: 10.1021/acs.jpcb.5b12742). Here some important further details of the reactIon path are presented, with specific emphasis on the water solvent’s role. The overall reactIon is barrierless and very rapid, on an ∼100 fs time scale, with the proton transfer (PT) event itself being very sudden (<10 fs). This transfer is preceded by the acid–base H-bond’s compressIon, while the water solvent changes little until the actual PT occurrence; this results from the very strong driving force for the reactIon, as indicated by the very favorable acid-protonated base ΔpKa difference. Further solvent rearrangement follows immediately the sudden PT’s productIon of an incipient Contact Ion Pair, stabilizing it by establis...
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dissociatIon of nitric acid at an aqueous surface large amplitude motIons in the Contact Ion Pair to solvent separated Ion Pair conversIon
Physical Chemistry Chemical Physics, 2010Co-Authors: Shuzhi Wang, James T Hynes, Roberto BiancoAbstract:Beyond its fundamental interest, the acid dissociatIon of nitric acid (HNO3) at an aqueous interface is of importance in a wide variety of atmospheric contexts. Here we present a Car–Parrinello molecular dynamics (CPMD) study of the second step of this process, the formatIon, via proton transfer (PT), of a solvent-separated Ion Pair (SSIP) from a Contact Ion Pair (CIP) of the hydronium (H3O+) and the nitrate (NO−3) Ions. This reactIon represents an extensIon of our earlier CPMD study of the first PT step to produce the CIP from molecular HNO3 at various locatIons at and below the aqueous surface (S. Wang, R. Bianco and J. T. Hynes, J. Phys. Chem. A, 2009, 113, 1295); it is important in establishing the Ionic distributIon in the aqueous interfacial regIon, with potential consequences for heterogeneous reactIons occurring in that regIon. We focus on the large amplitude, microscopic level motIons—such as the hydrogen-bonding coordinatIon number changes around the proton-donating and -accepting species—which are key for the CIP → SSIP PT conversIon.
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molecular mechanism of hcl acid IonizatIon in water ab initio potential energy surfaces and monte carlo simulatIons
Journal of Physical Chemistry B, 1997Co-Authors: Koji Ando And, James T HynesAbstract:The acid IonizatIon of HCl in water is examined via a combinatIon of electronic structure calculatIons with ab initio molecular orbital methods and Monte Carlo computer simulatIons. The following key features are taken into account in the modeling: the polarizatIon of the electronic structure of the solute reactIon system by the solvent, the quantum character of the proton nuclear motIon, the solvent fluctuatIon and reorganizatIon along with the solvent polarizatIon effects on the proton potential, and a Grotthuss mechanism of the aqueous proton transfer. The mechanism is found to involve the following: first, a nearly activatIonless motIon in a solvent coordinate, which is adiabatically followed by the quantum proton rather than tunneling, to produce a Contact Ion Pair Cl-−H3O+, which is stabilized by ∼7 kcal/mol; second, motIon in the solvent with a small activatIon barrier, as a second adiabatic proton transfer produces a solvent-separated Ion Pair from the Contact Ion Pair in a nearly thermoneutral ...
Rui Wang - One of the best experts on this subject based on the ideXlab platform.
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chiral organic Contact Ion Pairs in metal free catalytic enantioselective oxidative cross dehydrogenative coupling of tertiary amines to ketones
Chemical Science, 2013Co-Authors: Gen Zhang, Shoulei Wang, Weidong Kong, Rui WangAbstract:A novel chiral organic Contact Ion-Pair catalytic system has been developed for the transitIon-metal-free catalytic enantioselective oxidative cross-dehydrogenative coupling of tertiary amines to ketones for sp3 C–H functIonalizatIon. This new strategy provides an efficient and environmentally friendly way to access diversify optically active C1-alkylated tetrahydroisoquinoline derivatives from simple starting materials under mild conditIons.
Heather C Allen - One of the best experts on this subject based on the ideXlab platform.
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molecular level origin of the carboxylate head group response to divalent metal Ion complexatIon at the air water interface
Proceedings of the National Academy of Sciences of the United States of America, 2019Co-Authors: Joanna K Denton, Marcel D Baer, Christopher J Mundy, Patrick J Kelleher, Mark A Johnson, Shawn M Kathmann, Bethany Wellen A Rudd, Heather C AllenAbstract:We exploit gas-phase cluster Ion techniques to provide insight into the local interactIons underlying divalent metal Ion-driven changes in the spectra of carboxylic acids at the air–water interface. This informatIon clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentratIons of Ca2+ metal Ions. To this end, we report the evolutIon of the vibratIonal spectra of size-selected [Ca2+·RCO2−]+·(H2O)n=0to12 and RCO2−·(H2O)n=0to14 cluster Ions toward the features observed at the air–water interface. Surprisingly, not only does stepwise hydratIon of the RCO2− anIon and the [Ca2+·RCO2−]+ Contact Ion Pair yield solvatochromic responses in opposite directIons, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydratIon are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directIons. SimulatIons of the [Ca2+·RCO2−]+·(H2O)n clusters indicate that the metal Ion remains directly bound to the head group in a Contact Ion Pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexatIon or deprotonatIon can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C–C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.
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sodium carboxylate Contact Ion Pair formatIon induces stabilizatIon of palmitic acid monolayers at high ph
Physical Chemistry Chemical Physics, 2017Co-Authors: Ellen M Adams, Bethany A Wellen, Raphael Thiraux, Sandeep K Reddy, Andrew S Vidalis, Francesco Paesani, Heather C AllenAbstract:Sea spray aerosols (SSA) are known to have an organic coating that is mainly composed of fatty acids. In this study, the effect of pH and salt on the stability and organizatIon of a palmitic acid (PA) monolayer is investigated by surface vibratIonal spectroscopy and molecular dynamics simulatIons. Results indicate that alkyl chain packing becomes more disordered as the carboxylic headgroup becomes deprotonated. This is associated with packing mismatch of charged and neutral species as charged headgroups penetrate deeper into the solutIon phase. At pH 10.7, when the monolayer is ∼99% deprotonated, palmitate (PA-) molecules desorb and solubilize into the bulk solutIon where there is spectroscopic evidence for aggregate formatIon. Yet, additIon of 100 mM NaCl to the bulk solutIon is found to drive PA- molecules to the aqueous surface. Free energy calculatIons show that PA- molecules become stabilized within the interface with increasing NaCl concentratIon. FormatIon of Contact -COO-:Na+ Pairs alters the hydratIon state of PA- headgroups, thus increasing the surface propensity. As salts are highly concentrated in SSA, these results suggest that deprotonated fatty acids may be found at the air-aqueous interface of aerosol particles due to sea salt's role in surface stabilizatIon.
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aqueous divalent metal nitrate interactIons hydratIon versus Ion Pairing
Physical Chemistry Chemical Physics, 2008Co-Authors: James P Larentzos, Mazen Roshdy, Louise J Criscenti, Heather C AllenAbstract:Nitrate aqueous solutIons, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, and Pb(NO3)2, are investigated using Raman spectroscopy and free energy profiles from molecular dynamics (MD) simulatIons. Analysis of the in-plane deformatIon, symmetric stretch, and asymmetric stretch vibratIonal modes of the nitrate Ions reveal perturbatIon caused by the metal catIons and hydrating water molecules. Results show that Pb2+ has a strong tendency to form Contact Ion Pairs with nitrate relative to Sr2+, Ca2+, and Mg2+, and Contact Ion Pair formatIon decreases with decreasing catIon size and increasing catIon charge density: Pb2+ > Sr2+ > Ca2+ > Mg2+. In the case of Mg2+, the Mg2+–OH2 intermolecular modes indicate strong hydratIon by water molecules and no Contact Ion Pairing with nitrate. Free energy profiles provide evidence for the experimentally observed trend and clarificatIon between solvent-separated, solvent-shared, and Contact Ion Pairs, particularly for Mg2+ relative to other catIons.
Ioannis Skarmoutsos - One of the best experts on this subject based on the ideXlab platform.
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solvent and salt effect on lithium Ion solvatIon and Contact Ion Pair formatIon in organic carbonates a quantum chemical perspective
Journal of Physical Chemistry C, 2018Co-Authors: Veerapandian Ponnuchamy, Stefano Mossa, Ioannis SkarmoutsosAbstract:Quantum chemical calculatIons have been employed to investigate the solvatIon of lithium catIons in ethylene carbonate/propylene carbonate and propylene carbonate/dimethyl carbonate mixed electroly...
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solvent and salt effect on lithium Ion solvatIon and Contact Ion Pair formatIon in organic carbonates a quantum chemical perspective
Journal of Physical Chemistry C, 2018Co-Authors: Veerapandian Ponnuchamy, Stefano Mossa, Ioannis SkarmoutsosAbstract:Quantum chemical calculatIons have been employed to investigate the solvatIon of lithium catIons in ethylene carbonate/propylene carbonate and propylene carbonate/dimethyl carbonate mixed electrolytes. The impact of the presence of the counteranIon on the solvatIon of Li+ in pure propylene carbonate and dimethyl carbonate was also studied. The calculatIons revealed small free-energy changes for the transitIons between different preferred structures in mixed solvents. This implies that transitIons between distinct local arrangements can take place in the mixtures. The additIon of dimethyl carbonate causes a significant increase of the dipole moment of solvatIon clusters, indicating important molecular-scale modificatIons when dimethyl carbonate is used as a co-solvent. The presence of an anIon in the solvatIon shell of Li+ modifies the intermolecular structure comprising four carbonate molecules in dilute solutIons, allowing only two carbonate molecules to coordinate to Li+. The bidentate complexatIon of L...
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Solvent and Salt Effect on Lithium Ion SolvatIon and Contact Ion Pair FormatIon in Organic Carbonates: A Quantum Chemical Perspective
2018Co-Authors: Veerapandian Ponnuchamy, Stefano Mossa, Ioannis SkarmoutsosAbstract:Quantum chemical calculatIons have been employed to investigate the solvatIon of lithium catIons in ethylene carbonate/propylene carbonate and propylene carbonate/dimethyl carbonate mixed electrolytes. The impact of the presence of the counteranIon on the solvatIon of Li+ in pure propylene carbonate and dimethyl carbonate was also studied. The calculatIons revealed small free-energy changes for the transitIons between different preferred structures in mixed solvents. This implies that transitIons between distinct local arrangements can take place in the mixtures. The additIon of dimethyl carbonate causes a significant increase of the dipole moment of solvatIon clusters, indicating important molecular-scale modificatIons when dimethyl carbonate is used as a co-solvent. The presence of an anIon in the solvatIon shell of Li+ modifies the intermolecular structure comprising four carbonate molecules in dilute solutIons, allowing only two carbonate molecules to coordinate to Li+. The bidentate complexatIon of Li+ with the anIon’s electron donor atoms, however, maintains the local tetrahedral structure on interatomic length scales. The neutralizatIon of the solvatIon shell of Li+ due to Contact Ion Pair formatIon and the consequent implicatIons on the underlying mechanisms provide a ratIonal explanatIon for the Ionic conductivity drop of electrolyte solutIons at high salt concentratIons
Kelly J Gaffney - One of the best experts on this subject based on the ideXlab platform.
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Contact Ion Pair formatIon between hard acids and soft bases in aqueous solutIons observed with 2dir spectroscopy
Journal of Physical Chemistry B, 2013Co-Authors: Zheng Sun, W Zhang, Robert W Hartsock, Kelly J GaffneyAbstract:The interactIon of charged species in aqueous solutIon has important implicatIons for chemical, biological, and environmental processes. We have used 2DIR spectroscopy to study the equilibrium dynamics of thiocyanate chemical exchange between free Ion (NCS(-)) and Contact Ion Pair configuratIons (MNCS(+)), where M(2+) = Mg(2+) or Ca(2+). Detailed studies of the influence of anIon concentratIon and anIon speciatIon show that the chemical exchange observed with the 2DIR measurements results from NCS(-) exchanging with other anIon species in the first solvatIon shell surrounding Mg(2+) or Ca(2+). The presence of chemical exchange in the 2DIR spectra provides an indirect, but robust, determinant of Contact Ion Pair formatIon. We observe preferential Contact Ion Pair formatIon between soft Lewis base anIons and hard Lewis acid catIons. This observatIon cannot be easily reconciled with Pearson's acid-base concept or Collins' Law of Matching Water Affinities. The anIons that form Contact Ion Pairs also correspond to the Ions with an affinity for water and protein surfaces, so similar physical and chemical properties may control these distinct phenomena.
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ligand exchange dynamics in aqueous solutIon studied with 2dir spectroscopy
Journal of Physical Chemistry B, 2010Co-Authors: Sungnam Park, Kelly J GaffneyAbstract:We have used time-resolved multidimensIonal vibratIonal spectroscopy, generally termed 2DIR spectroscopy, to study the equilibrium dynamics of ligand exchange in an aqueous solutIon containing 3.4 M Mg(ClO4)2 and 1.2 M NaSCN. The sensitivity of the CN stretching frequency of thiocyanate (SCN−) to Contact Ion Pair formatIon with Mg2+ Ions generates distinct spectroscopic signatures for the MgNCS+ Contact Ion Pair and the free SCN−. We have utilized 2DIR spectroscopy to successfully resolve the interconversIon between these thiocyanate configuratIons and measured the MgNCS+ Contact Ion Pair dissociatIon time constant to be 52 ± 10 ps. We attribute the observed dynamics to perchlorate−thiocyanate anIon exchange in the first solvatIon shell of the Mg2+ catIon. Magnesium Ions in this concentrated Ionic solutIon will be coordinated by water molecules, as well as perchlorate and thiocyanate Ions. While prior studies have observed microsecond residence times for water ligands in the first coordinatIon sphere of M...