Depth-Dependent Dissociation of Nitric Acid at an Aqueous Surface: Car−Parrinello Molecular Dynamics

Molecular Insights into Chemical Reactions at Aqueous Aerosol Interfaces

Atmospheric aerosols facilitate reactions between ambient gases and dissolved species. Here, we review our efforts to interrogate the uptake of these gases and the mechanisms of their reactions both theoretically and experimentally. We highlight the fascinating behavior of N2O5 in solutions ranging from pure water to complex mixtures, chosen because its aerosol-mediated reactions significantly impact global ozone, hydroxyl, and methane concentrations. As a hydrophobic, weakly soluble, and highly reactive species, N2O5 is a sensitive probe of the chemical and physical properties of aerosol interfaces. We employ contemporary theory to disentangle the fate of N2O5 as it approaches pure and salty water, starting with adsorption and ending with hydrolysis to HNO3, chlorination to ClNO2, or evaporation. Flow reactor and gas-liquid scattering experiments probe even greater complexity as added ions, organic molecules, and surfactants alter the interfacial composition and reaction rates. Together, we reveal a new perspective on multiphase chemistry in the atmosphere.

Structure and dynamics of dissociated and undissociated forms of nitric acid and their implications in interfacial mass transfer: insights from molecular dynamics simulations

Nitric acid (HNO3) is widely used in various chemical and nuclear industries. Therefore, it is important to develop an understanding of the different forms of nitric acid for its practical applications. Molecular dynamics (MD) simulation is one of the best tools to investigate the behavior of concentrated nitric acid in aqueous solution with various forms together with pure nitric acid to identify a suitable model of nitric acid for use in simulations of biphasic systems for interfacial mass transfer. The Mulliken partial charge embedded OPLS-AA force field was used to model the neutral nitric acid, hydronium ion and nitrate ion, and it was found that the Mulliken partial charge embedded force field works quite well. The computed density of the dissociated and mixed-form acid was in good agreement with the experimental values. In water, the HNO3 molecule was seen to be coordinated with three water molecules in the first sphere of coordination. The distribution of water surrounding the HNO3 molecule and nitrate ion was corroborated by the DFT-optimized hydrated cluster. The calculated diffusivity values of the neutral acid and ions were significantly higher in the mixed form of nitric acid, which is an important dynamic quantity controlling the kinetics of the liquid-liquid interfacial extraction. The structural analysis revealed that the local aggregation is minimized when both forms of acid are present together in the solution. The water-ion and water-neutral acid interactions were predicted to be enhanced, as confirmed by H-bond studies. The shear viscosity of the mixed acid exhibited excellent agreement with the experimental values, which again confirms the consideration of the mixed form of nitric acid. The simulated value of surface tension for the mixed form of acid also appeared to be quite accurate based on the surface tension of water. The mixed form of nitric acid comprising both forms of acid is the best representation for nitric acid to be considered for MD simulations of biphasic systems. The mixed form of nitric acid established that the concentrated nitric acid may not be present either in the fully dissociated form or fully undissociated form in the solution.

Acids at the Edge: Why Nitric and Formic Acid Dissociations at Air-Water Interfaces Depend on Depth and on Interface Specific Area

Whether the air-water interface decreases or increases the acidity of simple organic and inorganic acids compared to the bulk is critically important in a broad range of environmental and biochemical processes. However, a consensus has not yet been achieved on this key question. Here we use machine learning-based reactive molecular dynamics simulations to study the dissociation of paradigmatic nitric and formic acids at the air-water interface. We show that the local acidity profile across the interface is determined by changes in acid and conjugate base solvation and that the acidity decreases abruptly over a transition region of a few molecular layers. At the interface, both acids are weaker than in the bulk due to desolvation. In contrast, acidities below the interface reach a plateau and are all the stronger compared to those in the bulk as the surface to volume ratio of the aqueous phase is large, due to the growing impact of the stabilization of the released proton at the surface of the water. These results imply that the measured degree of dissociation sensitively depends on the experimental probing length and system size and suggest a molecular explanation for the contrasting experimental results. The aerosol size dependence of acidity has important consequences for atmospheric chemistry.

Peptide Isomerization is Suppressed at the Air-Water Interface

We use molecular dynamics simulations to study the thermodynamics and kinetics of alanine dipeptide isomerization at the air-water interface. Thermodynamically, we find an affinity of the dipeptide to the interface. This affinity arises from stabilizing intramolecular interactions that become unshielded as the dipeptide is desolvated. Kinetically, we consider the rate of transitions between the α_L and β conformations of alanine dipeptide and evaluate it as a continuous function of the distance from the interface using a recent extension of transition path sampling, TPS+U. The rate of isomerization at the Gibbs dividing surface is suppressed relative to the bulk by a factor of 3. Examination of the ensemble of transition states elucidates the role of solvent degrees of freedom in mediating favorable intramolecular interactions along the reaction pathway of isomerization. Near the air-water interface, water is less effective at mediating these intramolecular interactions.

Size-Dependent Onset of Nitric Acid Dissociation in Cs+·(HNO3)(H2O)n=0-11 Clusters at 20 K

We report the water-mediated charge separation of nitric acid upon incorporation into size-selected Cs⁺·(HNO₃)(H₂O)_n=0-11 clusters at 20 K. Dramatic spectral changes are observed in the n = 7-9 range that are traced to the formation of many isomeric structures associated with intermediate transfer of the acidic proton to the water network. This transfer is complete by n = 10, which exhibits much simpler vibrational band patterns consistent with those expected for a tricoordinated hydronium ion (the Eigen motif) along with the NO stretching bands predicted for a hydrated NO₃^- anion that is directly complexed to the Cs⁺ cation. Theoretical analysis of the n = 10 spectrum indicates that the dissociated ions adopt a solvent-separated ion-pair configuration such that the Cs⁺ and H₃O⁺ cations flank the NO₃^- anion in a microhydrated salt bridge. This charge separation motif is evidently assisted by the electrostatic stabilization of the product NO₃^-/H₃O⁺ ion pair by the proximal metal ion.

Reactivity of Undissociated Molecular Nitric Acid at the Air-Water Interface

Recent experiments and theoretical calculations have shown that HNO₃ may exist in molecular form in aqueous environments, where in principle one would expect this strong acid to be completely dissociated. Much effort has been devoted to understanding this fact, which has huge environmental relevance since nitric acid is a component of acid rain and also contributes to renoxification processes in the atmosphere. Although the importance of heterogeneous processes such as oxidation and photolysis have been evidenced by experiments, most theoretical studies on hydrated molecular HNO₃ have focused on the acid dissociation mechanism. In the present work, we carry out calculations at various levels of theory to obtain insight into the properties of molecular nitric acid at the surface of liquid water (the air-water interface). Through multi-nanosecond combined quantum-classical molecular dynamics simulations, we analyze the interface affinity of nitric acid and provide an order of magnitude for its lifetime with regard to acid dissociation, which is close to the value deduced using thermodynamic data in the literature (∼0.3 ns). Moreover, we study the electronic absorption spectrum and calculate the rate constant for the photolytic process HNO₃ + hν → NO₂ + OH, leading to 2 × 10^-6 s^-1, about twice the value in the gas phase. Finally, we describe the reaction HNO₃ + OH → NO₃ + H₂O using a cluster model containing 21 water molecules with the help of high-level ab initio calculations. A large number of reaction paths are explored, and our study leads to the conclusion that the most favorable mechanism involves the formation of a pre-reactive complex (HNO₃)(OH) from which product are obtained through a coupled proton-electron transfer mechanism that has a free-energy barrier of 6.65 kcal·mol^-1. Kinetic calculations predict a rate constant increase by ∼4 orders of magnitude relative to the gas phase, and we conclude that at the air-water interface, a lower limit for the rate constant is k = 1.2 × 10^-9 cm³·molecule^-1·s^-1. The atmospheric significance of all these results is discussed.

Molecular reactions at aqueous interfaces

This Review aims to critically analyse the emerging field of chemical reactivity at aqueous interfaces. The subject has evolved rapidly since the discovery of the so-called 'on-water catalysis', alluding to the dramatic acceleration of reactions at the surface of water or at its interface with hydrophobic media. We review critical experimental studies in the fields of atmospheric and synthetic organic chemistry, as well as related research exploring the origins of life, to showcase the importance of this phenomenon. The physico-chemical aspects of these processes, such as the structure, dynamics and thermodynamics of adsorption and solvation processes at aqueous interfaces, are also discussed. We also present the basic theories intended to explain interface catalysis, followed by the results of advanced ab initio molecular-dynamics simulations. Although some topics addressed here have already been the focus of previous reviews, we aim at highlighting their interconnection across diverse disciplines, providing a common perspective that would help us to identify the most fundamental issues still incompletely understood in this fast-moving field.

Isomer-specific cryogenic ion vibrational spectroscopy of the D2 tagged Cs+(HNO3)(H2O)n=0-2 complexes: ion-driven enhancement of the acidic H-bond to water

We report how the binary HNO3(H2O) interaction is modified upon complexation with a nearby Cs+ ion. Isomer-selective IR photodissociation spectra of the D2-tagged, ternary Cs+(HNO3)H2O cation confirms that two structural isomers are generated in the cryogenic ion source. In one of these, both HNO3 and H2O are directly coordinated to the ion, while in the other, the water molecule is attached to the OH group of the acid, which in turn binds to Cs+ with its -NO2 group. The acidic OH stretching fundamental in the latter isomer displays a ∼300 cm-1 red-shift relative to that in the neutral H-bonded van der Waals complex, HNO3(H2O). This behavior is analyzed with the aid of electronic structure calculations and discussed in the context of the increased effective acidity of HNO3 in the presence of the cation.

Theoretical Insights into the Role of Water Molecules in the Guanidinium-Based Protein Denaturation Process in Specific to Aromatic Amino Acids

Noncovalent interactions between the guanidinium cation (Gdm⁺) and aromatic amino acids (AAs) in the water molecules have been studied using quantum chemical calculation and molecular dynamics (MD) simulations. Our studies show that there are two different modes of interactions between Gdm⁺ and AAs with and without water molecules. It is observed that nonhydrated Gdm⁺ interacts with AAs through N-H···π interactions, whereas hydrated clusters of Gdm⁺ are stabilized by stacking interactions with the help of the water-mediated hydrogen bond. Thus, different hydration patterns have significant effects on the predominant cation···π interactions in AAs-Gdm⁺ complexes. Findings from MD simulation elicit that the interaction pattern of Gdm⁺ with AAs varies as Phe < Tyr < Trp. Both the QM and MD calculations show a similar trend in the interaction of AAs with Gdm⁺. Moreover, the interaction of AAs with Gdm⁺ depends on the spatial orientation of AAs in the protein and the concomitant local structure, that is, the AAs present in the unstructured region of protein such as coils and bends exhibit higher binding for Gdm⁺ when compared to the AAs present in the structured region of the protein such as the α-helix and the β-sheet. Our study clearly reveals that H-bonded water molecules and the hydration pattern of Gdm⁺ as well as the positional presence of these AAs in the protein structure context play determining roles in the denaturation of protein by the Gdm⁺ cation.

Increased Acid Dissociation at the Quartz/Water Interface

As shown by a quite significant amount of literature, acids at the water surface tend to be "less" acid, meaning that their associated form is favored over the conjugated base. What happens at the solid/liquid interface? In the case of the silica/water interface, we show how the acidity of adsorbed molecules can instead increase. Using a free energy perturbation approach in combination with electronic structure-based molecular dynamics simulations, we show how the acidity of pyruvic acid at the quartz/water interface is increased by almost two units. Such increased acidity is the result of the specific microsolvation at the interface and, in particular, of the stabilization of the deprotonated form by the silanols on the quartz surface and the special interfacial water layer.

Ab initio and density functional theory (DFT) studies on triflic acid with water and protonated water clusters

The structure, stability and infrared spectral signatures of triflic acid (TA) with water clusters (W_n) and protonated water clusters (TAH⁺W_n, n = 1 - 6) were computed using DFT and MP2 methods. Our calculations show that a minimum of three water molecules are necessary to stabilize the dissociated zwitterionic form of TA. It can be seen from the results that there is no significant movement of protons in smaller (n = 1 and 2) and linear (n = 1 - 6) types of water clusters. Further, the geometries of TAW_n clusters first form a neutral pair (NP) to contact ion pair (CIP), then form a solvent separated ion pair (SSIP) in a water hexamer. These findings reveal that proton transfer may take place through NP to CIP and then CIP to SSIP. The calculated binding energies (BEs) of ion pair clusters is always higher than that of NP clusters (i.e., more stable than the NP). Existing excess proton linear chain clusters transfer a proton to adjacent water molecules via a Grotthuss mechanism, whereas the same isomers in the branched motifs do not conduct protons. Examination of geometrical parameters and infrared frequencies reveals hydronium ion (H₃O⁺ also called Eigen cation) formation in both TAW_n and protonated TAW_n clusters. The stability of Eigen water clusters is three times higher than that of other non-Eigen water clusters. Our study shows clearly that formation of ion pairs in TAW_n and TAH⁺W_n clusters greatly favors proton transfer to neighboring water molecules and also enhances the stability of these complexes.

Protons and Hydroxide Ions in Aqueous Systems

Understanding the structure and dynamics of water's constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution I: Acid and Base Coordinate and Charge Dynamics

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.

Deprotonation of formic acid in collisions with a liquid water surface studied by molecular dynamics and metadynamics simulations

Formic acid has a lower barrier to deprotonation at the air–water interface than in bulk liquid water.

Toward a unified picture of the water self-ions at the air-water interface: a density functional theory perspective

The propensities of the water self-ions, H3O(+) and OH(-), for the air-water interface have implications for interfacial acid-base chemistry. Despite numerous experimental and computational studies, no consensus has been reached on the question of whether or not H3O(+) and/or OH(-) prefer to be at the water surface or in the bulk. Here we report a molecular dynamics simulation study of the bulk vs interfacial behavior of H3O(+) and OH(-) that employs forces derived from density functional theory with a generalized gradient approximation exchange-correlation functional (specifically, BLYP) and empirical dispersion corrections. We computed the potential of mean force (PMF) for H3O(+) as a function of the position of the ion in the vicinity of an air-water interface. The PMF suggests that H3O(+) has equal propensity for the interface and the bulk. We compare the PMF for H3O(+) to our previously computed PMF for OH(-) adsorption, which contains a shallow minimum at the interface, and we explore how differences in solvation of each ion at the interface vs in the bulk are connected with interfacial propensity. We find that the solvation shell of H3O(+) is only slightly dependent on its position in the water slab, while OH(-) partially desolvates as it approaches the interface, and we examine how this difference in solvation behavior is manifested in the electronic structure and chemistry of the two ions.

Molecular dynamics simulations predict an accelerated dissociation of H2CO3 at the air–water interface

Ab initio molecular dynamics simulations of carbonic acid (H₂CO₃) at the air–water interface yield a lower dissociation barrier than in bulk water.

Ionization of Nitric Acid on Crystalline Ice: The Role of Defects and Collective Proton Movement

Ionization of nitric acid (HNO3) on a model ice surface is studied using ab initio molecular dynamics at temperatures of 200 and 40 K with a surface slab model that consists of the ideal ice basal plane with locally optimized and annealed defects. Pico- and subpicosecond ionization of nitric acid can be achieved in the defect sites. Key features of the rapid ionization are (a) the efficient solvation of the polyatomic nitrate anion, by stealing hydrogen bonds from the weakened hydrogen bonds at defect sites, (b) formation of contact ion pairs to stable "presolvated" molecular species that are present at the defects,

Production of gas phase NO₂ and halogens from the photochemical oxidation of aqueous mixtures of sea salt and nitrate ions at room temperature

Nitrate and halide ions coexist in a number of environmental systems, including sea salt particles, the Arctic snowpack, and alkaline dry lakes. However, little is known about potential synergisms between halide and nitrate ions. The effect of sea salt on NO(3)(-) photochemistry at 311 nm was investigated at 298 K using thin films of deliquesced NaNO(3)-synthetic sea salt mixtures. Gas phase NO(2), NO, and halogen products were measured as a function of photolysis time using NO(y) chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). The production of NO(2) increases with the halide-to-nitrate ratio, and is similar to that for mixtures of NaCl with NaNO(3). Gas phase halogen production also increased with the halide-to-nitrate ratio, consistent with NO(3)(-) photolysis yielding OH which oxidizes halide ions in the film. Yields of gas phase halogens and NO were strongly dependent on the acidity of the solution, while that of NO(2) was not. An additional halogen formation mechanism in the dark involving molecular HNO(3) is proposed that may be important in other systems such as reactions on surfaces. These studies show that the yield of Br(2) relative to NO(2) during photolysis of halide-nitrate mixtures could be as high as 35% under some atmospheric conditions.

Anions dramatically enhance proton transfer through aqueous interfaces

Proton transfer (PT) through and across aqueous interfaces is a fundamental process in chemistry and biology. Notwithstanding its importance, it is not generally realized that interfacial PT is quite different from conventional PT in bulk water. Here we show that, in contrast with the behavior of strong nitric acid in aqueous solution, gas-phase HNO(3) does not dissociate upon collision with the surface of water unless a few ions (> 1 per 10(6) H(2)O) are present. By applying online electrospray ionization mass spectrometry to monitor in situ the surface of aqueous jets exposed to HNO(3(g)) beams we found that NO(3)(-) production increases dramatically on > 30-μM inert electrolyte solutions. We also performed quantum mechanical calculations confirming that the sizable barrier hindering HNO(3) dissociation on the surface of small water clusters is drastically lowered in the presence of anions. Anions electrostatically assist in drawing the proton away from NO(3)(-) lingering outside the cluster, whose incorporation is hampered by the energetic cost of opening a cavity therein. Present results provide both direct experimental evidence and mechanistic insights on the counterintuitive slowness of PT at water-hydrophobe boundaries and its remarkable sensitivity to electrostatic effects.

Quantum proton transfer in hydrated sulfuric acid clusters: a perspective from semiempirical path integral simulations

We have carried out path-integral molecular dynamics simulations for hydrated sulfuric acid clusters to understand acid-dissociation and hydrogen-bonded structural rearrangement processes in these clusters from a quantum mechanical viewpoint. The simulations were performed using the PM6 semiempirical electronic structure level whose parameters were modified on the basis of the specific reaction parameters strategy so that relative energies of optimized structures, as well as water binding energies reproduce ab initio and density-functional theory calculations. We have found that the acid dissociation processes, first and second deprotonation, effectively occur in a hydrated cluster with a specific cluster size. The mechanisms of the proton-transfer processes were analyzed in detail and it was found that the distance between O in sulfuric acid and O in the proton-accepting water is playing an important role. We also found that the water coordination number of the poton-accepting water is important in the proton-transfer processes.

Acid epimerization of 20-keto pregnane glycosides is determined by 2D-NMR spectroscopy

Carbohydrates influence many essential biological events such as apoptosis, differentiation, tumor metastasis, cancer, neurobiology, immunology, development, host-pathogen interactions, diabetes, signal transduction, protein folding, and many other contexts. We now report on the structure determination of pregnane glycosides isolated from the aerial parts of Ceropegia fusca Bolle (Asclepiadaceae). The observation of cicatrizant, vulnerary and cytostatic activities in some humans and animals of Ceropegia fusca Bolle, a species endemic to the Canary Islands, encouraged us to begin a pharmacological study to determine their exact therapeutic properties. High resolution (1)H-NMR spectra of pregnane glycosides very often display well-resolved signals that can be used as starting points in several selective NMR experiments to study scalar (J coupling), and dipolar (NOE) interactions. ROESY is especially suited for molecules such that ωτ(c) ~ 1, where τ(c) are the motional correlation times and ω is the angular frequency. In these cases the NOE is nearly zero, while the rotating-frame Overhauser effect spectroscopy (ROESY) is always positive and increases monotonically for increasing values of τ(c). The ROESY shows dipolar interactions cross peaks even in medium-sized molecules which are helpful in unambiguous assignment of all the interglycosidic linkages. Selective excitation was carried out using a double pulsed-field gradient spin-echo sequence (DPFGSE) in which 180° Gaussian pulses are sandwiched between sine shaped z-gradients. Scalar interactions were studied by homonuclear DPFGSE-COSY and DPFGSE-TOCSY experiments, while DPFGSE-ROESY was used to monitor the spatial environment of the selectively excited proton. Dipolar interactions between nuclei close in space can be detected by the 1D GROESY experiment, which is a one-dimensional counterpart of the 2D ROESY method. The C-12 and C-17 configurations were determined by ROESY experiments.