Computer simulation study of HNO3 dissociation in aqueous clusters

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.

High throughput approach to investigating ternary solvents of aqueous non-stoichiometric protic ionic liquids

The use of ionic liquids (ILs) is limited for many applications due to their cost and/or viscosity. An efficient solution is to make mixtures of ILs with molecular solvents. However, it is well known that there are a large number of possible cation and anion combinations resulting in ILs, and this becomes a vast number when these are then combined with a molecular solvent. Therefore, we need structure-property relationships to design new IL-molecular solvent systems. In this work we have applied high throughput methods to investigate IL containing solutions to provide systematic data of a broad compositional space. We have principally focused on the surface tension, apparent pH and liquid nanostructure to identify potential self-assembly and protein stabilizing ability of solvent systems. Non-stoichiometric and aqueous IL-solvents were prepared in a high-throughput manner based on a deliberate experimental design approach such that 26 samples were prepared for each cation-anion-water combination. A selection of 8 protic ionic liquids (PILs) were used as starting materials, comprising ethanol-, ethyl-, butyl-, and pentylammonium cations combined with formate, acetate and nitrate anions. This resulted in a total of 208 different solvent systems. The measured solvent properties showed different trends in base-rich and acid-rich solvent combinations. Surface tensions of base-rich samples exhibited a relatively linear relationship with increasing excess amine, while acid-rich samples were more dominantly affected by the change in water content. Liquid nanostructure of acid-rich samples was retained upon water dilution, whereas a significant SAXS peak shift towards lower scattering angles was observed in the presence of excess amines, indicating larger nanosized aggregates were forming. The design of experiment approach used here is considered to be applicable to any multi-component solvent compositional space due to its suitability in using small data sets to cover large compositional spaces, and hence can be employed to decrease the time and sample quantities required.

Aggregation-induced chemical reactions: acid dissociation in growing water clusters

Understanding chemical reactivity at ultracold conditions, thus enabling molecular syntheses via interstellar and atmospheric processes, is a key issue in cryochemistry. In particular, acid dissociation and proton transfer reactions are ubiquitous in aqueous microsolvation environments. Here, the full dissociation of a HCl molecule upon stepwise solvation by a small number of water molecules at low temperatures, as relevant to helium nanodroplet isolation (HENDI) spectroscopy, is analyzed in mechanistic detail. It is found that upon successive aggregation of HCl with H(2)O molecules, a series of cyclic heteromolecular structures, up to and including HCl(H(2)O)(3), are initially obtained before a precursor state for dissociation, HCl(H(2)O)(3)···H(2)O, is observed upon addition of a fourth water molecule. The latter partially aggregated structure can be viewed as an "activated species", which readily leads to dissociation of HCl and to the formation of a solvent-shared ion pair, H(3)O(+)(H(2)O)(3)Cl(-). Overall, the process is mostly downhill in potential energy, and, in addition, small remaining barriers are overcome by using kinetic energy released as a result of forming hydrogen bonds due to aggregation. The associated barrier is not ruled by thermal equilibrium but is generated by athermal non-equilibrium dynamics. These "aggregation-induced chemical reactions" are expected to be of broad relevance to chemistry at ultralow temperature much beyond HENDI spectroscopy.

Theoretical study of the dissociation of nitric acid at a model aqueous surface

The issue of acid dissociation of nitric acid at an aqueous surface is relevant in various portions of the atmosphere in connection with ozone depletion. This proton-transfer reaction is studied here via electronic structure calculations at the HF/SBK+(d) level of theory on the HNO(3) x (H(2)O)(3) model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters with an increasing degree of solvation of the nitrate group. Free energy estimates for all the cases examined favor undissociated, molecular nitric acid over the 0-300 K temperature range, including that relevant for the upper troposphere, where it is connected to the issue of the mechanism of nitric acid uptake by water ice aerosols. The presence of molecular HNO(3) at 300 K at the surface is further supported by vibrational band assignments in good agreement with a very recent surface-sensitive vibrational spectroscopy study of diluted HNO(3)/H(2)O solutions.

Photochemistry of Molecules Relevant to the Atmosphere: Photodissociation of Nitric Acid in the Gas Phase

This Minireview gives an account of the photochemical decay of nitric acid HNO3 in the gas phase, which has been well investigated under bulk and molecular-beam conditions. Due to the importance of this molecule in atmospheric chemistry, attention was paid to the irradiation regions around 300 and 200 nm, where solar photolysis of HNO3 is expected to be particularly efficient. While the low-energy region is characterized by the products OH and NO2, the high-energy region gives rise to a variety of photochemical decay pathways, dominated by channels which lead to the products HONO + O in different electronic states.