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

Abstract

Recent experiments and theoretical calculations have shown that HNO 3 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 HNO 3 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 HNO 3 + hν → NO 2 + OH, leading to 2 × 10 −6 s −1 , about twice the value in the gas phase. Finally, we describe the reaction HNO 3 + OH → NO 3 + H 2 O 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 (HNO 3 )(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 cm 3 •molecule −1 •s −1 . The atmospheric significance of all these results is discussed.

Reaction between a NO2 Dimer and Dissolved SO2: A New Mechanism for ONSO3– Formation and its Fate in Aerosol

“…The next step is another proton transfer between the water molecule and NO 3 . Although the formation of H 3 O + and NO 3 – can be observed in other trajectories (Figures S4 and S5), the HNO 3 molecule formed at the surface is stable without dissociation observed in this trajectory, which is consistent with its weak acid characteristic . A similar reaction mechanism is observed for the anti conformation (Figure S3).…”

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

“…SFG, XPS, and our calculations thus concur that nitric and formic acids are weaker at the interface than in the bulk. While nitric acid remains a strong acid at the interface, the small interfacial undissociated fraction has important consequences for atmospherically relevant photochemical reactions …”

Effect of diluent on the extraction of europium(iii) and americium(iii) with N,N,N′,N′-tetraoctyl diglycolamide (TODGA)

“…54 There is some evidence from experimental spectroscopic data (mostly Raman and Infra-Red), Nuclear Magnetic Resonance data and molecular dynamic simulations for the existence of weak HNO 0 3 ion pairs in relativity concentrated aqueous solutions of nitric acid. [56][57][58][59][60][61] Comprehensive review of the experimental data is available 62 and according to this review values of HNO 0 3 stability (dissociation) constants at innite dilution (K 0 a ) vary signicantly, from 10 to 40. Such a large difference in K 0 a can be explained by the problems associated with experimental or theoretical determination of stability constants of weak ion pairs and strongly indicates that uncertainty associated with this value is large.…”