Normal modes for probing the local solvation environment of nitrate anion during step wise hydration: A theoretical study

DFT Study of Microsolvated [NO3·(H2O)n]- (n = 1-12) Clusters and Molecular Dynamics Simulation of Nitrate Solution

Investigation of the process of the NO₃^- anion solvation is central to understanding the chemical and physical properties of its aqueous solutions. The importance of this topic can be seen in atmospheric chemistry, as well as in nuclear waste processing research. In this work, we used a particle swarm optimization technique driven by density functional theory to sample the potential energy surface of various microsolvated [NO₃·(H₂O)_n]^- (n = 1-12) clusters. We found that the charge transfer plays a crucial role in the stabilization of the investigated species. Moreover, by conducting ab initio molecular dynamics simulations, we showed that at low concentrations (∼0.2 M) the NO₃^- species tend to be located on the surface of water solution. We also observed that the contact ion pair K⁺-NO₃^- undergoes a fast dissociation and each of the ions is solvated separately. As a result, from our calculations, we expect that at low concentration there could be oppositely signed concentration gradients for NO₃^- and K⁺ ions in a thin water film.

Infrared spectroscopy of CO3 •-(H2O)1,2 and CO4 •-(H2O)1,2

Hydrated molecular anions are present in the atmosphere. Revealing the structure of the microsolvation is key to understanding their chemical properties. The infrared spectra of CO₃ ^•-(H₂O)_1,2 and CO₄ ^•-(H₂O)_1,2 were measured via infrared multiple photon dissociation spectroscopy in both warm and cold environments. Redshifted from the free O-H stretch frequency, broad, structured spectra were observed in the O-H stretching region for all cluster ions, which provide information on the interaction of the hydrogen atoms with the central ion. In the C-O stretching region, the spectra exhibit clear maxima, but dissociation of CO₃ ^•-(H₂O)_1,2 was surprisingly inefficient. While CO₃ ^•-(H₂O)_1,2 and CO₄ ^•-(H₂O) dissociate via loss of water, CO₂ loss is the dominant dissociation channel for CO₄ ^•-(H₂O)₂. The experimental spectra are compared to calculated spectra within the harmonic approximation and from analysis of molecular dynamics simulations. The simulations support the hypothesis that many isomers contribute to the observed spectrum at finite temperatures. The highly fluxional nature of the clusters is the main reason for the spectral broadening, while water-water hydrogen bonding seems to play a minor role in the doubly hydrated species.

The unexpected effect of aqueous ion pairs on the forbidden n →π* transition in nitrate

Aqueous nitrate is ubiquitous in the environment, found for example in stratospheric clouds, tropospheric particulate matter, rain and snow, fertilized fields, rivers and the ocean. Its photolysis is initiated by absorption into the strongly forbidden n →π* transition. Photolysis reactivates deposited nitrate, releasing nitrogen oxides, and UV light is commonly used to break down nitrate pollution. The transition is doubly forbidden unless its symmetry is broken, giving a powerful means of probing the interactions of nitrate with its environment and of using experiment to validate the results of theory. In this study we demonstrate the remarkably different effects of the addition of a series of mono- and di-valent metal chlorides on the nitrate UV transition. While they all shift the transition to shorter wavelengths, the shift changes significantly from one to another. For the monovalent series Li⁺, Na⁺, K⁺, the blue shift decreases down the column being strongest for Li⁺ and weakest for K⁺. For the divalent series Mg²⁺, Ca²⁺, Ba²⁺, the opposite effect is observed with the energy shift of Ba²⁺ being an order of magnitude larger than for Mg²⁺. The absorption intensity also changes; the addition of Na⁺ and K⁺ decrease intensity whereas Li⁺ increases intensity. For the divalent cations an increase is seen for all three members of the series Mg²⁺, Ca²⁺ and Ba²⁺. Paradoxically, the effect of addition of CaCl₂ to the solution is to decrease the environmental photolysis rate of nitrate; despite the increase in intensity, Ca²⁺ blue shifts the peak position above the tropospheric photolysis threshold around 300 nm. Using computational chemistry we conclude that the effects are due to the microscopic interactions of the nitrate anion and not continuum effects. Two microscopic mechanisms are investigated in detail, the formation of a nitrate monohydrate cluster and a contact ion pair. The contact ion pair shows the potential for significant impact on the energy and intensity of the transition.