Search for: All records

The nitrate anion (NO3−) is abundant in environmental aqueous phases, including aerosols, surface waters, and snow, where its photolysis releases nitrogen oxides back into the atmosphere. Nitrate photolysis occurs via two channels: (1) the formation of NO2 and O− and (2) the formation of NO2− and O(3P). The occurrence of two reaction channels with very low quantum yield (∼1%) highlights the critical role of the solvation environment and spin-forbidden electronic transitions, which remain unexplained at the molecular level. We investigate the two photolysis channels in water using quantum chemical calculations and first-principles molecular dynamics simulations with hybrid density functional theory and enhanced sampling. We find that spin-forbidden absorption to the triplet state (T1) is possible but occurs at a rate ∼15 times weaker than the spin-allowed transition to the singlet state (S1). A metastable solvation cage complex requires additional thermal energy to dissociate the N–O bond, allowing for recombination or non-radiative deactivation. Our results explain the temperature dependence of photolysis, linked to hydrogen bond rearrangement in the solvation shell. This work provides new molecular insights into nitrate photolysis and its low quantum yield under environmental conditions. 

Abstract Liquid water can be supercooled up to about 50~K below the melting point before undergoing homogeneous ice nucleation. Based on experimental thermodynamic observations and computer simulations it was hypothesized that below this temperature and at pressures of several kbar water undergoes a liquid-liquid phase transition (LLPT) and the transition line ends at a second critical point. However, challenges in experiments and simulations at such deep cooling leave doubts about the nature of the LLPT and the existence of the critical point.Here we use molecular dynamics simulations with a highly accurate and computationally efficient polarizable water model to establish the character of the LLPT and identify the location of the second critical point. Our microsecond-long simulations provide the first direct evidence of a well-defined moving interface between low-density and high-density water at conditions near the phase boundary. This is the ultimate proof of a first-order transition between two liquid phases with distinct free energy basins separated by a barrier, resolving a long-standing debate. These results provide new perspectives on supercooled water under pressure simulated with an accurate and realistic model suitable for studies of water in confined geological and biological environments. 

We characterise the structural properties of the quasi-liquid layer (QLL) at two low-index ice surfaces in the presence of sodium chloride (Na + /Cl − ) ions by molecular dynamics simulations. We find that the presence of a high surface density of Na + /Cl − pairs changes the surface melting behaviour from step-wise to gradual melting. The ions lead to an overall increase of the thickness and the disorder of the QLL, and to a low-temperature roughening transition of the air–ice interface. The local molecular structure of the QLL is similar to that of liquid water, and the differences between the basal and primary prismatic surface are attenuated by the presence of Na + /Cl − pairs. These changes modify the crystal growth rates of different facets and the solvation environment at the surface of sea-water ice with a potential impact on light scattering and environmental chemical reactions. 

Outstanding questions about the RNA world hypothesis for the emergence of life on Earth concern the stability and self-replication of prebiotic aqueous RNA. Recent experimental work has suggested that solid substrates and low temperatures could help resolve these issues. Herein, we use classical molecular dynamics simulations to explore the possibility that the substrate is ice itself. Simulations at −20 °C show that an eight-nucleotide single strand of RNA, initially situated in the quasiliquid layer at the air/ice interface, exhibits a robust propensity to reorient itself -- its bases turn toward the (hydrophobic) air/ice interface, while its anionic phosphodiester oxygens align with the underlying ice lattice. Kinetic analysis of hydrogen bonding indicates resistance to hydrolysis that is greater than that of an aqueous single-strand RNA at the same temperature. This enhanced resistance, in turn, could increase the opportunities for polymerization and self-copying. These findings thus offer the possibility of a role for an ancient RNA world on ice distinct from that considered in extant elaborations of the RNA world hypothesis. This work is, to the best of our knowledge, the first molecular dynamics study of RNA on ice.