The Effect of Electric Fields on Oxidization Processes at the Air-Water Interface.

At the air-water interface, many reactions are accelerated, sometimes by several orders of magnitude. This phenomenon has proved to be particularly important in water microdroplets, where the spontaneous oxidation of many species stable in bulk has been experimentally demonstrated. Different theories have been proposed to explain this finding, but it is currently believed that the role of interfacial electric fields is key.

The Structure of Carbon Dioxide at the Air-Water Interface and its Chemical Implications.

The efficient reduction of CO into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO with promising results.

Triplet State Radical Chemistry: Significance of the Reaction of SO with HCOOH and HNO.

The triplet excited states of sulfur dioxide can be accessed in the UV region and have a lifetime large enough that they can react with atmospheric trace gases. In this work, we report high level ab initio calculations for the reaction of the aB and bA excited states of SO with weak and strong acidic species such as HCOOH and HNO, aimed to extend the chemistry reported in previous studies with nonacidic H atoms (water and alkanes). The reactions investigated in this work are very versatile and follow different kinds of mechanisms, namely, proton-coupled electron transfer () and conventional hydrogen atom transfer () mechanisms.

Reactivity of Monoethanolamine at the Air-Water Interface and Implications for CO Capture.

The development of CO-capture technologies is key to mitigating climate change due to anthropogenic greenhouse gas emissions. These cover a number of technologies designed to reduce the level of CO emitted into the atmosphere or to eliminate CO from ambient air. In this context, amine-based sorbents in aqueous solutions are broadly used in most advanced separation techniques currently implemented in industrial applications.

Electrostatics and Chemical Reactivity at the Air-Water Interface.

It has been recently discovered that chemical reactions at aqueous interfaces can be orders of magnitude faster compared to conventional bulk phase reactions, but despite its wide-ranging implications, which extend from atmospheric to synthetic chemistry or technological applications, the phenomenon is still incompletely understood. The role of strong electric fields due to space asymmetry and the accumulation of ions at the interface has been claimed as a possible cause from some experiments, but the reorganization of the solvent around the reactive system should provide even greater additional electrostatic contributions that have not yet been analyzed. In this study, with the help of first-principles molecular dynamics simulations, we go deeper into this issue by a careful assessment of solvation electrostatics at the air-water interface.

Disentangling reaction rate acceleration in microdroplets.

We have investigated the origin of the unexpected, recently discovered phenomenon of reaction rate acceleration in water microdroplets relative to bulk water. Acceleration factors for reactions of atmospheric and synthetic relevance can be dissected into elementary contributions thanks to the original and versatile kinetic model. The microdroplet is partitioned in two sub-volumes, the surface and the interior, operating as interconnected chemical reactors in the fast diffusion regime.

Fast Sulfate Formation Initiated by the Spin-Forbidden Excitation of SO at the Air-Water Interface.

The multiphase oxidation of SO to sulfate in aerosol particles is a key process in atmospheric chemistry. However, there is a large gap between the observed and simulated sulfate concentrations during severe haze events. To fill in the gaps in understanding SO oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO to its triplet states using UVA photons at an air-water interface, followed by reactions with water and O that facilitate the rapid formation of sulfate.

Photosensitization mechanisms at the air-water interface of aqueous aerosols.

Photosensitization reactions are believed to provide a key contribution to the overall oxidation chemistry of the Earth's atmosphere. Generally, these processes take place on the surface of aqueous aerosols, where organic surfactants accumulate and react, either directly or indirectly, with the activated photosensitizer. However, the mechanisms involved in these important interfacial phenomena are still poorly known.

Tight electrostatic regulation of the OH production rate from the photolysis of hydrogen peroxide adsorbed on surfaces.

Recently, experimental and theoretical works have reported evidence indicating that photochemical processes may significantly be accelerated at heterogeneous interfaces, although a complete understanding of the phenomenon is still lacking. We have carried out a theoretical study of interface and surface effects on the photochemistry of hydrogen peroxide (HO) using high-level ab initio methods and a variety of models. Hydrogen peroxide is an important oxidant that decomposes in the presence of light, forming two OH radicals.

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

Recent experiments and theoretical calculations have shown that HNO 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 have focused on the acid dissociation mechanism.

The Aqueous Surface as an Efficient Transient Stop for the Reactivity of Gaseous NO in Liquid Water.

The heterogeneous reaction of NO with water on diverse surfaces is broadly considered as a possible source of atmospheric HONO in dark conditions, but the associated mechanisms are not fully understood. We report data from first-principles simulations showing that the lifetime of the putative reactive NO dimer on the surface of pure water droplets is too small to host the whole process. One infers from our results that the hydrolysis of NO in clouds must be catalyzed by organic or inorganic species adsorbed on the droplets.

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.

Photoinduced Oxidation Reactions at the Air-Water Interface.

Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydrophobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemical and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions.

Isoprene Reactivity on Water Surfaces from ab initio QM/MM Molecular Dynamics Simulations.

Isoprene is the most abundant volatile organic compound in the atmosphere after methane. While gas-phase processes have been widely studied, the chemistry of isoprene in aqueous environments is less well known. Nevertheless, some experiments have reported unexpected reactivity at the air-water interface.

Vibrational Sum-Frequency Generation Spectroscopy in the Energy Representation from Dual-Level Molecular Dynamics Simulations.

We report a cost-effective molecular dynamics approach to calculate sum-frequency generation (SFG) vibrational spectra of molecular species at liquid interfaces in the energy representation formalism that brings together the instantaneous normal mode (INM) analysis at free-energy minima (FEM) and the dual-level free-energy perturbation (FEP) methods. This combined FEP-INM-FEM approach allows analyzing SFG spectra in terms of normal mode contributions at very-high ab initio levels, in contrast to standard time-correlation function (TCF)-based methods, from which it can be considered complementary. It is applied here to the study of the CH-stretching band of methanol at the air-water interface, which has been thoroughly studied in the literature.

Theoretical Investigation of the Photoexcited NO +H O reaction at the Air-Water Interface and Its Atmospheric Implications.

The atmospheric role of photochemical processes involving NO beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO beyond 420 nm reacts with water according to NO +H O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10 -3.

Vibrational Spectroscopy in Solution through Perturbative ab Initio Molecular Dynamics Simulations.

We have developed a method that allows computing the vibrational spectra at a high quantum mechanical level for molecules in solution or other complex systems. The method is based on the use of configurational samplings from combined QM/MM molecular dynamics simulations and the use of perturbation theory to calculate accurate molecular properties. Such calculations provide in addition accurate free energy gradient vectors and Hessian matrices and thus open the door for the characterization of stationary points in free energy landscapes and the study of chemical reaction mechanisms in large disordered systems.

Triplet state promoted reaction of SO with HO by competition between proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) processes.

The SO2 + H2O reaction is proposed to be the starting process for the oxidation of sulfur dioxide to sulfate in liquid water, although the thermal reaction displays a high activation barrier. Recent studies have suggested that the reaction can be promoted by light absorption in the near UV. We report ab initio calculations showing that the SO2 excited triplet state is unstable in water, as it immediately reacts with H2O through a water-assisted proton coupled electron transfer mechanism forming OH and HOSO radicals.

Photochemistry of SO at the Air-Water Interface: A Source of OH and HOSO Radicals.

The photochemistry of sulfur dioxide in the near UV-vis energy range has been studied in aqueous environments. The combination of previously reported experimental measurements with accurate quantum chemical calculations achieved in this work reveals that the process represents an important source of OH radicals in the troposphere. It implicates the reaction of the lowest triplet excited state of SO with a water molecule.

Cost-Effective Method for Free-Energy Minimization in Complex Systems with Elaborated Ab Initio Potentials.

We describe a method to locate stationary points in the free-energy hypersurface of complex molecular systems using high-level correlated ab initio potentials. In this work, we assume a combined QM/MM description of the system although generalization to full ab initio potentials or other theoretical schemes is straightforward. The free-energy gradient (FEG) is obtained as the mean force acting on relevant nuclei using a dual level strategy.

Computational Insights into the CH Cl+OH Chemical Reaction Dynamics at the Air-Water Interface.

The reaction of methyl chloride with the hydroxyl radical OH is an important process in the troposphere. The kinetics of this reaction has been thoroughly studied in the gas phase, both experimentally and theoretically, but little is known about the effect of water on this reaction. In particular, investigating the reaction mechanism at the air-water interface is key in order to better understand the role of cloud water droplets and aerosols on the overall oxidation capacity of the troposphere.

Reaching multi-nanosecond timescales in combined QM/MM molecular dynamics simulations through parallel horsetail sampling.

We report an enhanced sampling technique that allows to reach the multi-nanosecond timescale in quantum mechanics/molecular mechanics molecular dynamics simulations. The proposed technique, called horsetail sampling, is a specific type of multiple molecular dynamics approach exhibiting high parallel efficiency. It couples a main simulation with a large number of shorter trajectories launched on independent processors at periodic time intervals.

Reactivity of aldehydes at the air-water interface. Insights from molecular dynamics simulations and ab initio calculations.

Understanding the influence of solute-solvent interactions on chemical reactivity has been a subject of intense research in the last few decades. Theoretical studies have focused on bulk solvation phenomena and a variety of models and methods have been developed that are now widely used by both theoreticians and experimentalists. Much less attention has been paid, however, to processes that occur at liquid interfaces despite the important role such interfaces play in chemistry and biology.

Amino acid capture by aqueous interfaces. Implications for biological uptake.

The interactions of natural amino acids with water-hydrophobic interfaces are central to the control of key biological processes, such as passive transport, and to the overall structure and stability of membrane proteins. We still have a very poor knowledge of these interactions, and our aim in this work is to investigate the thermochemistry and dynamics properties of simple aliphatic amino acids (glycine and valine) across a water-organic interface. The study has been carried out by means of Born-Oppenheimer molecular dynamics simulations focusing on the role that the hydrophobicity of the side chain has on the phase transfer mechanism of the amino acid.