Unraveling nucleation pathway in methane clathrate formation

Methane clathrates are widespread on the ocean floor of the Earth. A better understanding of methane clathrate formation has important implications for natural-gas exploitation, storage, and transportation. A key step toward understanding clathrate formation is hydrate nucleation, which has been suggested to involve multiple evolution pathways. Herein, a unique nucleation/growth pathway for methane clathrate formation has been identified by analyzing the trajectories of large-scale molecular dynamics (MD) simulations. In particular, ternary water-ring aggregations (TWRAs) have been identified as fundamental structures for characterizing the nucleation pathway. Based on this nucleation pathway, the critical nucleus size and nucleation timescale can be quantitatively determined. Specifically, a methane hydration layer compression/shedding process is observed to be the critical step in (and driving) the nucleation/growth pathway, which is manifested through overlapping/compression of the surrounding hydration layers of the methane molecules, followed by detachment (shedding) of the hydration layer. As such, an effective way to control methane hydrate nucleation is to alter the hydration layer compression/shedding process during the course of nucleation.

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. In this Perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this Perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemistry generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitization. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the kinetics and thermodynamics of photoinduced oxidation reactions at aqueous interfaces and analyze a number of challenges that need to be addressed in future studies.

Computational Prediction of Novel Ice Phases: A Perspective

Although computational prediction of new ice phases is a niche field in water science, the scientific subject itself is representative of two important areas in physical chemistry, namely, statistical thermodynamics and molecular simulations. The prediction of a variety of novel ice phases has also attracted general public interest since the 1980s. In particular, the prediction of low-dimensional ice phases has gained momentum since the confirmation of a number of low-dimensional "computer ice" phases in the laboratory over the past decade. In this Perspective, the research advancements in computational prediction of novel ice phases over the past few years are reviewed. Particular attention is placed on new ice phases whose physical properties or dimensional structures are distinctly different from conventional bulk ices. Specific topics include the (i) formation of superionic ices, (ii) electrofreezing of water under high pressure and in a high external electric field, (iii) prediction of low-density porous ice at strongly negative pressure, (iv) ab initio computational study of two-dimensional (2D) ice under nanoscale confinement, and (v) 2D ices formed on a solid surface near ambient temperature without nanoscale confinement. Clearly, the formation of most of these novel ice phases demands certain extreme conditions. Ongoing challenges and new opportunities for predicting new ice phases from either classical molecular dynamics simulation or high-level ab initio computation are discussed.

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. The physico-chemical aspects of these processes, such as the structure, dynamics and thermodynamics of adsorption and solvation processes at aqueous interfaces, are also discussed. We also present the basic theories intended to explain interface catalysis, followed by the results of advanced ab initio molecular-dynamics simulations. Although some topics addressed here have already been the focus of previous reviews, we aim at highlighting their interconnection across diverse disciplines, providing a common perspective that would help us to identify the most fundamental issues still incompletely understood in this fast-moving field.

HIOx-IONO2 Dynamics at the Air-Water Interface: Revealing the Existence of a Halogen Bond at the Atmospheric Aerosol Surface

Iodine is enriched in marine aerosols, particularly in coastal mid-latitude atmospheric environments, where it initiates the formation of new aerosol particles with iodic acid (HIO₃) composition. However, particle formation in polluted and semipolluted locations is inhibited when the iodine monoxide radical (IO) is intercepted by NO₂ to form the iodine nitrate (IONO₂). The primary fate of IONO₂ is believed to be, besides photolysis, uptake by aerosol surfaces, leading to particulate iodine activation. Herein we have performed Born-Oppenheimer molecular dynamics (BOMD) simulations and gas-phase quantum chemical calculations to study the iodine acids-iodine nitrate [HIO_x (x = 2 and 3)-IONO₂] dynamics at the air-water interface modeled by a water droplet of 191 water molecules. The results indicate that IONO₂ does not react directly with these iodine acids, but forms an unusual kind of interaction with them within a few picoseconds, which is characterized as halogen bonding. The halogen bond-driven HIO₃-IONO₂ complex at the air-water interface undergoes deprotonation and exists as IO₃^--IONO₂ anion, whereas the HIO₂-IONO₂ complex does not exhibit any proton loss to the interfacial water molecules. The gas-phase quantum chemical calculations suggest that the HIO₃-IONO₂ and HIO₂-IONO₂ complexes have appreciable stabilization energies, which are significantly enhanced upon deprotonation of iodine acids, indicating that these halogen bonds are fairly stable. These IONO₂-induced halogen bonds explain the rapid loss of IONO₂ to background aerosol. Moreover, they appear to work against iodide formation. Thus, they may play an important role in enhancing the amount of atmospherically nonrecyclable iodine (iodate) in marine aerosol.

Energetic Properties, Spectroscopy, and Reactivity of NF3O

The heats of formation of NF₃O and similar C, S, and Si systems are predicted using the accurate composite computational chemistry Feller-Peterson-Dixon (FPD) method. The harmonic vibrational frequencies at the CCSD(T)/aug-cc-pVTZ level are reported and compared to the experimental values for NF₃O, its isoelectronic species CF₃O^- and NF₄⁺, and NF₃. The infrared intensities were calculated at the MP2/aug-cc-pVTZ level and show that the infrared absorption is predicted to be like those of CF₂Cl₂ and SF₆ within a factor of ∼2. The calculated heats of formation are in good agreement with the available experimental values. These heats of formation are used to calculate a range of bond dissociation energies (BDEs). It is predicted that NF₃O is unlikely to decompose either thermally or photolytically in the troposphere. The potential energy curves for the decomposition of NF₃O to NF₂O + F are all repulsive, as are the channels to form NF₃ and either O³P or O¹D. The predicted persistence of NF₃O in the troposphere is attributed to the high barrier of its reaction with the OH radical and that light with the wavelength needed for its photodissociation will not reach the troposphere. Reliable experimental measurements of the global warming potential of NF₃O are needed to confirm our predictions that NF₃O is like NF₃ in this respect.

New Insights into the Stability of Anhydrous 2H-Imidazolium Fluoride and its High Dissolution Capability toward a Strongly Hydrogen-Bonded Compound

Fluorides have been widely applied in pharmaceutical, medicinal, and materials science as well as in fine chemical manufacturing. The performance of fluorides, however, can be markedly affected by the water content. One previous study (Maiti, A.; et al. Phys. Chem. Chem. Phys. 2008, 10, 5050) suggested that anhydrous 1,3-dimethylimidazolium fluoride ([DMIm]F) was unstable since the fluoride undergoes a self-decomposition reaction. Herein we first show quantum-chemical calculation evidence that although gas-phase [DMIm]F is unstable, the bulk phase of anhydrous [DMIm]F is quite stable. We then demonstrate the successful synthesis of the anhydrous [DMIm]F compound via the reaction between 1,3-dimethylimidazolium iodide and silver fluoride. Importantly, we find that anhydrous [DMIm]F possesses a high dissolution capability toward 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), although it is known that TATB is hardly dissolved in many common organic solvents. Our Born-Oppenheimer molecular dynamics (BOMD) simulations further show that the high dissolving ability of anhydrous [DMIm]F toward TATB can be attributed to the chemical reaction between the F^- anion and the TATB molecules, which disrupts the strong hydrogen-bonding interaction among the TATB molecules. Alternatively, water molecules in hydrous [DMIm]F tend to form a hydration layer around the F^- anion, thereby preventing F^- from reacting with the TATB molecule. This result explains why TATB is barely dissolved in hydrous [DMIm]F.

Multiple Stable Isoprene-Ozone Complexes Reveal Complex Entrance Channel Dynamics in the Isoprene + Ozone Reaction

Accurately characterizing isoprene ozonolysis continues to challenge atmospheric chemists. The reaction is believed to be a spontaneous, concerted cycloaddition. However, little information is available about the entrance channel and isoprene-ozone complexes thought to define the long-range portion of the reaction coordinate. Our coupled cluster and auxiliary field quantum Monte Carlo calculations predict multiple stable isoprene-ozone van der Waals complexes for trans-isoprene in the gas phase with moderate association energies. These results indicate that long-range dynamics in the isoprene-ozone entrance channel can impact the overall reaction in the troposphere and provide the spectroscopic information necessary to extend the microwave characterization of isoprene ozonolysis to prereactive complexes. At the air-water interface, Born-Oppenheimer molecular dynamics simulations indicate that the cycloaddition reaction between ozone and trans-isoprene follows a stepwise mechanism, which is quite distinct from our proposed gas-phase mechanism and occurs on a femtosecond time scale. The stepwise nature of isoprene ozonolysis on the aqueous surface is more consistent with the DeMore mechanism than with the Criegee mechanism suggested by the gas-phase calculations, suggesting that the reaction media may play an important role in the reaction. Overall, these predictions aim to provide a missing fundamental piece of molecular insight into isoprene ozonolysis, which has broad tropospheric implications due to its critical role as a nighttime source of hydroxyl radicals.

Adsorption and isomerization of glyoxal and methylglyoxal at the air/hydroxylated silica surface

We present results from molecular dynamics simulations coupled with enhanced sampling techniques on the adsorption and isomerization of glyoxal (GL) and methylglyoxal (MG) at the air/hydroxylated silica (α-Quartz) interface. GL and MG are two organic compounds present in the atmosphere as oxidation products of both biogenic and anthropogenic precursors. By adsorption and hydration on liquid droplets or wetted dust particles, they can enable aerosol growth in the atmosphere. Moreover, thanks to the different polar characters of their trans and cis conformers, GL and MG have been suggested as possible molecular switches capable of responding to changes in solvent polarity. Here, we show that the hydroxylated silica surface does not significantly catalyze the trans-to-cis isomerization, but it stabilizes the cis-isomers, indicating a higher interfacial cis/trans relative concentration compared to the gas phase. Moreover, adsorbed GL prefers to lie parallel on the silica surface, while adsorbed MG shows a tilted orientation. In particular, we report the aldehyde group pointing upward (downward) to the gas phase (to the silica surface) in trans-MG (cis-MG). These results will help in the rationalization of upcoming experimental and modeling work on the adsorption of ketonic compounds on dust aerosols, while it clarifies the catalytic role of the solid substrate surface in promoting conformational changes.

Photochemistry from low-lying states of HOSO. J Chem Phys 2020;152:134302. [PMID: 32268736 DOI: 10.1063/5.0001867]

Using configuration interaction ab initio methods, the evolution of the lowest electronic states of singlet and triplet spin multiplicities of HOSO⁺ along the stretching and bending coordinates of is investigated. Equilibrium geometries, rotational constants, and harmonic vibrational frequencies of the lowest electronic states are calculated, i.e., X¹A', 1¹A″, 1³A', and 1³A″. The global minimum of the 1¹A″ state is located below the first dissociation limit and its calculated lifetime is predicted to be 0.40 µs, making it suitable for detection by laser-induced fluorescence. According to the potential energy surfaces, HOSO⁺ should produce SO₂ ⁺ and H after ultraviolet photon absorption to the 2¹A' state. This work opens the door to investigate the branching ratio and the production rates of SO₂ ⁺, SO⁺, and OH from HOSO⁺. These insights can help understand the SO₂ cycle in the earth's atmosphere and its effect on cooling our planet.

Hydration, Solvation, and Isomerization of Methylglyoxal at the Air/Water Interface: New Mechanistic Pathways

Aqueous-phase processing of methylglyoxal (MG) has been suggested to play a key role in the formation of secondary organic aerosols and catalyze particle growth in the atmosphere. However, the details of these processes remain speculative owing to the lack of a complete description of the physicochemical behavior of MG on atmospheric aerosols. Here, the solvation and hydrolysis of MG at the air/liquid water interface is studied via classical and first-principles molecular dynamics simulations combined with free-energy methods. Our results reveal that the polarity of the water solvent catalyzed the trans-to-cis isomerization of MG at the air/liquid water interface relative to the gas phase. Despite the presence of a hydrophobic group, MG often solvates with both the ketone and methyl groups parallel to the water interface. Analysis of the instantaneous water surface reveals that when MG is in the trans state, the methyl group repels interfacial water to maintain the planarity of the molecule, indicating that lateral and temporal inhomogeneities of interfacial environments are important for fully characterizing the solvation of MG. The counterintuitive behavior of the hydrophobic group is ascribed to a tendency to maximize the number of hydrogen bonds between MG and interfacial water while minimizing the torsional free energy. This drives MG hydration, and our simulations indicate that the formation of MG diol is catalyzed at the air/liquid water interface compared to the gas phase and occurs through nucleophilic attack of water on the carbonyl carbon.

Evidence of the Elusive Gold-Induced Non-classical Hydrogen Bonding in Aqueous Environments

The ability of a gold ion to act as a proton acceptor in hydrogen bonding continues to remain an open question. Heavy-atom effects and secondary competitive interactions in gold complexes make it challenging to precisely establish the identity of gold-ion-induced hydrogen bonding via experimental techniques. In such situations, computational chemistry may play an important role. Herein we have performed Born-Oppenheimer molecular dynamics simulations to study the behavior of [Au(CH₃)₂)]^- in bulk and interfacial aqueous environments. The simulation results suggest that the [Au(CH₃)₂)]^- complex forms one and two gold-ion-induced hydrogen bonds with the water molecules in interfacial and bulk environments, respectively. The calculated probabilities of key hydrogen-bonded configurations of [Au(CH₃)₂)]^-, combined distribution functions, and diffusion coefficients further support this unusual hydrogen-bonding interaction. In summary, the present results suggest that gold-ion-induced hydrogen bonding in an actual solvent environment may be feasible. These results will improve our understanding about the role of weak interactions in transition metal complexes and, thus, will have implications in catalysis and supramolecular assemblies.

Dihalogenated Methylperoxy Radicals: Spectroscopic Characterization and Photodecomposition by Release of HO

Two atmospherically relevant dihalogenated methylperoxy radicals CHX₂ OO^. (X=F and Cl) have been generated through O₂ -oxidation of the corresponding alkyl radicals CHX₂ ^. in the gas phase. The IR spectroscopic characterization of both radicals in cryogenic Ar- and N₂ -matrices (15 K) is supported by ¹⁸ O-labeling and ab initio calculations at the UCCSD(T)/aug-cc-pVTZ level. Upon 266 nm laser irradiation, both radicals decompose mainly by releasing hydroxyl radicals (→HO^. +X₂ CO) via the intermediacy of intriguing α-hydroperoxyalkyl radicals (^. CX₂ OOH), implying that the photooxidation of dihalogenated hydrocarbons might serve as important sources of HO^. radicals in the atmosphere.

A possible unaccounted source of atmospheric sulfate formation: amine-promoted hydrolysis and non-radical oxidation of sulfur dioxide

Numerous field and laboratory studies have shown that amines, especially dimethylamine (DMA), are crucial to atmospheric particulate nucleation. However, the molecular mechanism by which amines lead to atmospheric particulate formation is still not fully understood. Herein, we show that DMA molecules can also promote the conversion of atmospheric SO2 to sulfate. Based on ab initio simulations, we find that in the presence of DMA, the originally endothermic and kinetically unfavourable hydrolysis reaction between gaseous SO2 and water vapour can become both exothermic and kinetically favourable. The resulting product, bisulfite NH2(CH3)2 +·HSO3 -, can be readily oxidized by ozone under ambient conditions. Kinetic analysis suggests that the hydrolysis rate of SO2 and DMA with water vapour becomes highly competitive with and comparable to the rate of the reaction between SO2 and OH·, especially under the conditions of heavily polluted air and high humidity. We also find that the oxidants NO2 and N2O5 (whose role in sulfate formation is still under debate) appear to play a much less significant role than ozone in the aqueous oxidation reaction of SO2. The newly identified oxidation mechanism of SO2 promoted by both DMA and O3 provides another important new source of sulfate formation in the atmosphere.

Unraveling Molecular Mechanism on Dilute Surfactant Solution Controlled Ice Recrystallization

Ice recrystallization (IR) is ubiquitous, playing an important role in many areas of science, such as cryobiology, food science, and atmospheric physics. However, controllable ice recrystallization remains a challenging task largely due to an incomplete understanding of the physical mechanism associated with ice recrystallization. Herein, we explore the molecular mechanism underlying the controlling of ice recrystallization by using different small amphiphilic molecules (surfactants) through joint experimental measurements and molecular dynamics simulation. Our experiment shows that in nonionic/zwitterionic surfactant solutions, the mean size of the recrystallized ice grains increases monotonically with the concentration of surfactants, whereas in the ionic surfactant solutions, the mean size of the recrystallized ice grains tends to increase first and then decrease with increasing the concentration, yielding a peak typically at ∼5 μM. Further sequential ice affinity purification experiments and molecular dynamics simulations show that the surfactants actually do not bind to ice directly. Rather, the different spatial distributions of counter ions and molecular surfactants in the interfacial regions (ice-water interface and water-air interface) and bulk region can markedly affect the mean size of the recrystallized ice grain.

Spectroscopic characterization of the first excited state and photochemistry of the HO3 radical

We report the one-dimensional cuts of the six-dimensional potential energy surfaces (PESs) of the ground and lowest doublet and quartet electronic states of trans-HO₃ at the MRCI-F12/aug-cc-pVTZ level of theory. Theoretical calculations predict that the first excited state (A²A) presents a real minimum on its PES and possesses a nonplanar structure. The adiabatic excitation energy at the MRCI+Q and MRCI-F12 levels shows that the A²A state lies in the near-infrared region. Both the transition dipole moment and the oscillator strength were predicted to be weak, which suggests that photodissociation of HO₃ to produce OH and O₂ after UV-Vis absorption is not a plausible mechanism. The harmonic vibrational frequencies and rotational constants of the weakly bound complex OH-O₂ in the two electronic states were predicted to help in its detection. Our PES shows that the reactions of H + O₃ or HO₂ + O in their ground states do not lead to trans-HO₃ in its ground electronic state if one of the component fragments, i.e., HO₂(A²A') + O(³P) or H(²S) + O₃(³B₂), is excited.

Two-dimensional semiconducting Lu2CT2 (T = F, OH) MXene with low work function and high carrier mobility

As a new family of two-dimensional materials, MXenes have attracted increasing attention in recent years due to their widespread potential applications. In contrast to early transition metals in convention, here we expand the M element of MXene to the rare earth element lutetium. Based on the first-principles density functional calculations, the bare lutetium-based carbide MXene Lu₂C is determined to be stabilized in the T-type configuration. Furthermore, both fluorine and hydroxyl terminated configurations are found to be semiconductors, and their band gaps are suitable for use in semiconductors and visible and near-infrared optical devices. The Lu₂C(OH)₂ configuration shows a direct band gap and possesses an ultralow work function of 1.4 eV. Both Lu₂CT₂ (T = F, OH) MXenes exhibit high carrier mobilities. Particularly, the electron mobility of the Lu₂C(OH)₂ MXene is found to be anisotropic at room temperature, with values as high as 95.19 × 10³ and 217.1 × 10³ cm² V^-1·s^-1 in the zigzag and armchair directions, respectively, which makes Lu₂C(OH)₂ a promising material for nanodevices. Based on these predicted properties, our work widens the range of MXene materials and their applications in semiconducting devices.