The photochemical reaction of phenol becomes ultrafast at the air–water interface

Reaction acceleration at the surface of a levitated droplet by vapor dosing from a partner droplet

Chemical reactions in micrometer-sized droplets can be accelerated by up to six orders of magnitude. However, this acceleration factor (ratio of rate constants relative to bulk) drops to less than 10 for millimeter-sized droplets due to the reduction in surface/volume ratio. To enhance the acceleration in millimeter-sized droplets, we use a new synthesis platform that directly doses reagent vapor onto the reaction droplet surface from a second levitated droplet. Using Katritzky transamination as a model reaction, we made quantitative measurements on size-controlled vapor-dosed droplets, revealing a 31-fold increase in reaction rate constants when examining the entire droplet contents. This enhancement is attributed to a greater reaction rate constant in the droplet surface region (estimated as 105 times greater than that for the bulk). The capability for substantial reaction acceleration in large droplets highlights the potential for rapid synthesis of important chemicals at useful scales. For example, we successfully prepared 23 pyridinium salts within minutes. This efficiency positions droplets as an exceptional platform for rapid, in situ catalyst synthesis. This is illustrated by the preparation of pyridinium salts as photocatalysts and their subsequent use in mediation of amine oxidation both within the same droplet.

Building Functional Liquid-Based Interfaces: From Mechanism to Application

Functional liquid-based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid-based interfaces and provide a perspective on future research directions.

Conical Intersections at Interfaces Revealed by Phase-Cycling Interface-Specific Two-Dimensional Electronic Spectroscopy (i2D-ES)

Conical intersections (CIs) hold significant stake in manipulating and controlling photochemical reaction pathways of molecules at interfaces and surfaces by affecting molecular dynamics therein. Currently, there is no tool for characterizing CIs at interfaces and surfaces. To this end, we have developed phase-cycling interface-specific two-dimensional electronic spectroscopy (i2D-ES) and combined it with advanced computational modeling to explore nonadiabatic CI dynamics of molecules at the air/water interface. Specifically, we integrated the phase locked pump pulse pair with an interface-specific electronic probe to obtain the two-dimensional interface-specific responses. We demonstrate that the nonadiabatic transitions of an interface-active azo dye molecule that occur through the CIs at the interface have different kinetic pathways from those in the bulk water. Upon photoexcitation, two CIs are present: one from an intersection of an optically active S2 state with a dark S1 state and the other from the intersection of the progressed S1 with the ground state S0. We find that the molecular conformations in the ground state are different for interfacial molecules. The interfacial molecules are intimately correlated with the locally populated excited state S2 being farther away from the CI region. This leads to slower nonadiabatic dynamics at the interface than in bulk water. Moreover, we show that the nonadiabatic transition from the S1 dark state to the ground state is significantly longer at the interface than that in the bulk, which is likely due to the orientationally restricted configuration of the excited state at the interface. Our findings suggest that orientational configurations of molecules manipulate reaction pathways at interfaces and surfaces.

Catalyst-free selective oxidation of C(sp3)-H bonds in toluene on water

The anisotropic water interfaces provide an environment to drive various chemical reactions not seen in bulk solutions. However, catalytic reactions by the aqueous interfaces are still in their infancy, with the emphasis being on the reaction rate acceleration on water. Here, we report that the oil-water interface activates and oxidizes C(sp3)-H bonds in toluene, yielding benzaldehyde with high selectivity (>99%) and conversion (>99%) under mild, catalyst-free conditions. Collision at the interface between oil-dissolved toluene and hydroxyl radicals spontaneously generated near the water-side interfaces is responsible for the unexpectedly high selectivity. Protrusion of free OH groups from interfacial water destabilizes the transition state of the OH-addition by forming π-hydrogen bonds with toluene, while the H-abstraction remains unchanged to effectively activate C(sp3)-H bonds. Moreover, the exposed free OH groups form hydrogen bonds with the produced benzaldehyde, suppressing it from being overoxidized. Our investigation shows that the oil-water interface has considerable promise for chemoselective redox reactions on water without any catalysts.

The Role of Sum-Frequency Generation Spectroscopy in Understanding On-Surface Reactions and Dynamics in Atmospheric Model-Systems

Surfaces, both water/air and solid/water, play an important role in mediating a multitude of processes central to atmospheric chemistry, particularly in the aerosol phase. However, the study of both static and dynamic properties of surfaces is highly challenging from an experimental standpoint, leading to a lack of molecular level information about the processes that take place at these systems and how they differ from bulk. One of the few techniques that has been able to capture ultrafast surface phenomena is time-resolved sum-frequency generation (SFG) spectroscopy. Since it is both surface-specific and chemically sensitive, the extension of this spectroscopic technique to the time domain makes it possible to study dynamic processes on the femtosecond time scale. In this Perspective, we will explore recent advances made in the field both in terms of studying energy dissipation as well as chemical reactions and the role the surface geometry plays in these processes.

Protein-Mediated Changes in Membrane Fluidity and Ordering: Insights into the Molecular Mechanism and Implications for Cellular Function

Probing protein-membrane interactions is vital for understanding biological functionality for various applications such as drug development, targeted drug delivery, and creation of functional biomaterials for medical and industrial purposes. In this study, we have investigated interaction of Human Serum Albumin (HSA) with two different lipids, dipalmitoylphosphatidylglycerol (dDPPG) and dipalmitoylphosphatidylcholine (dDPPC), using Vibrational Sum Frequency Generation spectroscopy at different membrane fluidity values. In the liquid-expanded (LE) state of the lipid, HSA (at pH 3.5) deeply intercalated lipid chains through a combination of electrostatic and hydrophobic interactions, which resulted in more ordering of the lipid chains. However, in the liquid-condensed (LC) state, protein intercalation is decreased due to tighter lipid packing. Moreover, our findings revealed distinct differences in HSA's interaction with dDPPG and dDPPC lipids. The interaction with dDPPC remained relatively weak compared to dDPPG. These results shed light on the significance of protein mediated changes in lipid characteristics, which hold considerable implications for understanding membrane protein behavior, lipid-mediated cellular processes, and lipid-based biomaterial design.

Study on the Transport Properties of SO2 and NO at the Interface of H2O2 Solutions Using Molecular Dynamics

Gas-liquid interfaces have a unique structure different from the bulk phase due to the complex intermolecular interactions within them and are regarded as barriers that prevent gases from entering solution or as channels that affect gas reactions. In this study, the adsorption and mass-transfer mechanisms of sulfur dioxide and nitric oxide at the gas-liquid interface of a H2O2 solution were comprehensively analyzed using molecular dynamics (MD) simulations. The analysis on molecule angle showed that H2O molecules tended to align parallel to the solution surface on the surface of the H2O2 solution. Regardless of whether the gas was adsorbed on the surface of the solution or not, H2O2 molecules were always perpendicular to the interface of the solution. The analysis on molecule angle and radial distribution function revealed that the H atoms of H2O molecules had a corresponding turn, and SO2 molecules were greatly affected by the attraction of H2O2 molecules during the adsorption of gas molecules on the interface. Steered MD was utilized to investigate the mass-transfer process of SO2 and NO molecules across the gas-liquid interface. The S atoms of SO2 molecules were significantly influenced by H2O2 molecules, while the O atoms of NO molecules gradually transitioned from the gas phase to the liquid phase. The results provided information on how gas molecules interacted with the surface of the solution and the specific details of the molecular orientation at the solution surface.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

Surface stratification determines the interfacial water structure of simple electrolyte solutions

The distribution of ions at the air/water interface plays a decisive role in many natural processes. Several studies have reported that larger ions tend to be surface-active, implying ions are located on top of the water surface, thereby inducing electric fields that determine the interfacial water structure. Here we challenge this view by combining surface-specific heterodyne-detected vibrational sum-frequency generation with neural network-assisted ab initio molecular dynamics simulations. Our results show that ions in typical electrolyte solutions are, in fact, located in a subsurface region, leading to a stratification of such interfaces into two distinctive water layers. The outermost surface is ion-depleted, and the subsurface layer is ion-enriched. This surface stratification is a key element in explaining the ion-induced water reorganization at the outermost air/water interface.

Bubble-water/catalyst triphase interface microenvironment accelerates photocatalytic OER via optimizing semi-hydrophobic OH radical

Photocatalytic water splitting (PWS) as the holy grail reaction for solar-to-chemical energy conversion is challenged by sluggish oxygen evolution reaction (OER) at water/catalyst interface. Experimental evidence interestingly shows that temperature can significantly accelerate OER, but the atomic-level mechanism remains elusive in both experiment and theory. In contrast to the traditional Arrhenius-type temperature dependence, we quantitatively prove for the first time that the temperature-induced interface microenvironment variation, particularly the formation of bubble-water/TiO2(110) triphase interface, has a drastic influence on optimizing the OER kinetics. We demonstrate that liquid-vapor coexistence state creates a disordered and loose hydrogen-bond network while preserving the proton transfer channel, which greatly facilitates the formation of semi-hydrophobic •OH radical and O-O coupling, thereby accelerating OER. Furthermore, we propose that adding a hydrophobic substance onto TiO2(110) can manipulate the local microenvironment to enhance OER without additional thermal energy input. This result could open new possibilities for PWS catalyst design.

Noncollinear Optical Parametric Amplification of Broadband Infrared Sum Frequency Generation Vibrational Spectroscopy

Sum-frequency generation (SFG) vibrational spectroscopy is an invaluable tool in surface science, known for its specificity to surfaces and interfaces. Despite its wide application, it is often hampered by weak signal detection. Here, we present an innovative enhancement technique of postsample amplification, using a picosecond noncollinear optical parametric amplifier (NOPA). We conducted a systematical investigation into the impact of different intensities of pump and SFG seed light, as the input signal in NOPA, and demonstrated this method on the octadecanethiol (ODT) molecules on gold films. The amplified SFG by NOPA reproduced the SFG vibrational spectra, enhanced by about 4 orders of magnitude but with broader spectral resolution due to the short pulse width of the pump light in NOPA. This study makes it possible to realize highly sensitive SFG measurements, marking a significant advancement in spectroscopic analysis techniques.

The power of microdroplet photochemistry

This study presents compelling evidence demonstrating that irradiation of the air-solution interface, whether achieved through the spraying of microdroplets into the air or by bubbling air through a solution, significantly accelerates the rate of photochemical reactions by orders of magnitude compared to identical reaction conditions in bulk solutions. We propose this approach as a novel and versatile method for harnessing solar energy in chemical transformations.

Unified picture of vibrational relaxation of OH stretch at the air/water interface

The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface.

Charged Microdroplets as Microelectrochemical Cells for CO2 Reduction and C-C Coupling

Charged microdroplets offer novel electrochemical environments, distinct from traditional solid-liquid or solid-liquid-gas interfaces, due to the intense electric fields at liquid-gas interfaces. In this study, we propose that charged microdroplets serve as microelectrochemical cells (MECs), enabling unique electrochemical reactions at the gas-liquid interface. Using electrospray-generated microdroplets, we achieved multielectron CO2 reduction and C-C coupling to synthesize ethanol using molecular catalysts. These catalysts effectively harness and relay electrons, enhancing the longevity of solvated electrons and enabling multielectron reactions. Importantly, we revealed the intrinsic relationship between the size and charge density of a MEC and its reaction selectivity. Employing in situ mass spectrometry, we identified reaction intermediates (molecular catalyst adducts with HCOO) and oxidation products, elucidating the CO2 reduction mechanism and the comprehensive reaction procedure. Our research underscores the promising role of charged microdroplets in pioneering new electrochemical systems.

Realizing Symmetry-Breaking Architectures in Soap Films

We show here that soap films-typically expected to host symmetric molecular arrangements-can be constructed with differing opposite surfaces, breaking their symmetry, and making them reminiscent of functional biological motifs found in nature. Using fluorescent molecular probes as dopants on different sides of the film, resonance energy transfer could be employed to confirm the lack of symmetry, which was found to persist on timescales of several minutes. Further, a theoretical analysis of the main transport phenomena involved yielded good agreement with the experimental observations.

Efficient Photothermal Anti-/Deicing Enabled by 3D Cu2-x S Encapsulated Phase Change Materials Mixed Superhydrophobic Coatings

Photothermal superhydrophobic surfaces are one of the most promising anti-/deicing materials, yet they are limited by the low energy density and intermittent nature of solar energy. Here, a coupling solution based on microencapsulated phase change materials (MPCMs) that integrates photothermal effect and phase change thermal storage is proposed. Dual-shell octahedral MPCMs with Cu2 O as the first layer and 3D Cu2-x S as the second layer for the first time is designed. By morphology and phase manipulation of the Cu2-x S shell, the local surface plasmonic heating modulation of MPCMs is realized, and the MPCM reveals full-spectrum high absorption with a photothermal conversion efficiency up to 96.1%. The phase change temperature and enthalpy remain in good consistency after 200 cycles. Multifunctional photothermal phase-change superhydrophobic composite coatings are fabricated by combining the hydrolyzed and polycondensation products of octadecyl trichlorosilane and the dual-shell MPCM. The multifunctional coatings exhibit excellent anti-/deicing performance under low temperature and high humidity conditions. This work not only provides a new approach for the design of high-performance MPCMs but also opens up an avenue for the anti-icing application of photothermal phase-change superhydrophobic composite coatings.

Bulklike Vibrational Coupling of Surface Water Revealed by Sum-Frequency Generation Spectroscopy

Vibrational coupling between interfacial water molecules is important for energy dissipation after on-water chemistry, yet intensely debated. Here, we quantify the interfacial vibrational coupling strength through the linewidth of surface-specific vibrational spectra of the water's O─H (O─D) stretch region for neat H_{2}O/D_{2}O and their isotopic mixtures. The local-field-effect-corrected experimental SFG spectra reveal that the vibrational coupling between hydrogen-bonded interfacial water O─H groups is comparable to that in bulk water, despite the effective density reduction at the interface.

Photophysical Outcomes of Water-Solvated Heterocycles: Single-Conformation Ultraviolet and Infrared Spectroscopy of Microsolvated 2-Phenylpyrrole

The molecular chromophores within brown carbon (BrC) aerosols absorb solar radiation at visible and near-ultraviolet wavelengths. This contributes to the overall warming of the troposphere and the photochemical aging of aerosols. In this investigation, we combine a suite of experimental and theoretical methods to reveal the conformation-specific ultraviolet and infrared spectroscopy of 2-phenylpyrrole (2PhPy)─an extended π-conjugated pyrrole derivative and a model BrC chromophore─along with its water microsolvated molecular complexes (2PhPy:nH2O, n = 1-3). Using resonant two-photon ionization and double-resonance holeburning techniques alongside MP3 (ground state) and ADC(3) (excited state) torsional potential energy surfaces and discrete variable representation simulations, we characterized the ultraviolet spectra of 2PhPy and 2PhPy:1H2O. This analysis revealed evidence for Herzberg-Teller vibronic coupling along the CH wagging and NH stretching coordinates of the aromatic rings. Conformation-specific infrared spectroscopy revealed extended hydrogen-bonding networks of the 2PhPy:nH2O complexes. Upon stepwise addition of H2O solvation, the nearest H2O acceptor forms a strong, noncovalent interaction with the pyrrole NH donor, while the second and third H2O partners interface with the phenyl and pyrrole aromatic rings through growing van der Waals π/H atom stabilization. A local-mode Hamiltonian approach was employed for comparison with the experimental spectra, thus identifying the vibrational spectral signatures to specific 2PhPy:nH2O oscillators.

Aqueous Interfaces in Chemical Separations

Chemical separations play a vital role in refinery and reprocessing of critical materials, such as platinum group metals, rare earths, and actinides. The choice of separation system─whether it is liquid-liquid extraction (LLE), sorbents, or membranes─depends on specific needs and applications. In almost all separation processes, the desired metal ions adsorb or transfer across an aqueous interface, such as the solid/liquid interface in sorbents or oil/water interfaces in LLE. Despite these separation technologies being extensively used for decades, our understanding of the molecular-scale mechanisms governing ion adsorption and transport at interfaces remains limited. This knowledge gap presents a significant challenge in meeting the increasing demands for these critical materials due to their growing use in advanced technologies. Fortunately, recent advancements in surface-specific experimental and computational techniques offer promising avenues to bridge this gap and facilitate the development of next-generation separation systems. Interestingly, unanswered questions regarding interfacial phenomena in chemical separations hold great relevance to various fields, including energy storage, geochemistry, and atmospheric chemistry. Therefore, the model interfacial systems developed for studying chemical separations, such as amphiphilic molecules assembled at a solid/water, air/water, or oil/water interface, may have far-reaching implications, extending beyond separations and opening doors to addressing a wide range of scientific inquiries. This perspective discusses recent interfacial studies elucidating amphiphile-ion interactions in chemical separations of metal ions. These studies provide direct, molecular-scale information about solute and solvent behavior at aqueous interfaces, including multivalent and complex ions in highly concentrated solutions, which play key roles in LLE of critical materials.

Accelerated Photolysis of H2O2 at the Air-Water Interface of a Microdroplet

Photochemical homolysis of hydrogen peroxide (H2O2) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H2O2 photolysis play a pivotal role in determining the efficiency of ·OH production, which is currently mainly investigated in bulk systems. Here, we report considerably accelerated H2O2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 103 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H2O2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H2O2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H2O2. This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H2O2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO2 and volatile organic compounds.

A Surface Matrix of Au NPs Decorated Graphdiyne for Multifunctional Laser Desorption/Ionization Mass Spectrometry

The development of the valid strategy to enhance laser desorption/ionization efficiency gives rise to widespread concern in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) technology. Herein, a hybrid of Au NP-decorated graphdiyne (Au/GDY) was fabricated and employed as the SALDI-MS matrix for the first time, and a mechanism based on photothermal and photochemical energy conversions was proposed to understand LDI processes. Given theoretical simulations and microstructure characterizations, it was revealed that the formation of a coupled thermal field and internal electric field endow the as-prepared Au/GDY matrix with superior desorption and ionization efficiency, respectively. Moreover, laser-induced matrix ablation introduced strain and defect level into the Au/GDY hybrid, suppressing the recombination of charge carriers and thereby facilitating analyte ionization. The optimized Au/GDY matrix allowed for reliable detection of trace sulfacetamide and visualization of exogenous/endogenous components in biological tissues. This work offers an integrated solution to promote LDI efficiency based on collaborative photothermal conversion and internal electric field, and may inspire the design of novel semiconductor-based surface matrices.

"Nonlinear" pursuit of understanding pollutant accumulation and chemistry at environmental and biological interfaces

Over the past few decades, the public recognition of the prevalence of certain classes of pollutants, such as perfluoroalkyl substances and nanoplastics, within the environment, has sparked growing concerns over their potential impact on environmental and human health. Within both environmental and biological systems, the adsorption and structural organization of pollutants at aqueous interfaces can greatly impact the chemical reactivity and transformation. Experimentally probing chemical behavior at interfaces can often pose a problem due to bulk solvated molecules convoluting molecular signatures from interfacial molecules. To solve this problem, there exist interface-specific nonlinear spectroscopy techniques that can directly probe both macroscopic planar interfaces and nanoplastic interfaces in aqueous environments. These techniques can provide essential information such as chemical adsorption, structure, and reactivity at interfaces. In this perspective, these techniques are presented with obvious advantages for studying the chemical properties of pollutants adsorbed to environmental and biological interfaces.

Infrared Reflection-Absorption Spectroscopy of α-Keto Acids at the Air-Water Interface: Effects of Chain Length and Headgroup on Environmentally Relevant Surfactant Films

A variety of organic surfactants are found at air-water interfaces in natural environments, including on the surfaces of aqueous aerosols. The structure and morphology of these organic films can have profound impacts on material transfer between the gas and condensed phases, the optical properties of atmospheric aerosol, and chemical processing at air-water interfaces. Combined, these effects can have significant impacts on climate via radiative forcing, but our understanding of organic films at air-water interfaces is incomplete. Here, we examine the impact of the polar headgroup and alkyl tail length on the structure and morphology of organic monolayers at the air-water interfaces. First, we focus on the substituted carboxylic acids, α-keto acids, using Langmuir isotherms and infrared reflection absorption spectroscopy (IR-RAS) to elucidate key structures and phase behaviors of α-keto acids with a range of surface activities. We show that the structure of α-keto acids, both soluble and insoluble, at water surfaces is a compromise between van der Waals interactions of the hydrocarbon tail and hydrogen bonding interactions involving the polar headgroup. Then, we use this new data set regarding α-keto acid films at water surfaces to examine the role of the polar headgroup on organic films using a similar substituted carboxylic acid (α-hydroxystearic acid), an unsubstituted carboxylic acid (stearic acid), and an alcohol (stearyl alcohol). We show that the polar headgroup and its hydrogen bonding interactions can significantly affect the orientation of amphiphiles at air-water interfaces. Here, we provide side-by-side comparisons of Langmuir isotherms and IR-RA spectra for a set of environmentally relevant organic amphiphiles with a range of alkyl tail lengths and polar headgroup structures.

Ions Speciation at the Water-Air Interface

In typical aqueous systems, including naturally occurring sweet and salt water and tap water, multiple ion species are co-solvated. At the water-air interface, these ions are known to affect the chemical reactivity, aerosol formation, climate, and water odor. Yet, the composition of ions at the water interface has remained enigmatic. Here, using surface-specific heterodyne-detected sum-frequency generation spectroscopy, we quantify the relative surface activity of two co-solvated ions in solution. We find that more hydrophobic ions are speciated to the interface due to the hydrophilic ions. Quantitative analysis shows that the interfacial hydrophobic ion population increases with decreasing interfacial hydrophilic ion population at the interface. Simulations show that the solvation energy difference between the ions and the intrinsic surface propensity of ions determine the extent of an ion's speciation by other ions. This mechanism provides a unified view of the speciation of monatomic and polyatomic ions at electrolyte solution interfaces.

Photodegradation behavior and mechanism of dibutyl phthalate in water under flood discharge atomization

Flood discharge atomization is a prevalent hydraulics phenomenon in reservoir scheduling operations, however, its effect on the migration and transformation behavior of pollutants has not been examined. In this study, the behaviors and mechanisms of the direct photodegradation of dibutyl phthalate (DBP) in atomized water and the indirect photodegradation of DBP in the presence of ferric ions and nitrate were investigated. The results showed that the photodegradation rate of DBP was accelerated under atomization conditions by sunlight irradiation. The photodegradation efficiency of DBP in the presence of ferric ions and nitrate under atomization conditions was increased by 2.20 times and 1.82 times compared with no-atomization conditions, respectively. The quencher experiments indicated that the main active species for DBP photodegradation in the presence of ferric ions were hydroxyl radicals (·OH) and superoxide radicals (·O₂^-) with atomization, while the main active species in the presence of nitrate were ·OH, ·O₂^- and electrons (e^-). In addition, the differences were found in the photodegradation products and pathways of DBP between with and without atomization treatment. In the presence of ferric ions, the benzene ring of DBP was opened to produce fumaric acid, while phthalic acid bis(4-hydroxybutyl) ester was produced in the presence of nitrate under atomization conditions. The results of this study provide a scientific basis for assessing the effect of water conservancy projects on the migration and transformation behaviors of pollutants, which is of great theoretical significance and scientific value.

Momentum-dependent sum-frequency vibrational spectroscopy of bonded interface layer at charged water interfaces

Interface-specific hydrogen (H)-bonding network of water directly controls the energy transfer and chemical reaction pathway at many charged aqueous interfaces, yet to characterize these bonded water layer structures remains a challenge. We now develop a sum-frequency spectroscopic scheme with varying photon momenta as an all-optic solution for retrieving the vibrational spectra of the bonded water layer and the ion diffuse layer and, hence, microscopic structural and charging information about an interface. Application of the method to a model surfactant-water interface reveals a hidden weakly donor H-bonded water species, suggesting an asymmetric hydration-shell structure of fully solvated surfactant headgroups. In another application to a zwitterionic phosphatidylcholine lipid monolayer-water interface, we find a highly polarized bonded water layer structure associating to the phosphatidylcholine headgroup, while the diffuse layer contribution is experimentally proven to be negligible. Our all-optic method offers an in situ microscopic probe of electrochemical and biological interfaces and the route toward future imaging and ultrafast dynamics studies.

Experimental and Thermodynamic Viewpoints on Claims of a Spontaneous H2O2 Formation at the Air-Water Interface

Recent claims of the spontaneous H₂O₂ formation at the air-water interface of water microdroplets have sparked debates on its feasibility. New results from different research groups have provided more insight into these claims, but conclusive proofs are still far from realized. In this Perspective, thermodynamic viewpoints, potential experiments, and theoretical approaches are presented as references for future studies. We suggest that future work should seek for H₂ byproduct as indirect evidence to confirm the feasibility of this phenomenon. Examining potential energy surfaces for H₂O₂ formation reaction when moving from the bulk to the interface under the influence of the local electric fields is also critical to establish this phenomenon.

Counterintuitive Oxidation of Alcohols at Air-Water Interfaces

This study shows that the oxidation of alcohols can rapidly occur at air-water interfaces. It was found that methanediols (HOCH₂OH) orient at air-water interfaces with a H atom of the -CH₂- group pointing toward the gaseous phase. Counterintuitively, gaseous hydroxyl radicals do not prefer to attack the exposed -CH₂- group but the -OH group that forms hydrogen bonds with water molecules at the surface via a water-promoted mechanism, leading to the formation of formic acids. Compared with gaseous oxidation, the water-promoted mechanism at the air-water interface significantly lowers free-energy barriers from ∼10.7 to ∼4.3 kcal·mol^-1 and therefore accelerates the formation of formic acids. The study unveils a previously overlooked source of environmental organic acids that are bound up with aerosol formation and water acidity.

Analytic Gradient for Time-Dependent Density Functional Theory Combined with the Fragment Molecular Orbital Method

The analytic energy gradient of energy with respect to nuclear coordinates is derived for the fragment molecular orbital (FMO) method combined with time-dependent density functional theory (TDDFT). The response terms arising from the use of a polarizable embedding are derived. The obtained analytic FMO-TDDFT gradient is shown to be accurate in comparison to both numerical FMO-TDDFT and unfragmented TDDFT gradients, at the level of two- and three-body expansions. The gradients are used for geometry optimizations, molecular dynamics, vibrational calculations, and simulations of IR and Raman spectra of excited states. The developed method is used to optimize the geometry of the ground and excited electronic states of the photoactive yellow protein (PDB: 2PHY).

A perspective on iron (Fe) in the atmosphere: air quality, climate, and the ocean

As scientists engage in research motivated by climate change and the impacts of pollution on air, water, and human health, we increasingly recognize the need for the scientific community to improve communication and knowledge exchange across disciplines to address pressing and outstanding research questions holistically. Our professional paths have crossed because our research activities focus on the chemical reactivity of Fe-containing minerals in air and water, and at the air-sea interface. (Photo)chemical reactions driven by Fe can take place at the surface of the particles/droplets or within the condensed phase. The extent and rates of these reactions are influenced by water content and biogeochemical activity ubiquitous in these systems. One of these reactions is the production of reactive oxygen species (ROS) that cause damage to respiratory organs. Another is that the reactivity of Fe and organics in aerosol particles alter surficial physicochemical properties that impact aerosol-radiation and aerosol-cloud interactions. Also, upon deposition, aerosol particles influence ocean biogeochemical processes because micronutrients such as Fe or toxic elements such as copper become bioavailable. We provide a perspective on these topics and future research directions on the reactivity of Fe in atmospheric aerosol systems, from sources to short- and long-term impacts at the sinks with emphasis on needs to enhance the predictive power of atmospheric and ocean models.

Pushing Photochemistry into Water: Acceleration of the Di-π-Methane Rearrangement and the Paternó-Büchi Reaction "On-Water"

Two representative organic photoreactions, namely a bimolecular photocycloaddition and a monomolecular photorearrangement, are presented that are accelerated when the reaction is performed "on-water", that is, at the water-substrate interface with no solvation of the reaction components. According to the established models of ground-state reactions "on-water", the enhanced efficiency of the photoreactions is explained by hydrophobic effects (Paternó-Büchi reaction) or specific hydrogen bonding (di-π-methane rearrangement) at the water-substrate interface that decrease the energy of the respective transition state. These results point to the potential of this approach to conduct photoreactions more efficiently in an ecologically favorable medium.

Boosting Catalytic Selectivity through a Precise Spatial Control of Catalysts at Pickering Droplet Interfaces

Exploration of new methodologies to tune catalytic selectivity is a long-sought goal in catalytic community. In this work, oil-water interfaces of Pickering emulsions are developed to effectively regulate catalytic selectivity of hydrogenation reactions, which was achieved via a precise control of the spatial distribution of metal nanoparticles at the droplet interfaces. It was found that Pd nanoparticles located in the inner interfacial layer of Pickering droplets exhibited a significantly enhanced selectivity for p-chloroaniline (up to 99.6%) in the hydrogenation of p-chloronitrobenzene in comparison to those in the outer interfacial layer (63.6%) in pure water (68.5%) or in pure organic solvents (46.8%). Experimental and theoretical investigations indicated that such a remarkable interfacial microregion-dependent catalytic selectivity was attributed to the microenvironments of the coexistence of water and organic solvent at the droplet interfaces, which could provide unique interfacial hydrogen-bonding interactions and solvation effects so as to alter the adsorption patterns of p-chloronitrobenzene and p-chloroaniline on the Pd nanoparticles, thereby avoiding the unwanted contact of C-Cl bonds with the metal surfaces. Our strategy of precise spatial control of catalysts at liquid-liquid interfaces and the unprecedented interfacial effect reported here not only provide new insights into the liquid-liquid interfacial reactions but also open an avenue to boost catalytic selectivity.

On the Fresnel factor correction of sum-frequency generation spectra of interfacial water

Insights into the microscopic structure of aqueous interfaces are essential for understanding the chemical and physical processes on the water surface, including chemical synthesis, atmospheric chemistry, and events in biomolecular systems. These aqueous interfaces have been probed by heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. To obtain the molecular response from the measured HD-SFG spectra, one needs to correct the measured ssp spectra for local electromagnetic field effects at the interface due to a spatially varying dielectric function. This so-called Fresnel factor correction can change the inferred response substantially, and different ways of performing this correction lead to different conclusions about the interfacial water response. Here, we compare the simulated and experimental spectra at the air/water interface. We use three previously developed models to compare the experiment with theory: an advanced approach taking into account the detailed inhomogeneous interfacial dielectric profile and the Lorentz and slab models to approximate the interfacial dielectric function. Using the advanced model, we obtain an excellent quantitative agreement between theory and experiment, in both spectral shape and amplitude. Remarkably, we find that for the Fresnel factor correction of the ssp spectra, the Lorentz model for the interfacial dielectric function is equally accurate in the hydrogen (H)-bonded region of the response, while the slab model underestimates this response significantly. The Lorentz model, thus, provides a straightforward method to obtain the molecular response from the measured spectra of aqueous interfaces in the H-bonded region.

A Molecular Dynamic Study of the Effects of Surface Partitioning on the OH Radical Interactions with Solutes in Multicomponent Aqueous Aerosols

The surface-bulk partitioning of small saccharide and amide molecules in aqueous droplets was investigated using molecular dynamics. The air-particle interface was modeled using a 80 Å cubic water box containing a series of organic molecules and surrounded by gaseous OH radicals. The properties of the organic solutes within the interface and the water bulk were examined at a molecular level using density profiles and radial pair distribution functions. Molecules containing only polar functional groups such as urea and glucose are found predominantly in the water bulk, forming an exclusion layer near the water surface. Substitution of a single polar group by an alkyl group in sugars and amides leads to the migration of the molecule toward the interface. Within the first 2 nm from the water surface, surface-active solutes lose their rotational freedom and adopt a preferred orientation with the alkyl group pointing toward the surface. The different packing within the interface leads to different solvation shell structures and enhanced interaction between the organic molecules and absorbed OH radicals. The simulations provide quantitative information about the dimension, composition, and organization of the air-water interface as well as about the nonreactive interaction of the OH radicals with the organic solutes. It suggests that increased concentrations, preferred orientations, and decreased solvation near the air-water surface may lead to differences in reactivities between surface-active and surface-inactive molecules. The results are important to explain how heterogeneous oxidation mechanisms and kinetics within interfaces may differ from those of the bulk.

Spectroscopy from Machine Learning by Accurately Representing the Atomic Polar Tensor

Vibrational spectroscopy is a key technique to elucidate microscopic structure and dynamics. Without the aid of theoretical approaches, it is, however, often difficult to understand such spectra at a microscopic level. Ab initio molecular dynamics has repeatedly proved to be suitable for this purpose; however, the computational cost can be daunting. Here, the E(3)-equivariant neural network e3nn is used to fit the atomic polar tensor of liquid water a posteriori on top of existing molecular dynamics simulations. Notably, the introduced methodology is general and thus transferable to any other system as well. The target property is most fundamental and gives access to the IR spectrum, and more importantly, it is a highly powerful tool to directly assign IR spectral features to nuclear motion─a connection which has been pursued in the past but only using severe approximations due to the prohibitive computational cost. The herein introduced methodology overcomes this bottleneck. To benchmark the machine learning model, the IR spectrum of liquid water is calculated, indeed showing excellent agreement with the explicit reference calculation. In conclusion, the presented methodology gives a new route to calculate accurate IR spectra from molecular dynamics simulations and will facilitate the understanding of such spectra on a microscopic level.

Enhanced Sum Frequency Generation for Monolayers on Au Relative to Silica: Local Field Factors and SPR Effect

Using metals as signal magnified substrates, surface plasmon-enhanced sum frequency generation (SFG) vibrational spectroscopy is a promising technique to probe weak molecular-level signals at surfaces and interfaces. In this study, the vibrational signals of the n-alkane monolayer on the gold (Au) and silica substrates are investigated using the broadband femtosecond SFG. The enhancement factors are discovered to be up to ∼1076 and ∼31 for the methyl symmetric and asymmetric stretching (ss and as) modes of the monolayer, respectively. By systematically analyzing the second-order nonlinear susceptibility tensor components (χ_ijks), the Fresnel coefficients (F_ijks), and the surface plasmon resonance (SPR) effect, we find that the interplay between F_ijk and χ_ijk terms and the SPR effect dominate the SFG signal enhancement. Our study reveals that the relative contributions of different influencing factors (i.e., Fresnel coefficients and SPR) to the SFG signal enhancement provide an approach to interpreting enhanced SFG vibrational signals detected from probe molecules on distinct substrates and may finally guide the design of the experimental methodology to improve the detection sensitivity and signal-to-noise ratio.

Resonance-Induced Reduction of Interfacial Tension of Water-Methane and Improvement of Methane Solubility in Water

Molecular dynamics simulations were performed to study the effect of the periodic oscillating electric field on the interface between water and methane. We propose a new strategy that utilizes oscillating electric fields to reduce the interfacial tension (IFT) between water and methane and increase the solubility of methane in water simultaneously. These are attributed to the hydrogen bond resonance induced by an electric field with a frequency close to the natural frequency of the hydrogen bond. The resonance breaks the hydrogen bond network among water molecules to the maximum, which destroys the hydration shell and reduces the cohesive action of water, thus resulting in the decrease of IFT and the increase of methane solubility. As the frequency of the electric field is close to the optimum resonant frequency of hydrogen bonds, IFT decreases from 56.43 to 5.66 mN/m; water and methane are miscible because the solubility parameter of water reduces from 47.63 to 2.85 MPa^1/2, which is close to that of methane (3.43 MPa^1/2). Our results provide a new idea for reducing the water-gas IFT and improving the solubility of insoluble gas in water and theoretical guidance in the fields of natural gas exploitation, hydrate generation, and nanobubble nucleation.

Experimental and Theoretical Heterodyne-Detected Sum Frequency Generation Spectroscopy of Isotopically Pure and Diluted Water Surfaces

The χ⁽²⁾ (second-order nonlinear optical susceptibility) spectrum of the water surface has been a matter of debate for a few decades. Here, we report that we experimentally measured the isotopic dilution dependence of the χ⁽²⁾ spectrum and theoretically reproduced it by employing the quantum/classical mixed approach with a new idea to subtract an artifact. The present theoretical framework allows for clarifying the effects of the intramolecular, intermolecular, and Fermi resonance couplings on the OH-stretch vibrational spectra of water at the surface as well as in the bulk.

Microdroplet-Mediated Radical Polymerization

Micrometer-sized aqueous droplets serve as a unique reactor that drives various chemical reactions not seen in bulk solutions. However, their utilization has been limited to the synthesis of low molecular weight products at low reactant concentrations (nM to μM). Moreover, the nature of chemical reactions occurring outside the droplet remains unknown. This study demonstrated that oil-confined aqueous microdroplets continuously generated hydroxyl radicals near the interface and enabled the synthesis of polymers at high reactant concentrations (mM to M), thus successfully converting the interfacial energy into the synthesis of polymeric materials. The polymerized products maintained the properties of controlled radical polymerization, and a triblock copolymer with tapered interfaces was prepared by the sequential addition of different monomers into the aqueous microdroplets. Furthermore, a polymerization reaction in the continuous oil phase was effectively achieved by the transport of the hydroxyl radicals through the oil/water interface. This interfacial phenomenon is also successfully applied to the chain extension of a hydrophilic polymer with an oil-soluble monomer across the microdroplet interface. Our comprehensive study of radical polymerization using compartmentalization in microdroplets is expected to have important implications for the emerging field of microdroplet chemistry and polymerization in cellular biochemistry without any invasive chemical initiators.

Spectroscopy of Retinoic Acid at the Air-Water Interface

The spectroscopy of all-trans-retinoic acid (ATRA), an important molecule of biological origin that can be found in nature, is investigated at the air-water interface using UV-Vis and IR reflection spectroscopy. We employ a UV-Vis reflection absorption spectroscopy (RAS) experiment along with infrared reflection absorption spectroscopy (IR-RAS) to probe ATRA at the air-water interface. We elucidate the factors influencing the spectroscopy of ATRA at the air-water interface and compare its spectra at the water surface with results of bulk samples obtained with conventional spectroscopic methods and computational chemistry. Monolayers of pure ATRA as well as mixed ATRA with stearic-d₃₅ acid were prepared, and the spectroscopy reveals that ATRA forms J-aggregates with itself, causing a significant redshift of its S₀ to S₁ electronic transition. Pure ATRA monolayers are found to be unstable at the air-water interface and are lost from the surface over time due to the formation of aggregates. The mixture of ATRA and stearic-d₃₅ acid has been shown to stabilize the monolayers and inhibit the loss of surface ATRA. On the basis of our observations, we propose that ATRA could be a significant photosensitizer in natural aqueous environments.

Complete Inhibition of the Rotation in a Barrierless TICT Probe for Fluorescence-On Qualitative Analysis

Inhibition of twisting intramolecular charge transfer (TICT) is one of the most attractive methods for fluorescence-on analysis, whereas it remains enigmatic whether the fluorescence in a TICT-based probe could be thoroughly lightened. Here, for maximizing the fluorescence-on signal of the TICT-based probe, we develop a model by employing chemical reaction to directly cleave the linkage between the rotational electron donor and acceptor with a predisposed fluorescent signal close to zero. To validate this assumption, a nonfluorescent probe with barrierless rotation is successfully achieved by grafting acryloyl with -C═C- recognition sites onto coumarin, and 7-hydroxycoumarin with bright blue fluorescence could be released within 3 s upon probing KMnO₄ with an amount as low as 0.95 nM and 6.6 pg. We believe that the present strategy could not only deepen the insights of photochemistry but also facilitate the development of a theranostic drug delivery system, energy conversion, pollution control, and health risk reduction.

Photooxidation of the Phenolate Anion is Accelerated at the Water/Air Interface

Molecular photodynamics can be dramatically affected at the water/air interface. Probing such dynamics is challenging, with product formation often probed indirectly through its interaction with interfacial water molecules using time-resolved and phase-sensitive vibrational sum-frequency generation (SFG). Here, the photoproduct formation of the phenolate anion at the water/air interface is probed directly using time-resolved electronic SFG and compared to transient absorption spectra in bulk water. The mechanisms are broadly similar, but 2 to 4 times faster at the surface. An additional decay is observed at the surface which can be assigned to either diffusion of hydrated electrons from the surface into the bulk or due to increased geminate recombination at the surface. These overall results are in stark contrast to phenol, where dynamics were observed to be 10⁴ times faster and for which the hydrated electron was also a photoproduct. Our attempt to probe phenol showed no electron signal at the interface.

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

Whether the air-water interface decreases or increases the acidity of simple organic and inorganic acids compared to the bulk is critically important in a broad range of environmental and biochemical processes. However, a consensus has not yet been achieved on this key question. Here we use machine learning-based reactive molecular dynamics simulations to study the dissociation of paradigmatic nitric and formic acids at the air-water interface. We show that the local acidity profile across the interface is determined by changes in acid and conjugate base solvation and that the acidity decreases abruptly over a transition region of a few molecular layers. At the interface, both acids are weaker than in the bulk due to desolvation. In contrast, acidities below the interface reach a plateau and are all the stronger compared to those in the bulk as the surface to volume ratio of the aqueous phase is large, due to the growing impact of the stabilization of the released proton at the surface of the water. These results imply that the measured degree of dissociation sensitively depends on the experimental probing length and system size and suggest a molecular explanation for the contrasting experimental results. The aerosol size dependence of acidity has important consequences for atmospheric chemistry.

Non-Born-Oppenheimer effects in molecular photochemistry: an experimental perspective

Non-adiabatic couplings between Born-Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating 'reactive' outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase H₂O molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products. This article is part of the theme issue 'Chemistry without the Born-Oppenheimer approximation'.

From Dynamic Superwettability to Ionic/Molecular Superfluidity

Life systems present ultralow energy consumption in high-efficiency energy conversion, information transmission, and biosynthesis. The total energy intake of the human body is about 2000 kcal/day to maintain all of our activities, which is comparable to a power of ∼100 W. The energy required for the brain to work is equivalent to ∼20 W, and the rest of the energy (∼80 W) is used for other activities. All in vivo biosyntheses take place only at body temperature, which is much lower than that of in vitro reactions. To achieve these ultralow energy-consumption processes, there should be a kind of ultralow-resistivity matter transport in nanochannels (e.g., ionic and molecular channels), in which the directional collective motion of ions or molecules is a necessary condition rather than traditional Newton diffusion. The directional collective motion of ions and molecules is considered to be ionic/molecular superfluidity. The driving force of ionic/molecular superfluidity formation requires two necessary conditions: (1) Ions or molecules are confined at a certain distance (e.g., approximately twice Debye length (2λ_D) for ions or twice the van der Waals equilibrium distance (2d₀) for molecules). (2) When the attractive potential energy (E₀) is stronger than the thermal noise (k_BT_c), ionic/molecular superfluidity can be formed. The concept of ionic/molecular superfluidity will promote the understanding of energy conversion with ultralow energy consumption in biological systems. The swing of an eel's body generating electricity and cardiac resuscitation denote the conversion from mechanical energy to electrical energy, and mechanical modulation might result in a coherent resonance of ionic motion. The coherent resonance of Ca²⁺ in myocardium cells can induce a heartbeat, realizing the conversion from the electrical energy to the mechanical energy of a biological system. The macroscopic quantum state of ion channels is considered to be a carrier of neural information, and the environment field might play a significant role in regulating the macroscopic quantum states of various ion channels. In the biological ion channels system, the coupling of ion channels and their released photons might induce an environment wave which in turn regulates the ion oscillations in the channels to a coherent state. The states of decoherence and coherence might correspond to the states of sleep and action. We also demonstrated the decomposition of ATP to ADP released photons with a frequency of ∼34 THz, which could further drive DNA polymerization in the nanocavity of DNA polymerase. The photochemical (mid- and far-IR) reaction might be the driving force in high-efficiency biosynthesis. Quantized syntheses resonantly driven by multiple mid- and far-IR photons could be further designed in a tubular reactor with membranes of different microporous structures to achieve a high-efficiency synthesis with a low energy consumption. Finally, we point out that the Bose-Einstein condensate potentially widely exists. We expect that this Account will provide new ideas for the key problem in life science: how can life systems present ultralow energy consumption in high-efficiency energy conversion, information transmission, and biosynthesis?

Why the Photochemical Reaction of Phenol Becomes Ultrafast at the Air-Water Interface: The Effect of Surface Hydration

Photochemical reactions at the air-water interface can show remarkably different rates from those in bulk water. The present study elucidates the reaction mechanism of phenol characteristic at the air-water interface by the combination of molecular dynamics simulation and quantum chemical calculations of the excited states. We found that incomplete hydrogen bonding to phenol at the air-water interface affects excited states associated with the conical intersection and significantly reduces the reaction barrier, resulting in the distinctively facilitated rate in comparison with the bulk phase. The present study indicates that the reaction dynamics can be substantially different at the interfaces in general, reflecting the difference in the stabilization energy of the electronic states in markedly different solvation at the interface.

Selective Synthesis of the Aminobutadiene Intermediate and Mechanistic Analysis of 1,4-Dihydropyridine Formation Reaction in Water

We previously isolated an aminobutadiene derivative as a by-product in the synthesis of a 1,4-dihydropyridine (1,4-DHP) derivative by the reaction of methyl propiolate with excess ammonium acetate in water, and we proposed that it is an intermediate in the formation of 1,4-DHP. Here, to test this idea and to investigate the reaction mechanism, we selectively synthesized the aminobutadiene derivative in EtOH and examined its reactivity. The yield of the aminobutadiene derivative was increased in the presence of excess ammonium salt. X-Ray crystal structure analysis indicated the presence of an intramolecular hydrogen bond between the terminal amine and ester carbonyl oxygen, together with a short C-N bond length consistent with enamine-imine equilibrium. Direct cyclization of the aminobutadiene derivative with methyl propiolate to afford the 1,4-DHP derivative did not proceed well, but the yield was increased in the presence of morpholine salt as an additive. These results suggest that the predominant reaction pathway from the intermediate to 1,4-DHP in water involves Michael addition of a second amine molecule and reaction with methyl propiolate, followed by intramolecular cyclization and elimination of amine.