Spectroscopic characterization of neutral boron oxide clusters is insightful for understanding the structures and properties of the bulk but has proven to be extremely challenging due to the difficulty in size selection. Here, we report a size-specific infrared spectroscopy study of a series of neutral boron oxide clusters using near-threshold photoionization with a tunable vacuum ultraviolet free electron laser. Quantum chemical calculations were carried out to understand the structures and bonding of the clusters and to help assign the experimental spectral features. The BO3, B2O4, and B3O6 clusters focused in this study are found to have planar structures with BO, BO3, and B2O5 groups, which are key structural units in the two-dimensional network of the vitreous state. Chemical bonding analyses revealed structural stability arising from the synergy among the terminal B≡O groups and B-O σ bonds. This work provides spectral signatures for the key structural units of the bulk and paves the way for systematic studies on the stepwise formation and growth mechanisms of boron oxide materials.

Elucidating the hydration dynamics of polynuclear metals is vital for understanding the fundamental physicochemical properties of various interfacial processes. Nevertheless, the structural characterization of these neutral complexes is hindered by the inherent difficulties associated with experimental size selection. Here, the microhydration pathways and structural transformations of neutral Sr2(OH)3(H2O)n (n = 1-5) clusters were unveiled through size-selected infrared-vacuum ultraviolet spectroscopy, coupled with quantum chemical calculations as well as ab initio molecular dynamics simulations. The results reveal that three water molecules are sufficient to trigger the configuration transformation from Sr2(mu 2-OH)2(eta 1-OH) to Sr2(mu 2-OH)3. This intriguing transition is found to be driven by hydration-induced structural deformation, achieving thermodynamic stability through the rearrangement of the hydrogen-bonding network. The current system affords a model for clarifying the metal-oxygen skeleton and hydration dynamics in the microenvironment of perovskite catalysts and opens new avenues for systematic understanding of hydration-driven lattice rearrangements, active site modulation, and catalyst activation.

Investigating the dissociation of base in a microhydration environment is important for revealing various fundamental physical and chemical processes. In this study, neutral open-shell BaOH(H2O)n (n = 1-5) clusters were characterized by size-specific infrared-vacuum ultraviolet photoionization spectroscopy combined with quantum chemical calculations and ab initio molecular dynamics simulations. The results show that the Ba-OH bond is undissociated for n = 1 and 2, and the transition from contact ion pair (CIP) to solvent-shared ion pair (SIP) spontaneously starts at n = 3. The present findings clarify the previous debate that BaOH shows no sign of dissociation even at n = 5. This work reveals the microscopic mechanisms of base dissolution processes and lays a solid foundation for broader application across systems.

Elucidating the impacts of gaseous pollutants on secondary organic aerosol (SOA) formation mechanisms is critical for developing effective PM2.5 mitigation strategies. However, the combined effects of NOx (NOx = NO2 + NO) and SO2 on SOA compositions under complex photochemical conditions remain mechanistically unresolved. Here, we investigated the individual roles of NO2/NO and their combined effects with SO2 on β-myrcene photooxidation. Our results showed that NO2 enhanced the SOA yield and the O:C ratio through oxidant amplification, whereas NO suppressed nucleation and oxidation states of products but facilitated particle size growth via organic nitrate partitioning. SO2 exhibited significant synergistic effects with both NO2 and NO, facilitating particle formation and growth processes. The newly built vacuum ultraviolet free electron laser photoionization aerosol mass spectrometer enabled the observation of a series of new compounds (i.e., organic peroxides, organic nitrate, and organosulfates) and identified new mechanisms of SOA formation. Notably, a novel compound with a molecular weight of 287 was characterized to be a nitrogen- and sulfur-containing species. These findings highlight the critical roles of pollutants in particle formation in environmental processes and provide key scientific support for regional air quality management and climate change mitigation.

Elucidating the water splitting by neutral metals is crucial for understanding the structure-reactivity relationship of catalysts. However, experimental characterization of such neutral metal-water systems remains extremely challenging due to the difficulty in mass selection. Herein, the reactions of neutral tantalum with water were studied by size-specific infrared-vacuum ultraviolet spectroscopy with the combination of quantum chemical calculations and ab initio molecular dynamics simulations. The agreement between experimental and theoretical results identifies the key products of stepwise solvated hydrides HTa(OH)3(H2O)n (n = 1-3). The n = 1 monohydrate is stabilized by a cyclic double hydrogen bond motif between the HTa(OH)3 core and the water molecule, while subsequent hydration (n = 2 and 3) progressively extends this hydrogen bond network to form the first solvation shell. Significantly, the first water molecule in TaO(H2O)n inhibits the water dissociation reaction on TaO, whereas the second water molecule in TaO(H2O)n is able to trigger the water splitting facilitated by a six-membered hydrogen bond network. This work provides the direct spectral evidence of intrinsic structure and solvation effect of hydrolysis products and offers a molecular-level prelude to the macroscopic mechanism of single-atom catalysts for water splitting.

Gas-phase heteronuclear cerium-nickel carbonyl complexes, OCeNi(CO)3- and CeNi(CO)3-, were successfully synthesized and characterized via mass-selected infrared photodissociation spectroscopy in combination with global minimum energy searches. The agreement between experimental and theoretical spectra reveals the coexistence of multiple isomers, different from single-structured iron carbonyl analogues. Besides, these complexes possess the shortest Ce-Ni bond and a relatively higher bond order reported to date, along with a previously unobserved Ce≡O triple bond. Detailed chemical bonding analyses unravel the formation of multicenter covalent bonds or dative bonds between Ce-based moieties and the Ni(CO)3- fragment, highlighting that the Ni 4p (rather than 3d) orbital plays a key role in bonding with the Ce 6s or 5d orbital. Such diverse bonding modes are found to arise from the singly occupied molecular orbital (SOMO) of the Ni(CO)3- fragment, whose amphiphilic nature and multiple active sites facilitate interactions with Ce-based moieties. These findings advance the scope of metal-metal bond between f-block elements and transition metals, paving a way for exploring novel heterobimetallic interactions.

Metal carbonyl complexes provide unique platforms for probing metal-ligand bonding, electronic structures, and catalytic mechanisms. This review focusses on recent advances in a series of novel homometallic and hetero-bimetallic carbonyl complexes studied by infrared-vacuum ultraviolet spectroscopy, photoelectron spectroscopy, and quantum chemical calculations. This combined approach enables accurate determination of vibrational characteristics, electron detachment energies, and bonding motifs, allowing clear differentiation between sigma-donation, pi-back-donation, and metal-metal interactions. Investigations of group-3 homoleptic carbonyls identified the first neutral confinement-free species: Sc(CO)(7) and M(CO)(8) (M = Y, La). Spectroscopic observation of neutral OTiCCO(CO)(n) (n = 2-5) served as the fresh evidence for efficient C-O cleavages and concomitant C-C formations. Studies of heterobimetallic carbonyl complexes MFe(CO)(4)(-) (M = Ti, V, Cr, Si, Ge, Sn) and MNi(CO)(n)(-) (M = Sc, Y, Ti, Zr, Hf, V; n = 3-5) indicated coordination preferences dictated by both cluster size and metal identity, along with associated charge redistribution and CO-activation pathways, all of which bear direct relevance to surface catalysis. Collectively, these studies established the well-defined clusters as functional molecular analogues of catalytically active sites, effectively bridging fundamental bonding concepts with applications in CO/CO2 utilization, syngas chemistry, and energy-conversion processes.

Nitrogen fixation is a challenging target in chemistry. N2 adsorption on transition metal sites has been identified as a prerequisite for activating the stable N≡N triple bond in industrial and biological processes. The structural and bonding properties of the Rh2O2(N2)n- (n = 1-2) complexes have been investigated via mass-selected photoelectron velocity-map imaging spectroscopy combined with quantum chemical calculations. The experimental and theoretical results indicate that the N2 molecules in the Rh2O2(N2)n- (n = 1-2) complexes possess the end-on bonding motifs. Adsorption and activation of dinitrogen are facilitated by charge transfer from Rh and O to N2. The importance of π back-donation from the 4d orbital of the Rh atom to the antibonding π orbitals of N2 for dinitrogen activation is discussed in detail; these results identify Rh2O2(N2)n- (n = 1-2) as a key adsorbed species in the initial stage of dinitrogen activation by rhodium oxide clusters.

Catalytic alkyne semi-hydrogenation is a pivotal industrial transformation currently constrained by the limitations of existing heterogeneous catalysts: precious metals (e.g., Pd) face high costs, while non-precious alternatives (e.g., Ni, Cu, Fe) generally require harsh reaction conditions (100-200 degrees C, 10-40 bar H2). To address these persistent challenges, we propose a novel strategy integrating alkali metal hydrides, serving simultaneously as hydrogen donors and anionic "ligands" into transition metal systems. Herein, we report a NiFe-LiH composite catalyst that enables efficient and highly selective liquid-phase semi-hydrogenation of alkynes-including aliphatic, aromatic, and heteroatom-substituted substrates under very mild conditions (25 degrees C, <= 3 bar H2). Mechanistic investigations reveal that reactive hydride species (H-) are integral to the hydrogenation catalysis. Critically, the formation of unique Li-Ni-Fe-H interfacial species facilitate synergistic interactions wherein anionic hydrides coordinate with transition metals, modulating their electronic states to simultaneously enhance catalytic activity and selectivity. This work demonstrates room-temperature activation and transformation of alkynes enabled by a catalytic Li-Ni-Fe-H system, offering a promising new pathway for developing efficient nonnoble metal catalysts for selective hydrogenation reactions.

Investigating the influences of gaseous pollutants on volatile organic compound photooxidation mechanisms is essential for establishing robust air quality management strategies. Here, a set of well-defined experiments were conducted to elucidate the roles of NO and SO2 in Δ3-carene-derived secondary organic aerosol (SOA) formation. Employing electron ionization time-of-flight aerosol mass spectrometer, we found that high NO concentration significantly inhibited new particle formation (NPF) by reducing the O:C rate of products. With the presence of SO2 pollution, the trend of SOA yield exhibited two distinct regimes governed by SO2 concentration. At low SO2 concentrations ([SO2]0 ≤ 119 ppb), H2SO4 nucleation promoted the NPF and elevated the SOA yields; with the increase of SO2 pollution level (119-178 ppb), the consumption of stabilized Criegee intermediates increased, which reduced particle number concentrations and SOA yields. Significantly, two distinct NPF events were observed in the Δ3-carene photooxidation. Leveraging the unique capabilities of vacuum ultraviolet free electron laser photoionization online aerosol mass spectrometer, we detected the dynamic chemical evolution of SOA across reaction stages and elucidated the differences between such two NPF mechanisms. The results show that the introduction of SO2 progressively accelerated the onset of NPF, with a maximum shift of approximately 1 h H2SO4 formed from SO2 oxidation promoted earlier nucleation, while the generation of particulate formation was dominated by Δ3-carene oxidation. By integrating experimental observations with theoretical calculations, we identified a series of key organosulfates and elucidated their formation mechanisms. These results highlight the NO and SO2 pollution impacts on Δ3-carene photooxidation and advance our understanding of NPF in polluted environments.

Enhancing the mechanistic regulation of the oxygen evolution reaction (OER) is crucial for developing efficient and stable electrocatalysts. However, the dynamic variation of surface structure during the electrocatalytic process limits the accurate identification of the active source and underlying reaction mechanism. Herein, we report an iodine-doping strategy to direct the reconstruction of active species in CuS catalysts toward an unconventional oxygen vacancy oxidation mechanism, thereby overcoming the activity and stability limitations. Mechanistic analysis indicates that the electronic manipulation, weak coordination of Cu-S bonds, and lattice distortion induced by iodine-doping facilitate the thermodynamically favorable Cu2+ to Cu3+ oxidation during OER. The decisively formed oxygen vacancies are emphasized as a genuine active source to promote hydroxyl adsorption, with hypervalent Cu species acting as auxiliary sites to accelerate deprotonation by strengthening Cu-O covalent. Consequently, the optimal iodine-doped CuS exhibits a reduced overpotential of 189 mV at 10 mA cm-2 and superb stability prolonging to 1250 h. When used as a bifunctional electrode in a membrane electrode assembly electrolyzer, it also exhibits a low voltage of 1.65 V at 1 A cm-2, with electrolysis durability of 480 h and a low hydrogen cost of US$1.70/kg H2, outperforming the 2026 targets set by the U.S. Department of Energy.

Elucidating the mutual effects between the different volatile organic compounds (VOCs) is crucial for comprehending the formation mechanism of atmospheric secondary organic aerosols (SOA). Here, the mixed VOCs experiments of isoprene and Δ3-carene/β-caryophyllene were carried out in the presence of O3 using an indoor smog chamber. The suppression effect of isoprene was recognized by the scanning mobility particle sizer spectrometer, online vacuum ultraviolet free electron laser (VUV-FEL) photoionization aerosol mass spectrometry, and quantum chemical calculations. The results indicate that the suppression effect of isoprene on the ozonolysis of Δ3-carene and β-caryophyllene shows fluctuating and monotonous trends, respectively. The carbon content of the precursor could be the main factor for regulating the strength of the suppression effect. Plausible structures and formation mechanisms of several new products generated from the single VOC precursor and VOC-cross-reaction are proposed, which enrich the category of VOC oxidation products. Meanwhile, a new dimerization mechanism of the RO2 + R'O2 reaction is suggested, which offers an intriguing perspective on the gas phase formation process of particle phase accretion products. The present findings provide valuable insights into clarifying the pivotal roles played by isoprene in the interplay between different VOCs and understanding of SOA formation mechanisms of VOC mixtures, especially nearby the emission origins.

Structural characterization of archetypal water clusters is essential for exploring the nature of aqueous hydrogen-bonding interactions that are responsible for the properties of water. While spectroscopic measurement of interference-free neutral water clusters has been proven to be challenging due to the difficulty in size selection, recent studies have successfully measured the infrared spectra of small water clusters (H2O)n (n = 2−10). Thus far, experimental evidence for structural motifs of larger water clusters (H2O)n (n ≥ 11) without environmental perturbation such as an ultraviolet-chromophore label, a messenger tag, or a host matrix has been lacking. Here utilizing the recently-developed size-specific infrared spectroscopy apparatus with a tunable vacuum ultraviolet free electron laser (VUV-FEL) and quantum-chemical studies, we have provided experimental evidence to characterize the structure of interference-free neutral water undecamer (H2O)11. Distinct OH stretching bands provide the evidence for the three lowest-energy isomer families denoted as 515, 43′4, and 55′1 structural motifs. The 515 structure is found to be the dominant one, which features a “5 + 1 + 5” assembling of two stacked 5-membered rings with an additional H2O on the side. Formation mechanism of these three structural motifs is proposed based on calculated energetics. This work provides crucial insights into the microscopic development of hydrogen-bonding water networks and advances our capabilities toward size-dependence study of a diverse range of neutral hydrated clusters for exploring the stepwise mechanisms of solvation processes such as salt dissolution and acid dissociation. Using size-specific infrared spectroscopy and quantum-chemical calculations, structural motifs of interference-free neutral water undecamer (H2O)11 were characterized, revealing three isomer families and insights into hydrogen-bonding networks essential for understanding solvation processes.

Ammonia is the feedstock for nitrogen fertilizers and a potential carbon-free energy carrier, but the current production emits more CO2 than any other chemical producing reaction in the world. The demand for decarbonizing the ammonia industry by using renewable energy has renewed research interests into catalyst development for effective N2 reduction under mild conditions, a grand scientific challenge. Conventional heterogeneous catalysts based on metallic Fe or Ru mediate dinitrogen dissociation and hydrogenation through a relatively energy-costing pathway. The ternary ruthenium complex hydrides Li4RuH6 and Ba2RuH6 reported in this work, on the other hand, represent an entirely new class of compound catalysts, which are composed of the electron- and H-rich [RuH6] anionic centers for non-dissociative dinitrogen reduction, where hydridic H transports electron and proton between the centers, and the Li(Ba) cations for stabilizing NxHy (x: 0 to 2, y: 0 to 3) intermediates. The dynamic and synergistic involvement of all the components of the ternary complex hydrides facilitates a novel reaction mechanism with a narrow energy span and perfectly balanced kinetic barriers for the multi-step process, leading to ammonia production from N2+H2 with superior kinetics under mild conditions.

The continuous wavelength tunability of free-electron lasers (FELs) offers significant potential for research across various scientific fields. The Dalian Coherent Light Source (DCLS), when operating in the high-gain harmonic generation (HGHG) mode, has demonstrated wavelength tuning capability within the vacuum ultraviolet (VUV) range of 50–150 nm. To address diverse experimental demands, it is imperative to have the capability of fast wavelength switching and scanning. This study presents three wavelength tuning strategies experimentally implemented at DCLS, each characterized by specific tuning ranges and resolutions: (i) full-range continuous wavelength switching over 50–150 nm, typically requiring more than 2 h; (ii) wide-range coarse wavelength scanning with approximately 20% variation within several minutes; and (iii) narrow-range fine wavelength scanning with a range of about 0.44% and a scanning resolution of 0.01% in less than 1 min. Each strategy has its own advantages and limitations, and their combination significantly enhances the flexibility of wavelength manipulation at DCLS, accommodating diverse user requirements.

Despite major progress in the investigation of boron cluster anions, direct experimental study of neutral boron clusters remains a significant challenge because of the difficulty in size selection. Here we report a size-specific study of the neutral B9 cluster using threshold photoionization with a tunable vacuum ultraviolet free electron laser. The ionization potential of B9 is measured to be 8.45±0.02 eV and it is found to have a heptagonal bipyramid D7h structure, quite different from the planar molecular wheel of the B9 - anionic cluster. Chemical bonding analyses reveal superior stability of the bipyramidal structure arising from delocalized σ and π bonding interactions within the B7 ring and between the B7 ring and the capping atoms. Photoionization of B9 breaks the single-electron B-B bond of the capping atoms, which undergo off-axis distortion to enhance interactions with the B7 ring in the singlet ground state of B9 +. The single-electron B-B bond of the capping atoms appears to be crucial in stabilizing the D7h structure of B9. This work opens avenues for direct size-dependent experimental studies of a large variety of neutral boron clusters to explore the stepwise development of network structures.

Hydrogen bonding, as a relatively weak inter-or intra-molecular interaction, plays a crucial role not only in chemical compounds, biological structure, function, and conformational dynamics but also in chemical reactions such as proton transfer and catalysis. Therefore, hydrogen bonding has always been one of the important scientific issues in the fields of chemistry, material, and biology. The study of hydrogen bonds has a history of over 125 years since the first recognition of somewhat unique properties of certain hydrogen-containing substances. However, due to the complexity of hydrogen bonding itself, its concept has been continuously expanded and improved with the development of experimental techniques and chemical theories since its discovery. There is still some mild debate on the definition and essence of hydrogen bonding in current research, which needs to be further explored with the development of theory. Usually, hydrogen bonds are roughly divided into weak hydrogen bonds (5-10 kcal/mol) and strong hydrogen bonds (30-60 kcal/mol) based on their strength. At present, there are a large number of reviews and publications on hydrogen bonding in literature. Due to limitations of spaces, we will not try to be comprehensive, but rather only discuss some selected important literature. In this paper, we will briefly review the development history of the hydrogen bond concept over the past century. In 2011, the IUPAC updated the definition of hydrogen bond as: "The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragmentA-H in which A is more electronegative than H, and an X atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation." Although there are still different understandings of the nature of hydrogen bonds, most studies converge to the viewpoint that hydrogen bonds are usually driven by electrostatic interactions, and in some cases (such as short hydrogen bonds and hydrogen bonds involving ions), charge transfer covalent interactions may account for a larger proportion, indicating that hydrogen bonds have a certain degree of covalent characteristics. Hydrogen bonds have both ionic and partially covalent properties, especially in strong hydrogen bonding systems where the bond energy far exceeds that of typical weak hydrogen bonds, resulting in significant covalent bonding properties. It has more or less reached a consensus that the dominant interactions that contribute to hydrogen bonding include: (a) electrostatic interactions; (b) charge transfer interactions; (c) Pauli repulsion; and (d) London dispersion. Yet controversy sometimes remains over the nature and mechanism of hydrogen bonding, which deserves further in-depth exploration. Among the various models, using natural bond orbital (NBO) theory, Weinhold proposed a localized 3-center 2-electron (3c-2e) model for hydrogen bonding A-HX, which advances the delocalized 3-center/4-electron (3c-4e) model proposed by Pimentel. This 3c-2e model suggests that hydrogen bonding essentially originates from, in addition to electrostatic interaction, the donor-acceptor interaction between the non-bonded lone pair orbitals (nX) on the X atom and the empty antibonding orbitals of A-H (sigma*A-H <- nX). This covalent interaction and electrostatic interaction are the main sources of hydrogen bonding energy. This (3c-2e) model has been augmented with multiple (3c-2e) interactions between the donor and acceptor fragments in our work. In summary, this article reviews the historical evolution, current research status, and recent progress of hydrogen bonding research, and provides a brief introduction to the definition, classification, and characteristics of hydrogen bonding. By selectively reviewing and analyzing the relevant literature related to hydrogen bonding, the aim is to provide insights and understanding on the complex, multifaceted nature of such interactions, and promote further investigations on hydrogen bonding theory and application research.

The Si-O-H containing complexes are key interstellar species, but their structural characterization has been proven to be a challenging experimental target because of the difficulty in size selection of neutral clusters in general. Here, two series of products with the chemical formula of SiOnH2n and SiOnH2n-1 (n = 2-4) were prepared via the reactions of laser-ablated silicon atoms with the water molecules and characterized by using size-specific infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum chemical calculations. The SiOnH2n and SiOnH2n-1 (n = 2-4) products were identified to have the hydroxysilylene structures HSiOH(H2O)n and the silica hydroxide structures SiOH(H2O)n (n = 1-3), respectively. In particular, the HSiOH(H2O)n and SiOH(H2O)n (n = 2 and 3) complexes were found to have cyclic hydrogen-bonded networks. Two possible pathways for the formation of the observed products have been discussed. This work provides new insights into the astrochemically relevant Si-O-H molecular architectures that are helpful for understanding the formation mechanism of cosmic particle.

In this study, we investigated the structure and bonding of Au(CO2)n- (n = 2, 3) using photoelectron spectroscopy analysis, quantum chemical calculations, and weak interaction analysis. Quantum chemical calculations revealed that the geometries of the physisorbed structures closely aligned with experimental data, suggesting that these configurations were the most stable under the experimental conditions. Conversely, while chemisorbed structures exhibit stronger interactions and considerable CO2 activation, they show less agreement with the observed spectroscopic data. Using the interaction region indicator method, our weak interaction analysis confirmed that van der Waals forces were the dominant interaction in the physisorbed structures. Our experimental results indicate that these physically adsorbed structures are more stable under the conditions of this study. These findings shed light on the interaction mechanisms of Au(CO2)n- (n = 2, 3) at the molecular level and provide new insights into the potential for transition metals to catalytically activate CO2.