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Electronic energy relaxation and transfer shapes the photochemistry in molecules and materials that are exposed to UV radiation in areas ranging from astrochemistry to biology. The interaction between CO2 and polycyclic aromatic hydrocarbons (PAHs) specifically, is of paramount interest in astrochemically relevant ices, the transition to renewable energy and the development of green chemistry. We investigate the vacuum UV excitation of the naphthalene-CO2 complex and observe excited states of CO2 through a newly identified molecular electronic energy transfer ionization mechanism. We evaluate the spectral development upon cluster growth with time-dependent density functional theory and show that the photoionization spectrum of naphthalene-CO2 closely resembles the photon-stimulated desorption spectrum of CO2 ice. The molecular electronic energy transfer ionization mechanism may affect the energy redistribution and charge balance in the interstellar medium significantly and therefore we discuss its implications for astrochemical models.

In the interstellar medium, diols and other prebiotic molecules adsorb onto icy mantles surrounding dust grains. Water in the ice may affect the reactivity and photoionization of these diols. Ethylene glycol (EG), 1,2-propylene glycol, and 1,3-propylene glycol clusters with water clusters were used as a proxy to study these interactions. The diol-water clusters were generated in a continuous supersonic molecular beam, photoionized by synchrotron-based vacuum ultraviolet light from the Advanced Light Source, and subsequently detected by reflectron time-of-flight mass spectrometry. The appearance energies for the detected clusters were determined from the mass spectra, collected at increasing photon energy. Clusters of both diol fragments and unfragmented diols with water were detected. The lowest energy geometry optimized conformers for the observed EG-water clusters and EG fragment-water clusters have been visualized using density functional theory (DFT), providing insight into hydrogen bonding networks and how these affect fragmentation and appearance energy. As the number of water molecules clustered around EG fragments (m/z 31 and 32) increased, the appearance energy for the cluster decreased, indicating a stabilization by water. This trend was supported by DFT calculations. Fragment clusters from 1,2-propylene glycol exhibited a similar trend, but with a smaller energy decrease, and no trend was observed from 1,3-propylene glycol. We discuss and suggest that the reactivity and photoionization of diols in the presence of water depend on the size of the diol, the location of the hydroxyl group, and the number of waters clustered around the diol.

The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch+) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch+ into the anti-conformer in relatively lower ChCl concentrations. Finally, the depletion of Ch+ from the interface (probing depth = 0.4 nm) is demonstrated by aerosol-based velocity map imaging XPS measurements of glyceline and water mixtures. The nanostructure of liquid-vapor interfaces and structural rearrangement by hydration can provide critical insight into the molecular origin of the deep eutectic behavior and gas-capturing application of DESs.

Understanding the origins of structure and bonding at the molecular level in complex chemical systems spanning magnitudes in length and time is of paramount interest in physical chemistry. We have coupled vibrational spectroscopy and X-ray based techniques with a series of microreactors and aerosol beams to tease out intricate and sometimes subtle interactions, such as hydrogen bonding, proton transfer, and noncovalent interactions. This allows for unraveling the self-assembly of arginine-oleic acid complexes in an aqueous solution and growth processes in a metal-organic framework. Terahertz and infrared spectroscopy provide an intimate view of the hydrogen-bond network and associated phase changes with temperature in neopentyl glycol. The hydrogen-bond network in aqueous glycerol aerosols and levels of protonation of nicotine in aqueous aerosols are visualized. Future directions in probing the hydrogen-bond networks in deep eutectic solvents and organic frameworks are described, and we suggest how X-ray scattering coupled to X-ray spectroscopy can offer insight into the reactivity of organic aerosols.

The 1-indenyl (C9H7•) radical, a prototype aromatic and resonantly stabilized free radical carrying a six- and a five-membered ring, has emerged as a fundamental molecular building block of nonplanar polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanostructures in deep space and combustion systems. However, the underlying formation mechanisms have remained elusive. Here, we reveal an unconventional low-temperature gas-phase formation of 1-indenyl via barrierless ring annulation involving reactions of atomic carbon [C(3P)] with styrene (C6H5C2H3) and propargyl (C3H3•) with phenyl (C6H5•). Macroscopic environments like molecular clouds act as natural low-temperature laboratories, where rapid molecular mass growth to 1-indenyl and subsequently complex PAHs involving vinyl side-chained aromatics and aryl radicals can occur. These reactions may account for the formation of PAHs and their derivatives in the interstellar medium and carbonaceous chondrites and could close the gap of timescales of their production and destruction in our carbonaceous universe.

The stability and distributions of small water clusters generated in a supersonic beam expansion are interrogated by tunable vacuum ultraviolet (VUV) radiation generated at a synchrotron. Time-of-flight mass spectrometry reveals enhanced population of various protonated water clusters (H+(H2O)n) based upon ionization energy and photoionization distance from source, suggesting there are "magic" numbers below the traditional n = 21 that predominates in the literature. These intensity distributions suggest that VUV threshold photoionization (11.0-11.5 eV) of neutral water clusters close to the nozzle exit leads to a different nonequilibrium state compared to a skimmed molecular beam. This results in the appearance of a new magic number at 14. Metadynamics conformer searches coupled with modern density functional calculations are used to identify the global minimum energy structures of protonated water clusters between n = 2 and 21, as well as the manifold of low-lying metastable minima. New lowest energy structures are reported for the cases of n = 5, 6, 11, 12, 16, and 18, and special stability is identified by several measures. These theoretical results are in agreement with the experiments performed in this work in that n = 14 is shown to exhibit additional stability, based on the computed second-order stabilization energy relative to most cluster sizes, though not to the extent of the well-known n = 21 cluster. Other cluster sizes that show some additional energetic stability are n = 7, 9, 12, 17, and 19. To gain insight into the balance between ion-water and water-water interactions as a function of the cluster size, an analysis of the effective two-body interactions (which sum exactly to the total interaction energy) was performed. This analysis reveals a crossover as a function of cluster size between a water-hydronium-dominated regime for small clusters and a water-water-dominated regime for larger clusters around n = 17.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium and in meteorites such as Murchison and Allende and signify the missing link between resonantly stabilized free radicals and carbonaceous nanoparticles (soot particles, interstellar grains). However, the predicted lifetime of interstellar PAHs of some 108 years imply that PAHs should not exist in extraterrestrial environments suggesting that key mechanisms of their formation are elusive. Exploiting a microchemical reactor and coupling these data with computational fluid dynamics (CFD) simulations and kinetic modeling, we reveal through an isomer selective product detection that the reaction of the resonantly stabilized benzyl and the propargyl radicals synthesizes the simplest representative of PAHs - the 10π Hückel aromatic naphthalene (C10H8) molecule - via the novel Propargyl Addition-BenzAnnulation (PABA) mechanism. The gas-phase preparation of naphthalene affords a versatile concept of the reaction of combustion and astronomically abundant propargyl radicals with aromatic radicals carrying the radical center at the methylene moiety as a previously passed over source of aromatics in high temperature environments thus bringing us closer to an understanding of the aromatic universe we live in.

Criegee intermediates (CIs) are central to the ozonolysis of unsaturated organic compounds, controlling the formation of small fragmentation reaction products and higher molecular weight oligomeric compounds through competing unimolecular and bimolecular pathways. In particular, the reaction of thermalized CI with carboxylic acids (RCOOH) to produce oligomeric α-acyloxyalkyl hydroperoxides (AAHPs) is of interest to the chemical transformation of organic compounds in the atmosphere. In the gas phase, the CI + RCOOH reaction proceeds at supercollisional rates, consistent with a barrierless reaction mechanism. However, the rate of this reaction in condensed phases remains uncertain. Here, we report the results of aerosol flow tube studies of the ozonolysis of submicron organic aerosol containing mixtures of an internal n-alkene (Z-9-tricosene, tri) and a saturated acid (2-hexyldecanoic acid, HDA). The decay of the reactants and formation of reaction products are monitored using atmospheric pressure chemical ionization mass spectrometry (APCI-MS). A 6-fold decrease in the reactive uptake of ozone is observed with increasing acid concentration, consistent with a previous study using alcohol additives. The decay of HDA with ozone exposure shows that HDA scavenges CIs at rates comparable to competing unimolecular pathways involving CIs. The decay kinetics of HDA are analyzed using a simple kinetic model in order to constrain the ratio of the CI + RCOOH and unimolecular CI rate constants. Empirical fitting to this model, combined with theoretical estimates of the unimolecular loss rate, yields a CI + RCOOH rate constant of 1.85 ± 0.27 × 10-19 cm3 molec-1 s-1, which is six orders of magnitude slower than the expected diffusion limit in a liquid organic matrix. Stochastic kinetic simulations are conducted with different values of the CI + RCOOH rate constant to validate this result and show that the diffusion-limited rate is indeed much too rapid to reproduce the experimental results.

A new generation of electronic cigarettes is exacerbating the youth vaping epidemic by incorporating additives that increase the acidity of generated aerosols, which facilitate uptake of high nicotine levels. We need to better understand the chemical speciation of vaping aerosols to assess the impact of acidification. Here we used X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to probe the acid-base equilibria of nicotine in hydrated vaping aerosols. We show that, unlike the behavior observed in bulk water, nicotine in the core of aqueous particles was partially protonated when the pH of the nebulized solution was 10.4, with a fraction of free-base nicotine (αFB) of 0.34. Nicotine was further protonated by acidification with equimolar addition of benzoic acid (αFB = 0.17 at pH 6.2). By contrast, the degree of nicotine protonation at the particle surface was significantly lower, with 0.72 < αFB < 0.80 in the same pH range. The presence of propylene glycol and glycerol completely eliminated protonation of nicotine at the surface (αFB = 1) while not affecting significantly its acid-base equilibrium in the particle core. These results provide a better understanding of the role of acidifying additives in vaping aerosols, supporting public health policy interventions.

The confinement of water molecules is vital in fields from biology to nanotechnology. The conditions allowing confinement in small finite polycyclic aromatic hydrocarbons (PAHs) are unclear, yet are crucial for understanding confinement in larger systems. Here, we report a computational study of water cluster confinement within PAHs dimers. Our results serve as a model for larger carbon allotropes and for understanding molecular interactions in confined systems. We identified size and structural motifs allowing confinement and demonstrated the motifs in various PAHs systems. We show that optimal OH⋯π interactions between water clusters and the PAH dimer permit optimal confinement to occur. However, the lack of such interactions leads to the formation of CH⋯O interactions, resulting in less ideal confinement. Confinement of layered clusters is also possible, provided that the optimal OH⋯π interactions are conserved.

Nucleation and crystallization arising from liquid to solid phase are involved in a multitude of processes in fields ranging from materials science to biology. Controlling the thermodynamics and kinetics of growth is advantageous to help tune the formation of complex morphologies. Here, we harness wide-angle X-ray scattering and vibrational spectroscopy to elucidate the mechanism for crystallization and growth of the metal-organic framework Co-MOF-74 within microscopic volumes enclosed in a capillary and an attenuated total reflection microchip reactor. The experiments reveal molecular and structural details of the growth processes, while the results of plane wave density functional calculations allow identification of lattice and linker modes in the formed crystals. Synthesis of the metal-organic framework with microscopic volumes leads to monodisperse and micron-sized crystals, in contrast to those typically observed under bulk reaction conditions. Reduction in the volume of reagents within the microchip reactor was found to accelerate the reaction rate. The coupling of spectroscopy with scattering to probe reactions in microscopic volumes promises to be a useful tool in the synthetic chemist's kit to understand chemical bonding and has potential in designing complex materials.

The molecular origins of homochirality on Earth is not understood well, particularly how enantiomerically enriched molecules of astrobiological significance like sugars and amino acids might have been synthesized on icy grains in space preceding their delivery to Earth. Polycyclic aromatic hydrocarbons (PAHs) identified in carbonaceous chondrites could have been processed in molecular clouds by circularly polarized light prior to the depletion of enantiomerically enriched helicenes onto carbonaceous grains resulting in chiral islands. However, the fundamental low temperature reaction mechanisms leading to racemic helicenes are still unknown. Here, by exploiting synchrotron based molecular beam photoionization mass spectrometry combined with electronic structure calculations, we provide compelling testimony on barrierless, low temperature pathways leading to racemates of [5] and [6]helicene. Astrochemical modeling advocates that gas-phase reactions in molecular clouds lead to racemates of helicenes suggesting a pathway for future astronomical observation and providing a fundamental understanding for the origin of homochirality on early Earth.

Water-cluster interactions with polycyclic aromatic hydrocarbons (PAHs) are of paramount interest in many chemical and biological processes. We report a study of anthracene monomers and dimers with water (up to four)-cluster systems utilizing molecular beam vacuum-UV photoionization mass spectrometry and density functional calculations. Structural loss in photoionization efficiency curves when adding water indicates that various isomers are generated, while theory indicates only a slight shift in energy in photoionization states of different isomers. Calculations reveal that the energetic tendency of water is to remain clustered and not to disperse around the PAH. Theoretically, we observe water confinement exclusively in the case of four water clusters and only when the anthracenes are in a cross configuration due to optimal OH⋯π interactions, indicating dependence on the size and structure of the PAH. Furthermore theory sheds light on the structural changes that occur in water upon ionization of anthracene, due to the optimal interactions of the resulting hole and water hydrogen atoms.

A molecular-level understanding of phase changes in hydrogen-bonded solid-state systems is of great importance in fields spanning thermal science to medical therapeutics. Polyols have recently emerged as prime targets for deployment, given their versatility in phase-induced changes, and occupy a deep space in eutectic solvents. Here, we explore the hydrogen-bond network of neopentyl glycol (NPG) with terahertz time-domain spectroscopy, attenuated total reflection spectroscopy in the far- and mid-infrared regions augmented by electronic structure calculations. A picture emerges where vibrational spectroscopy can exquisitely probe a crystalline to amorphous solid-solid phase transition while spectroscopy in the mid-infrared region provides a molecular picture of the phase transition. These methods are then applied to understand the thermal properties and phase changes in NPG upon incorporation of bis(trifluoromethane)-sulfonimide lithium salt, to demonstrate that vibrational spectroscopy can directly probe the disruption of hydrogen-bond networks in plastic crystals.

Polyol-water clusters provide a template to probe ionization and solvation processes of paramount interest in atmospheric and interstellar chemistry. We generate glycerol water clusters in a continuous supersonic jet expansion and interrogate the neutral species with synchrotron-based tunable vacuum ultraviolet photoionization mass spectrometry. A series of glycerol fragments (m/z 44, 61, 62, and 74) clustered with water are observed. A judicious combination of backing pressure, nozzle temperature, and water vapor pressure allows for tuning the mol % of glycerol. The recorded appearance energies of the water cluster series m/z 62 and 74 are similar to that observed in pure glycerol, while the m/z 61 series shows a dependence on cluster composition. Furthermore, this series also tracks the water concentration of the beam. Theoretical calculations on neutral and ionized clusters visualize the hydrogen bond network in these water clusters and provide an assessment of the number of glycerol-glycerol, glycerol-water, and water-water hydrogen bonds in the cluster, as well as their interaction energies. This method of bond counting and interaction energy assessment explains the changes in the mass spectrum as a function of mol % and offers a glimpse of the disruption of the hydrogen bond network in glycerol-water clusters. The calculations also reveal interesting barrierless chemical processes in photoionized glycerol water clusters that are either activated or do not occur without the presence of water. Examples include spontaneous intramolecular proton transfer within glycerol to form a distonic ion, nonactivated breaking of a C-C bond, and spontaneous proton transfer from glycerol to water. These results appear relevant to radiation-induced chemical processing of alcohol-water ices in the interstellar medium.

Aqueous aerosols are important in atmospheric chemistry, drug delivery, and human health. X-ray photoelectron spectroscopy (XPS), a surface-sensitive technique and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy which informs on the bulk, is deployed to study the solvation of ammonium nitrate (NH4NO3). Aerosolised solutions of NH4NO3 were introduced into a photoelectron spectrometer via an aerodynamic lens, and interrogated with soft X-ray photons, the resulting electrons were imaged via velocity map imaging. Density functional theory calculations were performed to compare with the measured binding energies of NH4+ and NO3− solvated in water. The results reveal that the nitrate anion has a slight propensity for the surface, while both ammonium and nitrate ions are present in equal measure in the bulk.

Polycyclic aromatic hydrocarbons (PAHs) are prevalent in deep space and on Earth as products in combustion processes bearing direct relevance to energy efficiency and environmental remediation. Reactions between hydrocarbon radicals in particular have been invoked as critical molecular mass growth processes toward cyclization leading to these PAHs. However, the mechanism of the formation of PAHs through radical - radical reactions are largely elusive. Here, we report on a combined computational and experimental study of the benzyl (C7H7) radical self-reaction to phenanthrene and anthracene (C14H10) through unconventional, isomer-selective excited state dynamics. Whereas phenanthrene formation is initiated via a barrierless recombination of two benzyl radicals on the singlet ground state surface, formation of anthracene commences through an exotic transition state on the excited state triplet surface through cycloaddition. Our findings challenge conventional wisdom that PAH formation via radical-radical reactions solely operates on electronic ground state surfaces and open up a previously overlooked avenue for a more "rapid" synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.

The properties of bulk water and aqueous solutions are known to change in the vicinity of an interface and/or in a confined environment, including the thermodynamics of ion selectivity at interfaces, transition states and pathways of chemical reactions, and nucleation events and phase growth. Here we describe joint progress in identifying unifying concepts about how air, liquid, and solid interfaces can alter molecular properties and chemical reactivity compared to bulk water and multicomponent solutions. We also discuss progress made in interfacial chemistry through advancements in new theory, molecular simulation, and experiments.

The properties of aerosols are of paramount importance in atmospheric chemistry and human health. The hydrogen bond network of glycerol-water aerosols generated from an aqueous solution with different mixing ratios is probed directly with X-ray photoelectron spectroscopy. The carbon and oxygen X-ray spectra reveal contributions from gas and condensed phase components of the aerosol. It is shown that water suppresses glycerol evaporation up to a critical mixing ratio. A dielectric analysis using terahertz spectroscopy coupled with infrared spectroscopy of the bulk solutions provides a picture of the microscopic heterogeneity prevalent in the hydrogen bond network when combined with the photoelectron spectroscopy analysis. The hydrogen bond network is composed of three intertwined regions. At low concentrations, glycerol molecules are surrounded by water forming a solvated water network. Adding more glycerol leads to a confined water network, maximizing at 22 mol %, beyond which the aerosol resembles bulk glycerol. This microscopic view of hydrogen bonding networks holds promise in probing evaporation, diffusion dynamics, and reactivity in aqueous aerosols.

Polycyclic aromatic hydrocarbons (PAHs) have been invoked in fundamental molecular mass growth processes in our galaxy. We provide compelling evidence of the formation of the very first ringed aromatic and building block of PAHs-benzene-via the self-recombination of two resonantly stabilized propargyl (C3H3) radicals in dilute environments using isomer-selective synchrotron-based mass spectrometry coupled to theoretical calculations. Along with benzene, three other structural isomers (1,5-hexadiyne, fulvene, and 2-ethynyl-1,3-butadiene) and o-benzyne are detected, and their branching ratios are quantified experimentally and verified with the aid of computational fluid dynamics and kinetic simulations. These results uncover molecular growth pathways not only in interstellar, circumstellar, and solar systems environments but also in combustion systems, which help us gain a better understanding of the hydrocarbon chemistry of our universe.

Fullerenes (C60, C70) detected in planetary nebulae and carbonaceous chondrites have been implicated to play a key role in the astrochemical evolution of the interstellar medium. However, the formation mechanism of even their simplest molecular building block-the corannulene molecule (C20H10)-has remained elusive. Here we demonstrate via a combined molecular beams and ab initio investigation that corannulene can be synthesized in the gas phase through the reactions of 7-fluoranthenyl (C16H9˙) and benzo[ghi]fluoranthen-5-yl (C18H9˙) radicals with acetylene (C2H2) mimicking conditions in carbon-rich circumstellar envelopes. This reaction sequence reveals a reaction class in which a polycyclic aromatic hydrocarbon (PAH) radical undergoes ring expansion while simultaneously forming an out-of-plane carbon backbone central to 3D nanostructures such as buckybowls and buckyballs. These fundamental reaction mechanisms are critical in facilitating an intimate understanding of the origin and evolution of the molecular universe and, in particular, of carbon in our galaxy.

Air-liquid interfacial processing of volatile organic compound photooxidation has been suggested as an important source of secondary organic aerosols. However, owing to the lack of techniques for studying the air-liquid interface, the detailed interfacial mechanism remains speculative. To obviate this, we enabled in situ synchrotron-based vacuum ultraviolet single photon ionization mass spectrometry using the system for analysis at the liquid-vacuum interface microreactor to study glyoxal photooxidation at the air-liquid interface. Determination of reaction intermediates and new oxidation products, including polymers and oligomers, by mass spectral analysis and appearance energy measurements has been reported for the first time. Furthermore, an expanded reaction mechanism of photooxidation and free radical induced reactions as a source of aqueous secondary organic aerosol formation is proposed. Single photon ionization can provide new insights into interfacial chemistry.

Aqueous secondary organic aerosol (aqSOA) formation from volatile and semivolatile organic compounds at the air-liquid interface is considered as an important source of fine particles in the atmosphere. However, due to the lack of in situ detecting techniques, the detailed interfacial reaction mechanism and dynamics still remain uncertain. In this study, synchrotron-based vacuum ultraviolet single-photon ionization mass spectrometry (VUV SPI-MS) was coupled with the System for Analysis at the Liquid Vacuum Interface (SALVI) to investigate glyoxal dark oxidation products at the aqueous surface. Mass spectral analysis and determination of appearance energies (AEs) suggest that the main products of glyoxal dark interfacial aging are carboxylic acid related oligomers. Furthermore, the VUV SPI-MS results were compared and validated against those of in situ liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS). The reaction mechanisms of the dark glyoxal interfacial oxidation, obtained using two different approaches, indicate that differences in ionization and instrument operation principles could contribute to their abilities to detect different oligomers. Therefore, the mechanistic differences revealed between the VUV SPI-MS and ToF-SIMS indicate that more in situ and real-time techniques are needed to investigate the contribution of the air-liquid interfacial reactions leading to aqSOA formation.

A study of the formation of microstructures in the reaction of oleic acid with arginine elucidates dynamical self-assembly processes at the molecular level. Terahertz spectroscopy combined with density functional calculations reveals the initial hydrogen-bonding motifs in the assembly process, leading to the formation of micelles and vesicles. Small-angle X-ray scattering measurements allow for kinetic analysis of the growth processes of these nanostructures, revealing a prenucleation pathway of vesicles and micelles which lead to spongelike structures. This final stage of the assembly into spongelike aggregates is investigated with optical microscopy. The formed structures only occur at pH > 8 and are resistant to extreme acidic and basic conditions. A mechanistic pathway to the formation of the spongelike aggregates is described.

The tricyclic polycyclic aromatic hydrocarbons (PAHs) 3H-cyclopenta[a]naphthalene (C13H10), 1H-cyclopenta[b]naphthalene (C13H10) and 1H-cyclopenta[a]naphthalene (C13H10) along with their indene-based bicyclic isomers (E)-5-(but-1-en-3-yn-1-yl)-1H-indene, (E)-6-(but-1-en-3-yn-1-yl)-1H-indene, 5-(but-3-ene-1-yn-1-yl)-1H-in-dene, and 6-(but-3-ene-1-yn-1-yl)-1H-indene were formed via a "directed synthesis" in a high-temperature chemical micro reactor at the temperature of 1300 ± 10 K through the reactions of the 5- and 6-indenyl radicals (C9H7˙) with vinylacetylene (C4H4). The isomer distributions were probed utilizing tunable vacuum ultraviolet light by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C13H10) and 167 (13CC12H10) of the products in a supersonic molecular beam. The underlying reaction mechanisms involve the initial formation of van-der-Waals complexes followed by addition of the 5- and 6-indenyl radicals to vinylacetylene via submerged barriers, followed by isomerization (hydrogen shifts, ring closures), and termination via atomic hydrogen elimination accompanied by aromatization. All the barriers involved in the formation of 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene and 1H-cyclopenta[a]naphthalene are submerged with respect to the reactants indicating that the mechanisms are in fact barrierless, potentially forming PAHs via the hydrogen abstraction - vinylacetylene addition (HAVA) pathway in the cold molecular clouds such as Taurus Molecular Cloud-1 (TMC-1) at temperatures as low as 10 K.

Although static electric fields have been effective in controlling ice nucleation, the highest freezing temperature (Tf) of water that can be achieved in an electric field (E) is still uncertain. We performed a systematic study of the effect of an electric field on water freezing by varying the thickness of a dielectric layer and the voltage across it in an electrowetting system. Results show that Tf first increases sharply with E and then reaches saturation at -3.5 °C after a critical value E of 6 × 106 V/m. Using classical heterogeneous nucleation theory, it is revealed that this behavior is due to saturation in the contact angle of the ice embryo with the underlying substrate. Finally, we show that it is possible to overcome this freezing saturation by controlling the uniformity of the electric field using carbon nanotubes. We achieve a Tf of -0.6 °C using carbon nanotube-based electrodes with an E of 3 × 107 V/m. This work sheds new light on the control of ice nucleation and has the potential to impact many applications ranging from food freezing to ice production.

For the last few decades, the Hydrogen-Abstraction/aCetylene-Addition (HACA) mechanism has been fundamental in aiding our understanding of the source of polycyclic aromatic hydrocarbons (PAHs) in combustion processes and in circumstellar envelopes of carbon rich stars. However, the reaction mechanisms driving high temperature molecular mass growth beyond triphenylene (C18H12) along with the link between PAHs and graphene-type nanostructures as identified in carbonaceous meteorites such as in Murchison and Allende has remained elusive. By exploring the reaction of the 1-naphthyl radical (C10H7˙) with methylacetylene (CH3CCH) and allene (H2CCCH2) under conditions prevalent in carbon-rich circumstellar environments and combustion systems, we provide compelling evidence on a facile formation of 1H-phenalene (C13H10) - the central molecular building block of graphene-type nanostructures. Beyond PAHs, molecular mass growth processes from 1H-phenalene via ring-annulation through C3 molecular building blocks may ultimately lead to two-dimensional structures such as graphene nano flakes and after condensation of multiple layers to graphitized carbon. These fundamental reaction mechanisms are of crucial significance to facilitate an understanding of the origin and chemical evolution of carbon in our Galaxy.

We present synchrotron-based mass spectrometry to probe products formed in a lithium sulphide electrolyte. In operando analysis was carried out at two different potentials in a vacuum compatible microfluidic electrochemical cell. Mass spectral observations show that the charged electrolyte formed sulphur clusters under dynamic conditions, demonstrating electrolyte electron shuttling.

The irreversible dimerization of polycyclic aromatic hydrocarbons (PAHs)-typically pyrene (C16H10) dimerization-is widely used in combustion chemistry models to describe the soot particle inception step. This paper concerns itself with the detection and identification of dimers of flame-synthesized PAH radicals and closed-shell molecules and an experimental assessment of the role of these PAH dimers for the nucleation of soot. To this end, flame-generated species were extracted from an inverse co-flow flame of ethylene at atmospheric pressure and immediately diluted with excess nitrogen before the mixture was analyzed using flame-sampling tandem mass spectrometry with collision-induced fragmentation. Signal at m/z = 404.157 (C32H20) and m/z = 452.157 (C36H20) were detected and identified as dimers of closed-shell C16H10 and C18H10 monomers, respectively. A complex between a C13H9 radical and a C24H12 closed-shell PAH was observed at m/z = 465.164 (C37H21). However, a rigorous analysis of the flame-sampled mass spectra as a function of the dilution ratio, defined as the ratio of the flow rates of the diluent nitrogen to the sampled gases, indicates that the observed dimers are not flame-born, but are produced in the sampling line. In agreement with theoretical considerations, this paper provides experimental evidence that pyrene dimers cannot be a key intermediate in particle inception at elevated flame temperatures.

Electronic excitation and concomitant energy transfer leading to Penning ionization in argon-acetylene clusters generated in a supersonic expansion are investigated with synchrotron-based photoionization mass spectrometry and electronic structure calculations. Spectral features in the photoionization efficiency of the mixed argon-acetylene clusters reveal a blue shift from the 2P1/2 and 2P3/2 excited states of atomic argon. Analysis of this feature suggests that excited states of argon clusters transfer energy to acetylene, resulting in its ionization and successive evaporation of argon. Theoretically calculated Arn (n = 2-6) cluster spectra are in excellent agreement with experimental observations, and provide insight into the structure and ionization dynamics of the clusters. A comparison between argon-acetylene and argon-water clusters reveals that argon solvates water better, allowing for higher-order excitons and Rydberg states to be populated. These results are explained by theoretical calculations of respective binding energies and structures.

A unified low-temperature reaction mechanism on the formation of acenes, phenacenes, and helicenes-polycyclic aromatic hydrocarbons (PAHs) that are distinct via the linear, zigzag, and ortho-condensed arrangements of fused benzene rings-is revealed. This mechanism is mediated through a barrierless, vinylacetylene mediated gas-phase chemistry utilizing tetracene, [4]phenacene, and [4]helicene as benchmarks contesting established ideas that molecular mass growth processes to PAHs transpire at elevated temperatures. This mechanism opens up an isomer-selective route to aromatic structures involving submerged reaction barriers, resonantly stabilized free-radical intermediates, and systematic ring annulation potentially yielding molecular wires along with racemic mixtures of helicenes in deep space. Connecting helicene templates to the Origins of Life ultimately changes our hypothesis on interstellar carbon chemistry.

Tunable synchrotron radiation provides a universal yet selective scalpel to decipher molecular information in complex chemical systems when coupled to mass spectrometry and X-ray spectroscopy. At the Chemical Dynamics Beamline, the radiation emanating from the Advanced Light Source at Berkeley has been utilized by physical chemists and chemical physicists to probe chemical reactivity, energetics and spectroscopy for over two decades. Emerging themes are the study of molecular growth mechanisms, solvation, electronic structure and reactivity in clusters, complexes and nanoparticles. The ion-induced and neutral growth mechanisms in methanol and acetylene clusters are revealed by vacuum ultraviolet (VUV) single photon ionization mass spectrometry. The photoionization dynamics of glycerol show signatures of strong ionic hydrogen bonds, non-covalent interactions are explored in naphthalene water clusters, proton transfer pathways are revealed in acetaldehyde water clusters, and exciton charge transfer is probed in argon water clusters. X-ray spectroscopy provides a local probe of a sample's electronic structure with elemental and site-specificity and is thus ideally suited for probing solvation. Velocity map imaging X-ray photoelectron spectroscopy coupled to nanoparticle beams that allows for the visualization of dynamic processes in solvation and molecular growth processes is described. This technique is used to probe reactivity in aerosol chemistry, obtain phase and pH dependent information on aqueous nanoparticles and electron scattering cross-sections from hydrocarbon nanoparticles. We describe future opportunities in probing elusive radicals such as the cyclic 3,5-dehydroxyphenyl radical cation and excited states in water clusters formed in VUV photoionization, explore reactivity in confined spaces via X-ray spectroscopy and elucidate time dynamics with laser-synchrotron pump probe experiments.

Silica can take many forms, and its interaction with water can change dramatically at the interface. Silica based systems offer a rich tapestry to probe the confinement of water as size and volume can be controlled by various templating strategies and synthetic procedures. To this end, microporous silica nanoparticles have been developed by a reverse microemulsion method utilizing zinc nanoclusters encapsulated in hydroxyl-terminated polyamidoamine (PAMAM-OH) dendrimers as a soft template. These nanoparticles were made tunable within the outer diameter range of 20-50 nm with a core mesopore of 2-15 nm. Synthesized nanoparticles were used to study the effects of surface area and microporous volumes on the vibrational spectroscopy of water. These spectra reveal contributions from bulk interfacial/interparticle water, ice-like surface water, liquid-like water, and hydrated silica surfaces suggesting that microporous silica nanoparticles allow a way to probe silica water interactions at the molecular scale.

Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance-stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl-addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen-abstraction/acetylene-addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.

Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks leading to carbonaceous nanoparticles identified in combustion systems and extraterrestrial environments. However, the understanding of their formation and growth in these high temperature environments has remained elusive. We present a mechanism through laboratory experiments and computations revealing how the prototype PAH-naphthalene-can be efficiently formed via a rapid 1-indenyl radical-methyl radical reaction. This versatile route converts five- to six-membered rings and provides a detailed view of high temperature mass growth processes that can eventually lead to graphene-type PAHs and two-dimensional nanostructures providing a radical new view about the transformations of carbon in our universe.

The three-ring polycyclic aromatic hydrocarbons (PAHs) 3H-benz[e]indene (C13H10) and 1H-benz[f]indene (C13H10) along with their naphthalene-based isomers 2-(prop-2-yn-1-yl)naphthalene (C13H10), 2-(prop-1-yn-1-yl)naphthalene (C13H10), and 2-(propa-1,2-dien-1-yl)naphthalene (C13H10) were formed through a "directed synthesis"via a high temperature chemical micro reactor under combustion-like conditions (1300 ± 35 K) through the reactions of the 2-naphthyl isomer (C10H7˙) with allene (C3H4) and methylacetylene (C3H4). The isomer distributions were probed utilizing tunable vacuum ultraviolet radiation from the Advanced Light Source (ALS) by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C13H10) and 167 (13CC12H10) of the products in a supersonic molecular beam. Complemented by electronic structure calculations, our study reveals critical mass growth processes via annulation of a five-membered ring from the reaction between aryl radicals and distinct C3H4 isomers at elevated temperatures as present in combustion processes and in circumstellar envelopes of carbon stars. The underlying reaction mechanisms proceed through the initial addition of the 2-naphthyl radical with its radical center to the π-electron density of the allene and methylacetylene reactants via entrance barriers between 8 and 14 kJ mol-1, followed by isomerization (hydrogen shifts, ring closure), and termination via atomic hydrogen losses accompanied by aromatization. The reaction mechanisms reflect the formation of indene - the prototype PAH carrying a single five- and a single six-membered ring - synthesized through the reaction of the phenyl radical (C6H5˙) with allene and methylacetylene. This leads us to predict that aryl radicals - upon reaction with allene/methylacetylene - may undergo molecular mass growth processes via ring annulation and de facto addition of a five-membered ring to form molecular building blocks essential to transit planar PAHs out of the plane.

The three-ring polycyclic aromatic hydrocarbons (PAHs) 3H-benz[e]indene (C13H10) and 1H-benz[f]indene (C13H10) along with their naphthalene-based isomers 2-(prop-2-yn-1-yl)naphthalene (C13H10), 2-(prop-1-yn-1-yl)naphthalene (C13H10), and 2-(propa-1,2-dien-1-yl)naphthalene (C13H10) were formed through a "directed synthesis"via a high temperature chemical micro reactor under combustion-like conditions (1300 ± 35 K) through the reactions of the 2-naphthyl isomer (C10H7˙) with allene (C3H4) and methylacetylene (C3H4). The isomer distributions were probed utilizing tunable vacuum ultraviolet radiation from the Advanced Light Source (ALS) by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C13H10) and 167 (13CC12H10) of the products in a supersonic molecular beam. Complemented by electronic structure calculations, our study reveals critical mass growth processes via annulation of a five-membered ring from the reaction between aryl radicals and distinct C3H4 isomers at elevated temperatures as present in combustion processes and in circumstellar envelopes of carbon stars. The underlying reaction mechanisms proceed through the initial addition of the 2-naphthyl radical with its radical center to the π-electron density of the allene and methylacetylene reactants via entrance barriers between 8 and 14 kJ mol-1, followed by isomerization (hydrogen shifts, ring closure), and termination via atomic hydrogen losses accompanied by aromatization. The reaction mechanisms reflect the formation of indene - the prototype PAH carrying a single five- and a single six-membered ring - synthesized through the reaction of the phenyl radical (C6H5˙) with allene and methylacetylene. This leads us to predict that aryl radicals - upon reaction with allene/methylacetylene - may undergo molecular mass growth processes via ring annulation and de facto addition of a five-membered ring to form molecular building blocks essential to transit planar PAHs out of the plane.

Polycyclic aromatic hydrocarbons (PAHs) may comprise up to 20% of the carbon budget in our galaxy and most PAHs condense onto water-rich icy grain mantles. Benzene-water clusters have been invoked as model systems for studying the photo-processing of water ice mantles containing PAHs. However, there is a paucity of information on larger aromatics, where the extended π cloud could affect photo-processing. In this study, tunable vacuum ultraviolet (VUV) photoionization of naphthalene-water clusters Nx(H2O)y (N denotes naphthalene) is performed using synchrotron radiation and analyzed by reflectron time-of-flight mass spectrometry. Naphthalene clusters up to x = 4 are generated as are naphthalene-water clusters up to y = 25. At low photon energy (

For aerosol particles that exist in highly viscous, diffusion-limited states, steep chemical gradients are expected to form during photochemical aging in the atmosphere. Under these conditions, species at the aerosol surface are more rapidly transformed than molecules residing in the particle interior. To examine the formation and evolution of chemical gradients at aerosol interfaces, the heterogeneous reaction of hydroxyl radicals (OH) on ∼200 nm particles of pure squalane (a branched, liquid hydrocarbon) and octacosane (a linear, solid hydrocarbon) and binary mixtures of the two are used to understand how diffusion limitations and phase separation impact the particle reactivity. Aerosol mass spectrometry is used to measure the effective heterogeneous OH uptake coefficient (γeff) and oxidation kinetics in the bulk, which are compared with the elemental composition of the surface obtained using X-ray photoemission. When diffusion rates are fast relative to the reaction frequency, as is the case for squalane and low-viscosity squalane-octacosane mixtures, the reaction is efficient (γeff ∼ 0.3) and only limited by the arrival of OH to the interface. However, for cases, where the diffusion rates are slower than reaction rates, as in pure octacosane and higher-viscosity squalane-octacosane mixtures, the heterogeneous reaction occurs in a mixing-limited regime and is ∼10× slower (γeff ∼ 0.03). This is in contrast to carbon and oxygen K edge X-ray absorption measurements that show that the octacosane interface is oxidized much more rapidly than that of pure squalane particles. The O/C ratio of the surface (estimated to be the top 6-8 nm of the interface) is measured to change with rate constants of (3.0 ± 0.9) × 10-13 and (8.6 ± 1.2) × 10-13 cm3 molecule-1 s-1 for squalane and octacosane particles, respectively. The differences in surface oxidation rates are analyzed using a previously published reaction-diffusion model, which suggests that a 1-2 nm highly oxidized crust forms on octacosane particles, whereas in pure squalane, the reaction products are homogeneously mixed within the aerosol. This work illustrates how diffusion limitations can form particles with highly oxidized surfaces even at relatively low oxidant exposures, which is in turn expected to influence their microphysics in the atmosphere.

The reactions of the indenyl radicals with acetylene (C2 H2 ) and vinylacetylene (C4 H4 ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C9 H7. ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C6 H5. ) and acetylene forming phenylacetylene (C6 H5 CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol-1 ) and slow, contrary to the exoergic (-38 kJ mol-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.

The reactions of the indenyl radicals with acetylene (C2 H2 ) and vinylacetylene (C4 H4 ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C9 H7 . ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C6 H5 . ) and acetylene forming phenylacetylene (C6 H5 CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol-1 ) and slow, contrary to the exoergic (-38 kJ mol-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.

Application of photoionization mass spectroscopy, a technique capable of assessing protonation states in complex molecules in the gas phase, is challenging for arginine due to its fragility. We report photoionization efficiencies in the valence region of aqueous aerosol particles produced from arginine solutions under various pH and vaporization conditions. By using ab initio calculations, we investigate the stability of different conformers. Our results show that neutral arginine fragments upon ionization in the gas phase but solvation stabilizes the molecular ion, resulting in different photoionization dynamics. We also report the valence-band photoelectron spectra of the aerosol solutions obtained at different pH values.

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C14H9]•) with vinylacetylene (C4H4) yields [4]-helicene (C18H12) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C18H13). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.

Reaction of gold atoms with acetylene and ethylene in a laser ablation source produces a number of gold-containing species. Their photoionization efficiency (PIE) curves are measured using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. Their structures are assigned by comparing the experimental ionization energies and PIE curves to those of potential isomers calculated at the CAM-B3LYP/aug-cc-pVTZ level. For smaller molecules, the contribution of ionization to excited electronic states of the cation is also included using photoionization cross sections calculated using ePolyScat. Reaction with acetylene produces adducts Au(C2H2) and Au(C2H2)2, as well as HAu(C4H2). Reaction with ethylene leads to adducts Au(C2H4), Au(C2H4)2, an adduct with a gold dimer, Au2(C2H4), as well as the gold hydrides AuH, HAu(C2H4), and HAu(C4H4). [Au,C4,H7] is also observed, and it likely corresponds to a gold alkyl, H2C═C(H)-Au(C2H4). Reactions leading to production of odd-hydrogen species are endothermic and are likely due to translationally or electronically excited gold atoms. These measurements provide the first directly measured ionization energy for gold hydride, IE(AuH) = 10.25 ± 0.05 eV. Combining this value with the dissociation energy of AuH+ gives a dissociation energy D0(AuH) = 3.15 ± 0.12 eV. Several other ionization energies are measured: IE(Au2(C2H4)) = 8.42 ± 0.05 eV, IE(HAu(C2H4)) = 9.35 ± 0.05 eV, IE(HAu(C4H2)) = 8.8 ± 0.1 eV, and IE(HAu(C4H4)) = 8.8 ± 0.1 eV.

For the last decades, the hydrogen-abstraction-acetylene-addition (HACA) mechanism has been widely invoked to rationalize the high-temperature synthesis of PAHs as detected in carbonaceous meteorites (CM) and proposed to exist in the interstellar medium (ISM). By unravelling the chemistry of the 9-phenanthrenyl radical ([C14 H9 ]. ) with vinylacetylene (C4 H4 ), we present the first compelling evidence of a barrier-less pathway leading to a prototype tetracyclic PAH - triphenylene (C18 H12 ) - via an unconventional hydrogen abstraction-vinylacetylene addition (HAVA) mechanism operational at temperatures as low as 10 K. The barrier-less, exoergic nature of the reaction reveals HAVA as a versatile reaction mechanism that may drive molecular mass growth processes to PAHs and even two-dimensional, graphene-type nanostructures in cold environments in deep space thus leading to a better understanding of the carbon chemistry in our universe through the untangling of elementary reactions on the most fundamental level.

The detection of benzene in Titan’s atmosphere led to the emergence of polycyclic aromatic hydrocarbons (PAHs) as potential nucleation agents triggering the growth of Titan’s orange-brownish haze layers. However, the fundamental mechanisms leading to the formation of PAHs in Titan’s low-temperature atmosphere have remained elusive. We provide persuasive evidence through laboratory experiments and computations that prototype PAHs like anthracene and phenanthrene (C14H10) are synthesized via barrierless reactions involving naphthyl radicals (C10H7•) with vinylacetylene (CH2=CH–C≡CH) in low-temperature environments. These elementary reactions are rapid, have no entrance barriers, and synthesize anthracene and phenanthrene via van der Waals complexes and submerged barriers. This facile route to anthracene and phenanthrene—potential building blocks to complex PAHs and aerosols in Titan—signifies a critical shift in the perception that PAHs can only be formed under high-temperature conditions, providing a detailed understanding of the chemistry of Titan’s atmosphere by untangling elementary reactions on the most fundamental level.