Direct Observation of Covalently Bound Clusters in Resonantly Stabilized Radical Reactions and Implications for Carbonaceous Particle Growth.

Based on quantum mechanically guided experiments that observed elusive intermediates in the domain of inception that lies between large molecules and soot particles, we provide a new mechanism for the formation of carbonaceous particles from gas-phase molecular precursors. We investigated the clustering behavior of resonantly stabilized radicals (RSRs) and their interactions with unsaturated hydrocarbons through a combination of gas-phase reaction experiments and theoretical calculations. Our research directly observed a sequence of covalently bound clusters (CBCs) as key intermediates in the evolution from small RSRs, such as benzyl (C7H7), indenyl (C9H7), 1-methylnaphthyl (1-C11H9), and 2-methylnaphthyl (2-C11H9), to large polycyclic aromatic hydrocarbons (PAHs) consisting of 28 to 55 carbons. We found that hydrogen abstraction and RSR addition drive the formation and growth of CBCs, leading to progressive H-losses, the generation of large PAHs and PAH radicals, and the formation of white smoke (incipient carbonaceous particles). This mechanism of progressive H-losses from CBCs (PHLCBC) elucidates the crucial relationship among RSRs, CBCs, and PAHs, and this study provides an unprecedentedly seamless path of observed assembly from small RSRs to large nanoparticles. Understanding the PHLCBC mechanism over a wide temperature range may enhance the accuracy of multiscale models of soot formation, guide the synthesis of carbonaceous nanomaterials, and deepen our understanding of the origin and evolution of carbon within our galaxy.

Enhancing CO2 adsorption capacity of ZIF-8 by synergetic effect of high pressure and temperature.

Metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) are promising porous materials for adsorption and storage of greenhouse gases, especially CO2. In this study, guided by the CO2 phase diagram, we explore the adsorption behavior of solid CO2 loaded with ZIF-8 framework by heating the sample under high pressures, resulting in a drastic improvement in the CO2 uptake. The behavior of CO2 under simultaneous high temperature (T) and pressure (P) conditions is directly monitored by in situ FTIR spectroscopy. The remarkable enhancement in CO2 adsorption capability observed can be attributed to the synergetic effect of high T and P: high temperature greatly enhances the transport property of solid CO2 by facilitating its diffusion into the framework; high pressure effectively modifies the pore size and shape via changing the linker orientation and creating new adsorption sites within ZIF-8. Our study thus provides important new insights into the tunability and enhancement of CO2 adsorptive capability in MOFs/ZIFs using pressure and temperature combined as a synergetic approach.

Elucidating the toluene formation mechanism in the reaction of propargyl radical with 1,3-butadiene.

Toluene is one of the simplest mono-substituted benzene derivatives and an important precursor to form polycyclic aromatic hydrocarbons (PAHs) and soot. However, there is a lack of critical understanding of the formation mechanisms of the toluene molecule. In this work, we explore high-temperature reactions of propargyl radical addition to 1,3-butadiene in a tubular flow microreactor. We obtain experimental evidence for the distinct formations of three C7H8 isomers consisting of toluene, 1,3,5-cycloheptatriene, and 5-methylene-1,3-cyclohexadiene discriminated by synchrotron VUV photoionization efficiency curves. Toluene is identified as the dominant product, which shows strong contrast with the calculated results of the system. By performing theoretical calculations and kinetic simulations, we found that 5-methylene-1,3-cyclohexadiene is a key product of the primary reaction, and toluene formation is enhanced by unavoidable secondary reactions, such as unimolecular isomerization and/or H-assisted isomerization reactions in the SiC microreactor. The current work provides competitive pathways for the enhanced formation of toluene, and may further help disentangle the toluene-promoted molecular growth mechanism of PAHs in combustion environments.

Discriminating between the dissociative photoionization and thermal decomposition products of ethylene glycol by synchrotron VUV photoionization mass spectrometry and theoretical calculations.

Understanding the combustion chemistry of biofuel compounds is of great importance in the intelligent selection of next-generation alternative fuels. Ethylene glycol (C6H10O2) is a prototypical representative of potential biofuels. In this work, the thermal decompositions along with the dissociative ionization of ethylene glycol are studied by synchrotron VUV photoionization mass spectrometry. As a part of the dissociative ionization study, the appearance energies of seven fragments are measured. Using the theoretical calculation results, the possible formation channels of these fragments are proposed. In particular, the productions of CH3OH+ and CH3OH2+ are suggested to be from the isomerization/dissociation process, where double proton transfer processes are highlighted. Using a tunable VUV source, the high-temperature pyrolysis products of ethylene glycol are differentiated from the dissociative ionization products. Specifically, two isomeric products vinyl alcohol and acetaldehyde by H2O elimination are obtained. Formaldehyde and methanol from direct C-C bond cleavage are identified. The fragmentations of fragile radicals such as hydroxymethyl, methoxy and ethoxy are used to explain the missing products from the direct C-C and C-O bond dissociation reactions. There is no experimental evidence for the occurrence of the H and H2 elimination reactions which may have not been accessed under the present temperature conditions.

A Facile Route for the Formation of Complex Nitrogen-Containing Prebiotic Molecules in the Interstellar Medium.

Prebiotic molecules have often been identified in the interstellar medium and meteorite samples. However, we still have only a fragmentary knowledge of the mechanism of the evolutionary process of these prebiotic molecules. With the aid of state-of-the-art vacuum ultraviolet (VUV)-infrared (IR) spectroscopy and ab initio calculations, we reveal a new pathway leading to the formation of the biorelevant molecules carrying amine groups or peptide bonds via the single-photon ionization induced Michael/cyclization reaction of acrylonitrile (AN)-alcohol heterodimer complexes in the gas phase. In the reactions, not only N-H nitrilium cations with H+-N≡C-R Lewis structure but also cyclic amine cations with a peptide bond can be formed when the AN reacts with alcohols of increasing molecular size (such as ethanol, propanol, or butanol). This study suggests the possibility of unsaturated nitriles being reduced by ionized alcohols in space, which can further drive sequential Michael addition/cyclization reactions to form more complex biorelevant molecules.

Gas-Phase Optical Spectra of the Indenyl Radical and Its Quantitative Detection in a Jet-Stirred Reactor.

Resonance-stabilized radicals (RSRs), such as the indenyl radical (C9H7), are proposed to be initiator radicals in soot inception and growth in hydrocarbon combustion processes, but spectroscopic data for many RSRs are still lacking. In this work, the gas-phase optical absorption spectra of the B̃2A2-X̃2A2 electronic transition of indenyl were identified in a supersonic indene/argon plasma jet. Spectroscopic parameters, including the transition energy, rotational constants, and upper-state lifetime broadening, were obtained from analysis of the experimental spectra. The results were readily applied to the quantitative detection of indenyl produced from high-temperature reactions in a jet-stirred reactor. This study now makes indenyl optically accessible in further reaction kinetics studies and in situ spectroscopic diagnostics of hydrocarbon combustion processes.

Predictive Combustion Kinetics of OH Radical Reactions with a C5 Unsaturated Alcohol: The Competitive H-Abstraction and OH-Addition Reactions of 2-Methyl-3-buten-2-ol.

2-Methyl-3-buten-2-ol (MBO232) is a potential biofuel and renewable fuel additive. In a combustion environment, the consumption of MBO232 is mainly through the reaction with a OH radical, one of the most important oxidants. Here, we predict the intricate reactions of MBO232 and OH radicals under a broad range of combustion conditions, that is, 230-2500 K and 0.01-1000 atm. The potential energy surfaces of H-abstraction and OH-addition have been investigated at the CCSD(T)/CBS//M06-2X/def2-TZVP level, and the rate constants were calculated via Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) theory. The decomposition reactions of the critical intermediates from the OH-addition reactions have also been studied. Our results show that OH-addition reactions are dominant below 850 K, while H-abstraction reactions, especially the channel-abstracting H atoms from the methyl groups, are more competitive at higher temperatures. We found that it is necessary to discriminate H atoms attached to the same C atom, as their abstraction rates can differ by up to 1 order of magnitude. The calculated results show good agreement with the reported experimental data. We have provided the modified Arrhenius expressions for rate constants of the dominant channels. The kinetic data determined in this work are of much value for constructing the combustion models of MBO232 and understanding the combustion kinetics and mechanism of other unsaturated alcohols.

Preparation of individual magnetic sub-levels of 4He(23S1) in a supersonic beam using laser optical pumping and magnetic hexapole focusing.

We compare two different experimental techniques for the magnetic-sub-level preparation of metastable 4He in the 23S1 level in a supersonic beam, namely, magnetic hexapole focusing and optical pumping by laser radiation. At a beam velocity of v = 830 m/s, we deduce from a comparison with a particle trajectory simulation that up to 99% of the metastable atoms are in the MJ″ = +1 sub-level after magnetic hexapole focusing. Using laser optical pumping via the 23P2-23S1 transition, we achieve a maximum efficiency of 94% ± 3% for the population of the MJ″ = +1 sub-level. For the first time, we show that laser optical pumping via the 23P1-23S1 transition can be used to selectively populate each of the three MJ″ sub-levels (MJ″ = -1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific MJ″ sub-levels than magnetic hexapole focusing.

Mechanistic Insights of Multiplexed Effects of Pressure, Physical States, and Laser on the Polymerization Kinetics of Phenylacetylene Probed by In Situ FTIR Spectroscopy.

High-pressure tuned polymerization kinetics have been examined to elucidate the (photon-assisted) polymerization mechanism of phenylacetylene (PA, C6H5C≡CH3) under different stimuli including high-pressure, physical state, and UV radiation effects by using in situ FTIR spectroscopy. The pressure-induced glass-forming and crystalline states of PA are found to be formed at different compression rates. The experimental results suggest that the polymerization is induced in the glass-forming state at a very low pressure in contrast to that in the crystal phase where much higher threshold pressure is required. The measured rate constants were found to strongly depend on the pressure, the physical state, and UV radiation. In particular, the rate constant reduced to different extents either by UV irradiation or upon phase transition. The derived activation volumes from the rate constants allow the direct comparison and elucidation of photoactivation and environmental effects on the polymerization. Finally, the diffusion-controlled 1D polymer growth processes were suggested for the glass-forming state or the crystal state at specific pressures. Overall, the mechanistic insights of this work provide guidance of optimizing the multiplexed reaction conditions for the production of the conducting poly(PA).

Intramolecular CH3-migration-controlled cation reactions in the VUV photochemistry of 2-methyl-3-buten-2-ol investigated by synchrotron photoionization mass spectrometry and theoretical calculations.

2-Methyl-3-buten-2-ol (MBO232) is a biogenic volatile organic compound (BVOC), and has a large percentage of emission into the atmosphere. The vacuum ultraviolet (VUV) photochemistry of BVOCs is of great importance for atmospheric chemistry. Studies have been carried out on several BVOCs but have not extended to MBO232. In the present report, the photoionization and dissociation processes of MBO232 in the energy range of 8.0-15.0 eV have been studied by tunable VUV synchrotron radiation coupled with a time-of-flight mass spectrometer. By measuring the photoionization spectra, the adiabatic ionization energy (AIE) of MBO232 and the appearance energies (AEs) of the eight identified fragment ions (i.e., C4H7O+, C3H7O+, C5H9+, C3H6O+, CH3CO+, CH3O+, C4H5+, and C3H5+) were determined. High-level quantum chemistry calculations suggest that there are 3 direct channels and 5 indirect channels via transition states and intermediates accountable for these fragments. Among the reaction channels, the direct elimination of CH3 is the most dominant channel and produces the resonance-stabilized radical cation. Most interestingly, our results show that the CH3 selectively migrates towards the cation, which leads to the different indirect channels. The CH3 migration is a rare process in the dissociative photoionization of metal-free organic molecules. We explain the process by molecular orbital calculations and electron localization function analysis and explore the non-conventional dissociation channels via the CH3 roaming mechanism. We further perform kinetics analysis using RRKM theory for the channels of interest. The activation barrier, and rate constants are analyzed for the branching fractions of the products. These results provide important implications for the VUV photochemistry of BVOCs in the atmosphere.

Pressure-induced structural transformations and new polymorphs in BiVO4.

BiVO4 has attracted much attention in recent years due to its active photocatalytic and microwave dielectric properties. BiVO4 exhibits a rich structural polymorphism, and its properties strongly depend on the crystalline phase. Therefore, it is of great importance to achieve an easy control of its crystalline phase. In the present work, phase stability and vibrational properties of fergusonite- and zircon-type BiVO4 are investigated up to 41.6 GPa by in situ synchrotron X-ray diffraction (XRD), Raman spectroscopy, and first principles calculation. Upon compression, although having different initial structures, both types of BiVO4 consecutively transform to scheelite- and ß-fergusonite structures. For the first time reported for BiVO4, the ß-fergusonite structure is determined using first principles computational techniques and from refinement of the XRD data. Along the way, one new phase of BiVO4 is theoretically predicted at higher pressures. Moreover, both the fergusonite-to-scheelite and scheelite-to-ß-fergusonite transitions are reversible, while the zircon-to-scheelite transition is irreversible. A large volume collapse is observed associated with each phase transition, and the equations of state for different types of BiVO4 have been determined. These results provide new insights into the relationship between different structural types in the AVO4 family.

Pressure-induced amorphization and reactivity of solid dimethyl acetylene probed by in situ FTIR and Raman spectroscopy.

Conjugated polymers are prominent semiconductors that have unique electric conductivity and photoluminescence. Synthesis of conjugated polymers under high pressure is extremely appealing because it does not require a catalyst or solvent used in conventional chemical methods. Transformation of acetylene and many of its derivatives to conjugated polymers using high pressure has been successfully achieved, but not with dimethyl acetylene (DMA). In this work, we present a high-pressure study on solid DMA using a diamond anvil cell up to 24.4 GPa at room temperature characterized by in situ Fourier transform infrared and Raman spectroscopy. Our results show that solid DMA exists in a phase II crystal structure and is stable up to 12 GPa. Above this pressure, amorphization was initiated and the process was completed at 24.4 GPa. The expected polymeric transformation was not evident upon compression, but only observed upon decompression from a threshold compression pressure (e.g. 14.4 GPa). In situ florescence measurements suggest excimer formation via crystal defects, which induces the chemical reactions. The vibrational spectral analysis suggests the products contain the amorphous poly(DMA) and possibly additional amorphous hydrogenated carbon material.

Pressure-Induced Polymorphic, Optical, and Electronic Transitions of Formamidinium Lead Iodide Perovskite.

Formamidinium lead iodide (FAPbI3) perovskite as a superior solar cell material was investigated in two polymorphs at high pressures using in situ synchrotron X-ray diffraction, FTIR spectroscopy, photoluminescence (PL) spectroscopy, electrical conductivity (EC) measurements, and ab initio calculations. We identified two new structures (i.e., Imm2 and Immm) for α-FAPbI3 but only a structural distortion (in C2/c) for δ-FAPbI3 upon compression. A pressure-enhanced hydrogen bond plays a prominent role in structural modifications, as corroborated by FTIR spectroscopy. PL measurements and calculations consistently show the structure and pressure dependences of the band gap energies. Finally, EC measurements reveal drastically different transport properties of α- and δ-FAPbI3 at low pressures but a common trend to metallic states at high pressures. All of these observations suggest strongly contrasting structural stabilities and pressure-tuned optoelectric properties of the two FAPbI3 polymorphs.

Highly efficient and selective pressure-assisted photon-induced polymerization of styrene.

The polymerization process of condensed styrene to produce polystyrene as an industrially important polymeric material was investigated using a novel approach by combining external compression with ultraviolet radiation. The reaction evolution was monitored as a function of time and the reaction products were characterized by in situ Fourier transform infrared spectroscopy. By optimizing the loading pressures, we observed highly efficient and selective production of polystyrene of different tacticities. Specifically, at relatively low loading pressures, infrared spectra suggest that styrene monomers transform to amorphous atactic polystyrene (APS) with minor crystalline isotactic polystyrene. In contrast, APS was found to be the sole product when polymerization occurs at relatively higher loading pressures. The time-dependent reaction profiles allow the examination of the polymerization kinetics by analyzing the rate constant and activation volume as a function of pressure. As a result, an optimized pressure condition, which allows a barrierless reaction to proceed, was identified and attributed to the very desirable reaction yield and kinetics. Finally, the photoinitiated reaction mechanism and the growth geometry of the polymer chains were investigated from the energy diagram of styrene and by the topology analysis of the crystal styrene. This study shows strong promise to produce functional polymeric materials in a highly efficient and controlled manner.

Dissociative ionization of the 1-propanol dimer in a supersonic expansion under tunable synchrotron VUV radiation.

Photoionization and dissociation of the 1-propanol dimer and subsequent fragmentations have been investigated by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. Besides the protonated monomer cation (C3H7OH)·H(+) (m/z = 61) and Cα-Cß bond cleavage fragment CH2O·(C3H7OH)H(+) (m/z = 91), the measured mass spectrum at an incident photon energy of 13 eV suggests a new dissociation channel resulting in the formation of the (C3H7OH)·H(+)·(C2H5OH) (m/z = 107) fragment. The appearance energies of the fragments (C3H7OH)·H(+), CH2O·(C3H7OH)H(+) and (C3H7OH)·H(+)·(C2H5OH) are measured at 10.05 ± 0.05 eV, 9.48 ± 0.05 eV, and 12.8 ± 0.1 eV, respectively, by scanning photoionization efficiency (PIE) spectra. The 1-propanol ion fragments as a function of VUV photon energy were interpreted with the aid of theoretical calculations. In addition to O-H and Cα-Cß bond cleavage, a new dissociation channel related to Cß-Cγ bond cleavage opens. In this channel, molecular rearrangement (proton transfer and hydrogen transfer after surmounting an energy barrier) gives rise to the generated complex, which then dissociates to produce the mixed propanol/ethanol proton bound cation (C3H7OH)·H(+)·(C2H5OH). This new dissociation channel has not been reported in previous studies of ethanol and acetic acid dimers. The photoionization and dissociation processes of the 1-propanol dimer are described in the photon energy range of 9-15 eV.

Unexpected methyl migrations of ethanol dimer under synchrotron VUV radiation.

While methyl transfer is well known to occur in the enzyme- and metal-catalyzed reactions, the methyl transfer in the metal-free organic molecules induced by the photon ionization has been less concerned. Herein, vacuum ultraviolet single photon ionization and dissociation of ethanol dimer are investigated with synchrotron radiation photoionization mass spectroscopy and theoretical methods. Besides the protonated clusters cation (C2H5OH)⋅H(+) (m/z = 47) and the ß-carbon-carbon bond cleavage fragment CH2O⋅(C2H5OH)H(+) (m/z = 77), the measured mass spectra revealed that a new fragment (C2H5OH)⋅(CH3)(+) (m/z = 61) appeared at the photon energy of 12.1 and 15.0 eV, where the neutral dimer could be vertically ionized to higher ionic state. Thereafter, the generated carbonium ions are followed by a Wagner-Meerwein rearrangement and then dissociate to produce this new fragment, which is considered to generate after surmounting a few barriers including intra- and inter-molecular methyl migrations by the aid of theoretical calculations. The appearance energy of this new fragment is measured as 11.55 ± 0.05 eV by scanning photoionization efficiency curve. While the signal intensity of fragment m/z = 61 starts to increase, the fragments m/z = 47 and 77 tend to slowly incline around 11.55 eV photon energy. This suggests that the additional fragment channels other than (C2H5OH)⋅H(+) and CH2O⋅(C2H5OH)H(+) have also been opened, which consume some dimer cations. The present report provides a clear description of the photoionization and dissociation processes of the ethanol dimer in the range of the photon energy 12-15 eV.

Pressure selected reactivity and kinetics deduced from photoinduced dissociation of ethylene glycol.

The photon-induced reactivity of liquid ethylene glycol (EG) was investigated in a diamond anvil cell at pressures up to ∼4 GPa and ambient temperature. The near-UV radiation at λ = 350 nm was employed to photodissociate EG via the two-photon absorption processes. The reaction evolution was monitored as a function of time and the reaction products were characterized by using in situ FTIR spectroscopy. At low initial loading pressures, the IR spectra show two distinctive sets of profile evaluations indicating sequential photoinduced chemical reactions, which are designated as primary and secondary photochemical processes, respectively. By careful examination of the characteristic IR bands and possible reaction pathways, over ten species as the primary and secondary reaction products were unambiguously identified. Significantly, we found that one of the photodissociation product CO2 forms specific clathrate hydrate structures or clusters that are both time- and pressure-dependent, indicating interesting and unique sequestration behavior of CO2 at high pressures. Quantitative analysis on selective reaction products allows detailed reaction kinetics involving competitive reaction channels to be probed. In particular, the type and quantity of reaction products as well as the kinetics were found highly pressure dependent. Moreover, the pressure variation of the system along the reaction progression allows the interpretation of possible reaction mechanisms of photodissociation of EG under high pressures.

Mass-selected IR-VUV (118 nm) spectroscopic studies of radicals, aliphatic molecules, and their clusters.

Mass-selected IR plus UV/VUV spectroscopy and mass spectrometry have been coupled into a powerful technique to investigate chemical, physical, structural, and electronic properties of radicals, molecules, and clusters. Advantages of the use of vacuum ultraviolet (VUV) radiation to create ions for mass spectrometry are its application to nearly all compounds with ionization potentials below the energy of a single VUV photon, its circumventing the requirement of UV chromophore group, its inability to ionize background gases, and its greatly reduced fragmenting capabilities. In this review, mass-selected IR plus VUV (118 nm) spectroscopy is introduced first in a general manner. Selected application examples of this spectroscopy are presented, which include the detections and structural analysis of radicals, molecules, and molecular clusters in a supersonic jet.

Site-selective ionization of ethanol dimer under the tunable synchrotron VUV radiation and its subsequent fragmentation.

Site-selective ionization of ethanol dimer and the subsequent fragmentation were studied by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. With photoionization efficiency spectra measurements and theoretical calculations, the detailed mechanisms of the ionization-dissociation processes of ethanol dimer under VUV irradiation were explored. In 9.49-10.89 eV photon energy range, it was found that the ejection of the highest occupied molecular orbital (HOMO) electron from hydrogen bond donor induces a rapid barrierless proton-transfer process followed by two competitive dissociation channels, generating (C2H5OH)[middle dot]H(+) and CH2O[middle dot](C2H5OH)H(+), respectively. The latter comes from a carbon-carbon bond cleavage in the donor. While the photon energy is 10.9-11.58 eV, the electron of HOMO-1 of the hydrogen bond acceptor, is removed. Besides the dissociation channel to produce C2H5OH and C2H5OH(+), a new channel to generate (C2H5OH)[middle dot]CH2OH(+) is opened, where the cleavage of the carbon-carbon bond occurs in the acceptor. When the photon energy increases to 11.58 eV, the electron from HOMO-2 is ejected.