Absolute photoionization cross-section of the propargyl radical

The isomer distribution of C6H6 products from the propargyl radical gas-phase recombination investigated by threshold-photoelectron spectroscopy

The resonance-stabilization of the propargyl radical (C3H3) makes it among the most important reactive intermediates in extreme environments and grants it a long enough lifetime to recombine in both terrestrial combustion media and cold molecular clouds in space. This makes the propargyl self-reaction a pivotal step in the formation of benzene, the first aromatic ring, to eventually lead to polycyclic aromatic hydrocarbons in a variety of environments. In this work, by producing propargyl radicals in a flow tube where propyne reacted with F atoms and probing the reaction products by mass-selected threshold-photoelectron spectroscopy (TPES), we identified eight C6H6 products in total, including benzene. On top of providing the first comprehensive measurements of the branching ratios of the eight identified C6H6 isomers in the propargyl self reaction products (4 mbar, 298 K conditions), this study also highlights the advantages and disadvantages of using isomer-selective TPES to identify and quantify reaction products.

Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation

Carbene species play an integral role in high-energy chemistry, transition-metal-carbene chemistry, catalysis, photolytic formation of carbohydrates, and possibly even the formation of interstellar sugars. In 1921, "reactive formaldehyde"─now known as hydroxymethylene (HCOH)─was first implicated as an intermediate in photocatalytic processes. However, due to its transient nature, direct observation of HCOH has predominantly been attained using cryogenic isolation methods. As a result, HCOH gas-phase reactivity measurements have been limited. We directly observed HCOH using photoionization spectroscopy following UV photodissociation of methanol. Our measurements show it reacts slowly with O2 at room temperature. This work provides evidence for the formation mechanism of HCOH from CH3OH and its subsequent reactivity under gas-phase reaction conditions.

Estimate of the C-Cl photoionization cross section and absolute photoionization cross sections of chlorinated organic compounds

Chlorinated organic compounds are prominently used for industrial production, but their vapors and emission byproducts can cause detrimental effects to human health and the environment. To accurately quantify organochlorine compounds, the absolute photoionization cross section of tetrachloroethylene, chlorobenzene, 1,2-dichlorobenzene, and chloroacetone are measured using multiplexed synchrotron photoionization mass spectrometry at the Advanced Light Source at Lawrence Berkeley National Laboratory. These measurements allow for the estimation of the C-Cl photoionization cross section, increasing quantification accuracy of chlorinated emissions for kinetic modeling and pollutant mitigation. CBS-QB3 calculations of adiabatic ionization energies and thermochemical appearance energies are also presented and agree well with the experimental results.

Time-resolved quantification of key species and mechanistic insights in low-temperature tetrahydrofuran oxidation

We investigate the kinetics and report the time-resolved concentrations of key chemical species in the oxidation of tetrahydrofuran (THF) at 7500 torr and 450-675 K. Experiments are carried out using high-pressure multiplexed photoionization mass spectrometry (MPIMS) combined with tunable vacuum ultraviolet radiation from the Berkely Lab Advanced Light Source. Intermediates and products are quantified using reference photoionization (PI) cross sections, when available, and constrained by a global carbon balance tracking approach at all experimental temperatures simultaneously for the species without reference cross sections. From carbon balancing, we determine time-resolved concentrations for the ROO˙ and ˙OOQOOH radical intermediates, butanedial, and the combined concentration of ketohydroperoxide (KHP) and unsaturated hydroperoxide (UHP) products stemming from the ˙QOOH + O2 reaction. Furthermore, we quantify a product that we tentatively assign as fumaraldehyde, which arises from UHP decomposition via H2O or ˙OH + H loss. The experimentally derived species concentrations are compared with model predictions using the most recent literature THF oxidation mechanism of Fenard et al., (Combust. Flame, 2018, 191, 252-269). Our results indicate that the literature mechanism significantly overestimates THF consumption and the UHP + KHP concentration at our conditions. The model predictions are sensitive to the rate coefficient for the ROO˙ isomerization to ˙QOOH, which is the gateway for radical chain propagating and branching pathways. Comparisons with our recent results for cyclopentane (Demireva et al., Combust. Flame, 2023, 257, 112506) provide insights into the effect of the ether group on reactivity and highlight the need to determine accurate rate coefficients of ROO˙ isomerization and subsequent reactions.

Photoionization of the Propargyl Radical Using the R-Matrix Method

The absolute photoionization cross section of the propargyl radical measured by Savee et al. [J. Chem. Phys. 2012, 136, 134307] is about 2-3 times larger than the experimental result from Robinson et al. [J. Chem. Phys. 2003, 119, 5311]. Meanwhile, there is a significant discrepancy in the shapes of the propargyl photoionization spectra between these two measurements and the result measured by Zhang et al. [J. Chem. Phys. 2006, 124, 074302]. To estimate the accuracy of the various results, we carried out comprehensive calculations with the multichannel R-matrix method, which uses multiconfigurational wave functions to generate accurate initial and final states. Three scattering models including static-exchange, correlated 1 state, and 80 state close-coupling (CC) approximations are employed to reveal the dynamic interaction. The autoionization peaks near the ionization threshold are predicted for the first time. Two resonances are detected with the photon energy around 41.4 and 49.6 eV in the 80 state CC model. We checked the sensitivity of the results to change descriptions of the active spaces, the different partial waves, and the R-matrix radii in the theoretical model. Our best results are compared with the experimental measurements, and discussions are provided. The cross sections and asymmetry parameters for the valence orbitals are reported for photon energies from ionization thresholds of 70 eV.

Modeling-Experiment-Theory Analysis of Reactions Initiated from Cl + Methyl Formate

Methyl formate (MF; CH3OCHO) is the smallest representative of esters, which are common components of biodiesel. The present study characterizes the thermal dissociation kinetics of the radicals formed by H atom abstraction from MF─CH3OCO and CH2OCHO─through a combination of modeling, experiment, and theory. For the experimental effort, excimer laser photolysis of Cl2 was used as a source of Cl atoms to initiate reactions with MF in the gas phase. Time-resolved species profiles of MF, Cl2, HCl, CO2, CH3, CH3Cl, CH2O, and CH2ClOCHO were measured and quantified using photoionization mass spectrometry at temperatures of 400-750 K and 10 Torr. The experimental data were simulated using a kinetic model, which was informed by ab initio-based theoretical kinetics calculations and included chlorine chemistry and secondary reactions of radical decomposition products. We calculated the rate coefficients for the H-abstraction reactions Cl + MF → HCl + CH3OCO (R1a) and Cl + MF → HCl + CH2OCHO (R1b): k1a,theory = 6.71 × 10-15·T1.14·exp(-606/T) cm3/molecule·s; k1b,theory = 4.67 × 10-18·T2.21·exp(-245/T) cm3/molecule·s over T = 200-2000 K. Electronic structure calculations indicate that the barriers to CH3OCO and CH2OCHO dissociation are 13.7 and 31.6 kcal/mol and lead to CH3 + CO2 (R3) and CH2O + HCO (R5), respectively. The master equation-based theoretical rate coefficients are k3,theory (P = ∞) = 2.94 × 109·T1.21·exp(-6209/T) s-1 and k5,theory (P = ∞) = 8.45 × 108·T1.39·exp(-15132/T) s-1 over T = 300-1500 K. The calculated branching fractions into R1a and R1b and the rate coefficient for R5 were validated by modeling of the experimental species time profiles and found to be in excellent agreement with theory. Additionally, we found that the bimolecular reactions CH2OCHO + Cl, CH2OCHO + Cl2, and CH3 + Cl2 were critical to accurately model the experimental data and constrain the kinetics of MF-radicals. Inclusion of the kinetic parameters determined in this study showed a significant impact on combustion simulations of larger methyl esters, which are considered as biodiesel surrogates.

Riemannian Trust Region Method for Minimization of the Fourth Central Moment for Localized Molecular Orbitals

The importance of localized molecular orbitals (MOs) in correlation treatments beyond mean-field calculation and in the illustration of chemical bonding (and antibonding) can hardly be overstated. However, the generation of orthonormal localized occupied MOs is significantly more straightforward than obtaining orthonormal localized virtual MOs. Orthonormal MOs allow facile use of highly efficient group theoretical methods (e.g., graphical unitary group approach) for calculation of Hamiltonian matrix elements in multireference configuration interaction calculations (such as MRCISD) and in quasi-degenerate perturbation treatments, such as the Generalized Van Vleck Perturbation Theory. Moreover, localized MOs can elucidate qualitative understanding of bonding in molecules, in addition to high-accuracy quantitative descriptions. We adopt the powers of the fourth moment cost function introduced by Jørgensen and coworkers. Because the fourth moment cost functions are prone to having multiple negative Hessian eigenvalues when starting from easily available canonical (or near-canonical) MOs, standard optimization algorithms can fail to obtain the orbitals of the virtual or partially occupied spaces. To overcome this drawback, we applied a trust region algorithm on an orthonormal Riemannian manifold with an approximate retraction from the tangent space built into the first and second derivatives of the cost function. Moreover, the Riemannian trust region outer iterations were coupled to truncated Conjugate Gradient inner loops, which avoided any costly solutions of simultaneous linear equations or eigenvector/eigenvalue solutions. Numerical examples are provided on model systems, including the high-connectivity H₁₀ set in 1-, 2-, and 3-dimensional arrangements, and on a chemically realistic description of cyclobutadiene (c-C₄H₄) and the propargyl radical (C₃H₃). In addition to demonstrating the algorithm on occupied and virtual blocks of orbitals, the method is also shown to work on the active space at the MCSCF level of theory.

Radical-Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C₆H₅) with propargyl radical (H₂CCCH) provides an important prototype for radical-radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300-1000 K and pressure range of 4-10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C₉H₈ and C₉H₇ + H product channels and report experimental isomer-resolved product branching fractions for the C₉H₈ product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical-radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C₉H₇ + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C₉H₇) recombination to indene and H-assisted isomerization that converts less stable C₉H₈ isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons.

On the absolute photoionization cross section and threshold photoelectron spectrum of two reactive ketenes in lignin valorization: fulvenone and 2-carbonyl cyclohexadienone

We report the absolute photoionization cross section (PICS) of fulvenone and 2-carbonyl cyclohexadienone, two crucial ketene intermediates in lignin pyrolysis, combustion and organic synthesis. Both species were generated in situ by pyrolyzing salicylamide and dectected via imaging photoelectron photoion coincidence spectroscopy. In a deamination reaction, salicylamide loses ammonia yielding 2-carbonyl cyclohexadienone, a ketoketene, which further decarbonylates at higher pyrolysis temperatures to form fulvenone. We recorded the threshold photoelectron spectrum of the ketoketene and assigned the ground state (X̃⁺²A′′ ← X̃¹A′) and excited state (Ã⁺²A′ ← X̃¹A′) bands with the help of Franck–Condon simulations. Adiabatic ionization energies are 8.35 ± 0.01 and 9.19 ± 0.01 eV. In a minor reaction channel, the ketoketene isomerizes to benzpropiolactone, which decomposes subsequently to benzyne by CO₂ loss. Potential energy surface and RRKM rate constant calculations agree with our experimental observations that the decarbonylation to fulvenone outcompetes the decarboxylation to benzyne by almost two orders of magnitude. The absolute PICS of fulvenone at 10.48 eV was determined to be 18.8 ± 3.8 Mb using NH₃ as a calibrant. The PICS of 2-carbonyl cyclohexadienone was found to be 21.5 ± 8.6 Mb at 9 eV. Our PICS measument will enable the quantification of reactive ketenes in lignin valorization and combustion processes using photoionization techniques and provide advanced mechanistic and kinetics insights to aid the bottom-up optimization of such processes.

The absolute photoionization cross section (PICS) of these crucial ketene intermediates supports their quantification in lignin pyrolysis, combustion and organic synthesis.

Thermal and Catalytic Decomposition of 2-Hydroxyethylhydrazine and 2-Hydroxyethylhydrazinium Nitrate Ionic Liquid

To develop chemical kinetics models for the combustion of ionic liquid-based monopropellants, identification of the elementary steps in the thermal and catalytic decomposition of components such as 2-hydroxyethylhydrazinium nitrate (HEHN) is needed but is currently not well understood. The first decomposition step in protic ionic liquids such as HEHN is typically the proton transfer from the cation to the anion, resulting in the formation of 2-hydroxyethylhydrazine (HEH) and HNO₃. In the first part of this investigation, the high-temperature thermal decomposition of HEH is probed with flash pyrolysis (<1400 K) and vacuum ultraviolet (10.45 eV) photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS). Next, the investigation into the thermal and catalytic decomposition of HEHN includes two mass spectrometric techniques: (1) tunable VUV-PI-TOFMS (7.4-15 eV) and (2) ambient ionization mass spectrometry utilizing both plasma and laser ionization techniques whereby HEHN is introduced onto a heated inert or iridium catalytic surface and the products are probed. The products can be identified by their masses, their ionization energies, and their collision-induced fragmentation patterns. Formation of product species indicates that catalytic surface recombination is an important reaction process in the decomposition mechanism of HEHN. The products and their possible elementary reaction mechanisms are discussed.

A New Pathway for Intersystem Crossing: Unexpected Products in the O(3P) + Cyclopentene Reaction

We investigated the reaction of O(³P) with cyclopentene at 4 Torr and 298 K using time-resolved multiplexed photoionization mass spectrometry, where O(³P) radicals were generated by 351 nm photolysis of NO₂ and reacted with excess cyclopentene in He under pseudo-first-order conditions. The resulting products were sampled, ionized, and detected by tunable synchrotron vacuum ultraviolet radiation and an orthogonal acceleration time-of-flight mass spectrometer. This technique enabled measurement of both mass spectra and photoionization spectra as functions of time following the initiation of the reaction. We observe propylketene (41%), acrolein + ethene (37%), 1-butene + CO (19%), and cyclopentene oxide (3%), of which the propylketene pathway was previously unidentified experimentally and theoretically. The automatically explored reactive potential energy landscape at the CCSD(T)-F12a/cc-pVTZ//ωB97X-D/6-311++G(d,p) level and the related master equation calculations predict that cyclopentene oxide is formed on the singlet potential energy surface, whereas propylketene is first formed on the triplet surface. These calculations provide evidence that significant intersystem crossing can happen in this reaction not only around the geometry of the initial triplet adduct but also around that of triplet propylketene. The formation of 1-butene + CO is initiated on the triplet surface, with bond cleavage and hydrogen transfer occurring during intersystem crossing to the singlet surface. At present, we are unable to explain the mechanistic origins of the acrolein + ethene channel, and we thus refrain from assigning singlet or triplet reactivity to this channel. Overall, at least 60% of the products result from triplet reactivity. We propose that the reactivity of cyclic alkenes with O(³P) is influenced by their greater effective degree of unsaturation compared with acyclic alkenes. This work also suggests that searches for minimum-energy crossing points that connect triplet surfaces to singlet surfaces should extend beyond the initial adducts.

Theoretical Calculations of the 242 nm Absorption of Propargyl Radical

The propargyl radical, the most stable isomer of neutral C₃H₃, is important in combustion reactions, and a number of spectroscopic and reaction dynamics studies have been performed over the years. However, theoretical calculations have never been able to find a state that can generate strong absorption around 242 nm as seen in experiments. In this study, we calculated the low-lying electronic energy levels of the propargyl radical using the highly accurate multireference configuration interaction singles and doubles method with triples and quadruples treated perturbatively [denoted as MRCISD(TQ)]. Calculations indicate that this absorption can be attributed to a Franck-Condon-allowed electronic transition from the ground ²B₁ state to the Rydberg-like excited state 1²A₁. Further insight into the behavior of the multireference perturbative theory methods, GVVPT2 and GVVPT3, on a very challenging system are also obtained.

Absolute Photoionization Cross Section of the Simplest Enol, Vinyl Alcohol

The absolute photoionization cross section of vinyl alcohol was determined by multiplexed photoionization mass spectrometry of the Norrish type II photodissociation of butanal at 308 nm. The measured cross sections at 10.005 and 10.205 eV are 7.5 ± 1.9 and 8.1 ± 1.9 MB, respectively. A higher signal-to-noise ratio photoionization spectrum of vinyl alcohol was recorded via the pyrolysis of 2-chloroethanol and scaled to the absolute cross sections measured using the Norrish type II method. From a comparison of our spectrum with previously reported photoelectron spectra we conclude that vinyl alcohol is mainly ionized by direct ionization in the energy range of 9-9.6 eV, whereas autoionization is responsible for the steady rise in the photoionization spectrum above the end of the Franck-Condon envelope at 9.9 eV.

Product Detection of the CH(X2Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

The reaction of the methylidyne radical (CH(X²Π)) with cyclopentadiene (c-C₅H₆) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C₆H₆ + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C₆H₆ isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C₆H₆ isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C₆H₇ potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

Study of the Synchrotron Photoionization Oxidation of Alpha-Angelica Lactone (AAL) Initiated by O(3P) at 298, 550, and 700 K

In recent years, biofuels have been receiving significant attention because of their potential for decreasing carbon emissions and providing a long-term renewable solution to unsustainable fossil fuels. Currently, lactones are some of the alternatives being produced. Many lactones occur in a range of natural substances and have many advantages over bioethanol. In this study, the oxidation of alpha-angelica lactone initiated by ground-state atomic oxygen, O(³P), was studied at 298, 550, and 700 K using synchrotron radiation coupled with multiplexed photoionization mass spectrometry at the Lawrence Berkeley National Lab (LBNL). Photoionization spectra and kinetic time traces were measured to identify the primary products. Ketene, acetaldehyde, methyl vinyl ketone, methylglyoxal, dimethyl glyoxal, and 5-methyl-2,4-furandione were characterized as major reaction products, with ketene being the most abundant at all three temperatures. Possible reaction pathways for the formation of the observed primary products were computed using the CBS–QB3 composite method.

Quantitative Detection of Products and Radical Intermediates in Low-Temperature Oxidation of Cyclopentane

We present a combined experimental and theoretical investigation of the autoignition chemistry of a prototypical cyclic hydrocarbon, cyclopentane. Experiments using a high-pressure photolysis reactor coupled to time-resolved synchrotron VUV photoionization mass spectrometry directly probe the short-lived radical intermediates and products in cyclopentane oxidation reactions. We detect key peroxy radical intermediates ROO and OOQOOH, as well as several hydroperoxides, formed by second O₂ addition. Automated quantum chemical calculations map out the R + O₂ + O₂ reaction channels and demonstrate that the detected intermediates belong to the dominant radical chain-branching pathway: ROO (+ O₂) → γ-QOOH + O₂ → γ-OOQOOH → products. ROO, OOQOOH, and hydroperoxide products of second-O₂ addition undergo extensive dissociative ionization, making their experimental assignment challenging. We use photoionization dynamics calculations to aid in their characterization and report the absolute photoionization spectra of isomerically pure ROO and γ-OOQOOH. A global statistical fit of the observed kinetics enables reliable quantification of the time-resolved concentrations of these elusive, yet critical species, paving the way for detailed comparisons with theoretical predictions from master-equation-based models.

Valence Photoionization and Autoionization of the Formyl Radical

We have used 308 nm photolysis of acetaldehyde to measure a photoionization spectrum of the formyl (HCO) radical between 8 and 11.5 eV using an 11 meV FWHM photoionization energy resolution. We have confirmed that the formyl radical is the carrier of the spectrum by generating an identical spectrum of the HCO product in the Cl + H₂CO reaction. The spectrum of HCO and its deuterated isotopologue (DCO) have several resolved autoionizing resonances above the Franck-Condon envelope, which we assign to autoionization after initial excitation into neutral 3sσ and 3p Rydberg states converging to the first triplet excited state of HCO⁺(ã ³A'). The quantum defects for these states are δ_3sσ = 1.06 ± 0.02 and δ_3p = 0.821 ± 0.019. We report absolute photoionization cross-section measurements of σ_HCO^PI(9.907 eV) = 4.5 ± 0.9 Mb, σ_HCO^PI(10.007 eV) = 4.8 ± 1.0 Mb, σ_HCO^PI(10.107 eV) = 6.0 ± 1.2 Mb, σ_HCO^PI(10.107 eV) = 5.7 ± 1.2 Mb, and σ_HCO^PI(10.304 eV) = 10.6 ± 2.2 Mb relative to the photoionization cross section of the methyl radical. The absolute cross-section measurements are a factor of ∼1.5 larger than those determined in past studies, although the presence of strong autoionizing features supports a dependence on photoionization energy resolution. We propose that the semiempirical model of Xu and Pratt for estimation of free radical photoionization cross sections is more accurate when applied with a reference species containing the same atoms as the free radical rather than isoelectronic species with different atoms.

Gas-phase synthesis of benzene via the propargyl radical self-reaction

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.

Reinterpreting the vibrational structure in the electronic spectrum of the propargyl cation (H2C3H+) using an efficient and accurate quantum model

The B̃¹A₁ ← X̃¹A₁ absorption spectra of propargyl cations H₂C₃H⁺ and D₂C₃D⁺ were simulated by an efficient two-dimensional (2D) quantum model, which includes the C-C stretch (v₅) and the C≡C stretch (v₃) vibrational modes. The choice of two modes was based on a scheme that can identify the active modes quantitively by examining the normal coordinate displacements (∆Q) directly based on the ab initio equilibrium geometries and frequencies of the X̃¹A₁ and B̃¹A₁ states of H₂C₃H⁺. The spectrum calculated by the 2D model was found to be very close to those calculated by all the higher three-dimensional (3D) quantum models (including v₅, v₃, and another one in 12 modes of H₂C₃H⁺), which validates the 2D model. The calculated B̃¹A₁ ← X̃¹A₁ absorption spectra of both H₂C₃H⁺ and D₂C₃D⁺ are in fairly good agreement with experimental results.

Five vs. six membered-ring PAH products from reaction of o-methylphenyl radical and two C3H4 isomers

Gas-phase reactions of the o-methylphenyl (o-CH3C6H4) radical with the C3H4 isomers allene (H2C[double bond, length as m-dash]C[double bond, length as m-dash]CH2) and propyne (HC[triple bond, length as m-dash]C-CH3) are studied at 600 K and 4 Torr (533 Pa) using VUV synchrotron photoionisation mass spectrometry, quantum chemical calculations and RRKM modelling. Two major dissociation product ions arise following C3H4 addition: m/z 116 (CH3 loss) and 130 (H loss). These products correspond to small polycyclic aromatic hydrocarbons (PAHs). The m/z 116 signal for both reactions is conclusively assigned to indene (C9H8) and is the dominant product for the propyne reaction. Signal at m/z 130 for the propyne case is attributed to isomers of bicyclic methylindene (C10H10) + H, which contains a newly-formed methylated five-membered ring. The m/z 130 signal for allene, however, is dominated by the 1,2-dihydronaphthalene isomer arising from a newly created six-membered ring. Our results show that new ring formation from C3H4 addition to the methylphenyl radical requires an ortho-CH3 group - similar to o-methylphenyl radical oxidation. These reactions characteristically lead to bicyclic aromatic products, but the structure of the C3H4 co-reactant dictates the structure of the PAH product, with allene preferentially leading to the formation of two six-membered ring bicyclics and propyne resulting in the formation of six and five-membered bicyclic structures.

Determination of absolute photoionization cross-sections of some aromatic hydrocarbons

RATIONALE

Aromatic hydrocarbons play an important role in the formation and growth of polycyclic aromatic hydrocarbon (PAH) and soot particles. Measurements of their absolute photoionization cross-sections (PICSs), that benefit the quantitative investigation of mass spectrometry, are still lacking, however.

METHODS

PICSs of some aromatic hydrocarbons were measured with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Nitric oxide and benzene were chosen as standard references for PICS calibration, since their photoionization cross-sections are well documented in the literature. Binary liquid mixtures of the investigated molecules and their specific solvents were used in the measurements.

RESULTS

The investigated aromatics include naphthalene, phenanthrene, 1-methylnaphthalene, indene, 2-/3-/4-methylphenylacetylene, 2-methylindene, diphenylacetylene, 1-/2-ethynylnaphthalene and acenaphthylene. Photo-induced fragments from the molecules were also observed with increasing photon energy.

CONCLUSIONS

Based on our measurements and literature data, PICSs of most aromatic molecules have very similar values beyond their ionization energies. However, molecules that contain the phenylacetylene structure have PICSs higher than other aromatics.

Direct time-resolved detection and quantification of key reactive intermediates in diethyl ether oxidation at T = 450-600 K

High-pressure multiplexed photoionization mass spectrometry (MPIMS) with tunable vacuum ultraviolet (VUV) ionization radiation from the Lawrence Berkeley Labs Advanced Light Source is used to investigate the oxidation of diethyl ether (DEE). Kinetics and photoionization (PI) spectra are simultaneously measured for the species formed. Several stable products from DEE oxidation are identified and quantified using reference PI cross-sections. In addition, we directly detect and quantify three key chemical intermediates: peroxy (ROO˙), hydroperoxyalkyl peroxy (˙OOQOOH), and ketohydroperoxide (HOOP[double bond, length as m-dash]O, KHP). These intermediates undergo dissociative ionization (DI) into smaller fragments, making their identification by mass spectrometry challenging. With the aid of quantum chemical calculations, we identify the DI channels of these key chemical species and quantify their time-resolved concentrations from the overall carbon atom balance at T = 450 K and P = 7500 torr. This allows the determination of the absolute PI cross-sections of ROO˙, ˙OOQOOH, and KHP into each DI channel directly from experiment. The PI cross-sections in turn enable the quantification of ROO˙, ˙OOQOOH, and KHP from DEE oxidation over a range of experimental conditions that reveal the effects of pressure, O₂ concentration, and temperature on the competition among radical decomposition and second O₂ addition pathways.

Photoionization of C4H5 Isomers

Single-photon, photoelectron-photoion coincidence spectroscopy is used to record the mass-selected ion spectra and slow photoelectron spectra of C₄H₅ radicals produced by the abstraction of hydrogen atoms from three C₄H₆ precursors by fluorine atoms generated by a microwave discharge. Three different C₄H₅ isomers are identified, with the relative abundances depending on the nature of the precursor (1-butyne, 1,2-butadiene, and 1,3-butadiene). The results are compared with our previous work using 2-butyne as a precursor [Hrodmarsson, H. R. J. Phys. Chem. A 2019, 123, 1521-1528]. The slow photoelectron spectra provide new information on the three radical isomers that is in good agreement with previous experimental and theoretical results [Lang, M. J. Phys. Chem. A 2015, 119, 3995-4000; Hansen, N. J. Phys. Chem. A 2006, 110, 3670-3678]. The energy scans of the C₄H₅ photoionization signal are recorded with substantially better resolution and signal-to-noise ratio than those in earlier work, allowing the observation of autoionizing resonances based on excited states of the C₄H₅ cation. Photoelectron images recorded at several energies are also reported, providing insight into the decay processes of these excited states. Finally, in contrast to the earlier work using 2-butyne as a precursor, where H-atom abstraction was the only observed process, F- and H-atom additions to the present precursors are also observed through the detection of C₄H₆F, C₄H₅F, and C₄H₇.

Calculation of the absolute photoionization cross-sections for C1-C4 Criegee intermediates and vinyl hydroperoxides

Criegee Intermediates (CIs) and their isomer Vinyl Hydroperoxides (VHPs) are crucial intermediates in the ozonolysis of alkenes. To better understand the underlying chemistry of CIs and VHPs, progress has been made to detect and identify them by photoionization mass spectrometric experiments. Further reliable quantitative information about these elusive intermediates requires their photoionization cross sections. The present work systematically investigated the near-threshold absolute photoionization cross-sections for ten C1-C4 CIs and VHPs, i.e., formaldehyde oxide (CH₂OO), acetaldehyde oxide (syn-/anti-CH₃CHOO), acetone oxide ((CH₃)₂COO), syn-CH₃-anti-(cis-CH=CH₂)COO, syn-CH₃-anti-(trans-CH=CH₂)COO and vinyl hydroperoxide (CH₂CHOOH), 2-hydroperoxypropene (CH₂=C(CH₃)OOH), syn-CH₂ = anti-(cis-CH=CH₂)-COOH, syn-CH₂ = anti-(trans-CH=CH₂)COOH. The adiabatic ionization energies (AIEs) were calculated at the DLPNO-CCSD(T)/CBS level with uncertainties of less than 0.05 eV. The calculated AIEs for C1-C4 CIs and VHPs vary from 8.75 to 10.0 eV with the AIEs decreasing as the substitutions increase. Franck-Condon factors were calculated with the double Duschinsky approximation and the ionization spectra were obtained based on the calculated ionization energies. Pure electronic photoionization cross sections are calculated by the frozen-core Hartree-Fock (FCHF) approximation. The final determined absolute cross sections are around 4.5-6 Mb for the first and second ionization of CIs and 15-25 Mb for VHPs. It is found that the addition of a methyl group or an unsaturated vinyl substitution for the CIs does not substantially change the absolute value of their cross sections.

An experimental study of indene pyrolysis with synchrotron vacuum ultraviolet photoionization mass spectrometry

Pyrolytic kinetics of indene was studied in a flow reactor at 30 and 760 Torr. Indene and its decomposition products, as well as polycyclic aromatic hydrocarbons (PAHs), were measured with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Five literature models were selected to reproduce the experimental data and analyze the reaction kinetics of indene. The experimental and predicted results illustrate that an indenyl radical is the dominant decomposition intermediate and also the main contributor to the further growth of aromatic rings in the pyrolysis of indene. The indene consumption process needs further precise characterization, especially the subsequent dissociation reactions of indanyl and indenyl radicals. A self-recombination reaction of the indenyl radical and the combination reactions between indenyl and other radicals are found to be necessary for the efficient formation of large PAHs. The absence of these pathways leads to the underprediction of experimental measurements. In contrast, literature models adopting indenyl global reactions for PAH formation generally overestimate the system reactivity. Proper radical combination pathways proposed in a future model should consider not only the PAH formation efficiency but also its impact on system reactivity.

Product detection study of the gas-phase oxidation of methylphenyl radicals using synchrotron photoionisation mass spectrometry

Reactions of ortho and meta-methylphenyl radicals with oxygen form products that depend acutely on the position of the methyl group.

Investigation of Oxidation Reaction Products of 2-Phenylethanol Using Synchrotron Photoionization

A photolytically Cl-initiated oxidation reaction of 2-phenylethanol (2PE) was carried out at the Advanced Light Source (ALS) in the Lawrence Berkeley National Laboratory. Using the multiplex photoionization mass spectrometer, coupled with the tunable vacuum ultraviolet radiation of the ALS, data were collected at low pressure (4-6 Torr) and temperature (298-550 K) regimes. Data analysis was performed via characterization of the reaction species photoionization spectra and kinetic traces. Products and reaction pathways are also computed using the CBS-QB3 composite method. The present results suggest primary products m/ z = 30 (formaldehyde), 106 (benzaldehyde), and 120 (phenylacetaldehyde) at 298 K, and m/ z = 120 (phenylacetaldehyde) at 550 K. Branching fractions at room temperature are 27 ± 6.5% for formaldehyde, 24 ± 4.5% for benzaldehyde, and 25 ± 5.8% for phenylacetaldehyde and 60 ± 14% for phenylacetaldehyde at 550 K.

The reaction of Criegee intermediate CH2OO with water dimer: primary products and atmospheric impact

The rapid reaction of the smallest Criegee intermediate, CH₂OO, with water dimers is the dominant removal mechanism for CH₂OO in the Earth's atmosphere, but its products are not well understood. This reaction was recently suggested as a significant source of the most abundant tropospheric organic acid, formic acid (HCOOH), which is consistently underpredicted by atmospheric models. However, using time-resolved measurements of reaction kinetics by UV absorption and product analysis by photoionization mass spectrometry, we show that the primary products of this reaction are formaldehyde and hydroxymethyl hydroperoxide (HMHP), with direct HCOOH yields of less than 10%. Incorporating our results into a global chemistry-transport model further reduces HCOOH levels by 10-90%, relative to previous modeling assumptions, which indicates that the reaction CH₂OO + water dimer by itself cannot resolve the discrepancy between the measured and predicted HCOOH levels.

Valence shell threshold photoelectron spectroscopy of C3Hx (x = 0–3)

We present the photoelectron spectra of C₃H_x (x = 0–3) formed in a microwave discharge flow-tube reactor by consecutive H abstractions from C₃H₄ (C₃H_x + F → C₃H_x−1 + HF (x = 1–4)), but also from F + CH₄ schemes by secondary reactions.

Investigation on the absolute and relative photoionization cross sections of 3 potential propargylic fuels

Absolute photoionization cross sections for 2 potential propargylic fuels (propargylamine and dipropargyl ether) along with the partial ionization cross sections for their dissociative fragments are measured and presented for the first time via synchrotron photoionization mass spectrometry. The experimental setup consists of a multiplexed orthogonal time-of-flight mass spectrometer and is located at the Advanced Light Source facility of the Lawrence Berkeley National Laboratory in Berkeley, California. Data for a third propargylic compound (propargyl alcohol) were taken; however, because of its low signal, due to its weakly bound cation, only the dissociative ionization fragment from the H-loss channel is observed and presented. Suggested pathways leading to formation of dissociative photoionization fragments along with CBS-QB3 calculated adiabatic ionization energies and appearance energies for the dissociative fragments are also presented.

Study of the Synchrotron Photoionization Oxidation of 2-Methylfuran Initiated by O(3P) under Low-Temperature Conditions at 550 and 650 K

The O-(³P)-initiated oxidation of 2-methylfuran (2-MF) was investigated using vacuum-ultraviolet synchrotron radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. Reaction species were studied by multiplexed photoionization mass spectrometry at 550 and 650 K. The oxygen addition pathway is favored in this reaction, forming four triplet diradicals that undergo intersystem crossing into singlet epoxide species that lead to the formation of products at m/z 30 (formaldehyde), 42 (propene), 54 (1-butyne, 1,3-butadiene, and 2-butyne), and 70 (2-butenal, methyl vinyl ketone, and 3-butenal). Mass-to-charge ratios, photoionization spectra, and adiabatic ionization energies for each primary reaction species were obtained and used to characterize their identities. In addition, by means of electronic structure calculations, potential energy surface scans of the different species produced throughout the oxidation were examined to further validate the primary chemistry occurring. Branching fractions for the formation of the primary products were calculated at the two temperatures and contribute 81.0 ± 21.4% at 550 K and 92.1 ± 25.5% at 650 K.

Absolute photoionization cross sections of two cyclic ketones: cyclopentanone and cyclohexanone

Absolute photoionization cross sections for cyclopentanone and cyclohexanone, as well as partial ionization cross sections for the dissociative ionized fragments, are presented in this investigation. Experiments are performed via a multiplexed photoionization mass spectrometer utilizing vacuum ultraviolet (VUV) synchrotron radiation supplied by the Advanced Light Source of Lawrence Berkeley National Laboratory. These results allow the quantification of these species that is relevant to investigate the kinetics and combustion reactions of potential biofuels. The CBS-QB3 calculated values for the adiabatic ionization energies agree well with the experimental values, and the identification of possible dissociative fragments is discussed for both systems. Copyright © 2017 John Wiley & Sons, Ltd.

Low-Temperature Synchrotron Photoionization Study of 2-Methyl-3-buten-2-ol (MBO) Oxidation Initiated by O(3P) Atoms in the 298-650 K Range

This work studies the oxidation of 2-methyl-3-buten-2-ol initiated by O(³P) atoms. The oxidation was investigated at room temperature, 550, and 650 K. Using the synchrotron radiation from the Advanced Light Source (ALS) of the Lawrence Berkley National Laboratory, reaction intermediates and products were studied by multiplexed photoionization mass spectrometry. Mass-to-charge ratios, kinetic time traces, photoionization spectra, and adiabatic ionization energies for each primary reaction species were obtained and used to characterize their identity. Using electronic structure calculations, potential energy surface scans of the different species produced throughout the oxidation were examined and presented in this paper to further validate the primary chemistry occurring. Branching fractions of primary products at all three temperatures were also provided. At room temperature only three primary products formed: ethenol (26.6%), acetaldehyde (4.2%), and acetone (53.4%). At 550 and 650 K the same primary products were observed in addition to propene (5.1%, 11.2%), ethenol (18.1%, 2.8%), acetaldehyde (8.9%, 5.7%), cyclobutene (1.6%, 10.8%), 1-butene (2.0%, 10.9%), trans-2-butene (3.2%, 23.1%), acetone (50.4%, 16.8%), 3-penten-2-one (1.0%, 11.5%), and 3-methyl-2-butenal (0.9%, 2.5%), where the first branching fraction value in parentheses corresponds to the 550 K data. At the highest temperature, a small amount of propyne (1.0%) was also observed.

Products of Criegee intermediate reactions with NO2: experimental measurements and tropospheric implications

The reactions of Criegee intermediates with NO₂ have been proposed as a potentially significant source of the important nighttime oxidant NO₃, particularly in urban environments where concentrations of ozone, alkenes and NO_x are high. However, previous efforts to characterize the yield of NO₃ from these reactions have been inconclusive, with many studies failing to detect NO₃. In the present work, the reactions of formaldehyde oxide (CH₂OO) and acetaldehyde oxide (CH₃CHOO) with NO₂ are revisited to further explore the product formation over a pressure range of 4–40 Torr. NO₃ is not observed; however, temporally resolved and [NO₂]-dependent signal is observed at the mass of the Criegee–NO₂ adduct for both formaldehyde- and acetaldehyde-oxide systems, and the structure of this adduct is explored through ab initio calculations. The atmospheric implications of the title reaction are investigated through global modelling.

On the absolute photoionization cross section and dissociative photoionization of cyclopropenylidene

We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C₃H₂, and the heat of formation of the C₃H radical and ion derived by the dissociative ionization of the carbene.

Absolute photoionization cross sections of furanic fuels: 2-ethylfuran, 2-acetylfuran and furfural

Absolute photoionization cross sections of the molecules 2-ethylfuran, 2-acetylfuran and furfural, including partial ionization cross sections for the dissociative ionized fragments, are measured for the first time. These measurements are important because they allow fuel quantification via photoionization mass spectrometry and the development of quantitative kinetic modeling for the complex combustion of potential fuels. The experiments are carried out using synchrotron photoionization mass spectrometry with an orthogonal time-of-flight spectrometer used for mass analysis at the Advanced Light Source of Lawrence Berkeley National Laboratory. The CBS-QB3 calculations of adiabatic ionization energies and appearance energies agree well with the experimental results. Several bond dissociation energies are also derived and presented.

Are the three hydroxyphenyl radical isomers created equal?--The role of the phenoxy radical

We have investigated the thermal decomposition of the three hydroxyphenyl radicals (˙C6H4OH) in a heated microtubular reactor. Intermediates and products were identified isomer-selectively applying photoion mass-selected threshold photoelectron spectroscopy with vacuum ultraviolet synchrotron radiation. Similarly to the phenoxy radical (C6H5-O˙), hydroxyphenyl decomposition yields cyclopentadienyl (c-C5H5) radicals in a decarbonylation reaction at elevated temperatures. This finding suggests that all hydroxyphenyl isomers first rearrange to form phenoxy species, which subsequently decarbonylate, a mechanism which we also investigate computationally. Meta- and para-radicals were selectively produced and spectroscopically detectable, whereas the ortho isomer could not be traced due to its fast rethermalization and rapid decomposition in the reactor. A smaller barrier to isomerization to phenoxy was found to be the reason for this observation. Since hydroxyphenyl species may be present under typical sooting conditions in flames, the resonantly stabilized cyclopentadienyl radical adds to the hydrocarbon pool and can contribute to the formation of polycyclic aromatic hydrocarbons, which are precursors in soot formation.