Photoionization of C7H6 and C7H5: Observation of the Fulvenallenyl Radical

Fulvenallenyl Radical (C7H5·)-Mediated Gas-Phase Synthesis of Bicyclic Aromatic C10H8 Isomers: Can Fulvenallenyl Efficiently React with Closed-Shell Hydrocarbons?

To understand the reactivity of resonantly stabilized radicals, often found in relevant concentrations in gaseous environments, it is important to determine their main reaction pathways. Here, it is investigated whether the fulvenallenyl radical (C7H5·) reacts preferentially with closed-shell molecules or radicals. Electronic structure calculations on the C10H9 potential energy surface accessed by the reactions of C7H5· with methylacetylene (CH3CCH) and allene (H2CCCH2) were combined with RRKM-ME calculations of temperature- and pressure-dependent rate constants using the automated EStokTP software suite and kinetic modeling to assess the reactivity of C7H5· with closed-shell unsaturated hydrocarbons. Experimentally, the reactions were attempted in a chemical microreactor heated to 998 ± 10 K by preparing fulvenallenyl radicals via pyrolysis of trichloromethylbenzene (C7H5Cl3) and seeding the radicals in methylacetylene or allene carrier gas, with product identification by means of photoionization mass spectrometry. The measured photoionization efficiency curve of m/z = 128 was assigned to a linear combination of the reference curves of two C10H8 isomers, azulene (minor) and naphthalene (major), presumably resulting from the C7H5· plus C3H4 reactions. However, the calculations demonstrated that these reactions are too slow, and kinetic modeling of processes in the reactor allowed us to conclude that the observation of naphthalene and azulene is due to the C7H5· plus C3H3· reaction, where propargyl is produced by direct hydrogen atom abstraction by chlorine (Cl) atoms from allene or methylacetylene and Cl stem from the pyrolysis of C7H5Cl3. Modeling results under the copyrolysis conditions of toluene and methylacetylene in high-temperature shock tube experiments confirmed the prevalence of the fulvenallenyl reaction with propargyl over its reactions with C3H4 even when the concentrations of allene and methylacetylene largely exceed that of propargyl. Overall, the reactions of fulvenallenyl with both allene and methylacetylene were found to be noncompetitive in the formation of naphthalene and azulene thus attesting the inefficiency of the fulvenallenyl radical reactions with the prototype closed-shell hydrocarbon species. In the meantime, the new reaction pathways revealed, including H-assisted isomerizations between C10H8 isomers and decomposition reactions of various C10H9 isomers, emerge as relevant and are recommended for inclusion in combustion kinetic models for naphthalene formation.

Thermal Decomposition of 2- and 4-Iodobenzyl Iodide Yields Fulvenallene and Ethynylcyclopentadienes: A Joint Threshold Photoelectron and Matrix Isolation Spectroscopic Study

The thermal decomposition of 2- and 4-iodobenzyl iodide at high temperatures was investigated by mass-selective threshold photoelectron spectroscopy (ms-TPES) in the gas phase, as well as by matrix isolation infrared spectroscopy in cryogenic matrices. Scission of the benzylic C-I bond in the precursors at 850 K affords 2- and 4-iodobenzyl radicals (ortho- and para-IC6H4CH2•), respectively, in high yields. The adiabatic ionization energies of ortho-IC6H4CH2• to the X̃+(1A') and ã+(3A') cation states were determined to be 7.31 ± 0.01 and 8.78 ± 0.01 eV, whereas those of para-IC6H4CH2• were measured to be 7.17 ± 0.01 eV for X̃+(1A1) and 8.98 ± 0.01 eV for ã+(3A1). Vibrational frequencies of the ring breathing mode were measured to be 560 ± 80 and 240 ± 80 cm-1 for the X̃+(1A') and ã+(3A') cation states of ortho-IC6H4CH2•, respectively. At higher temperatures, subsequent aryl C-I cleavage takes place to form α,2- and α,4-didehydrotoluene diradicals, which rapidly undergo ring contraction to a stable product, fulvenallene. Nevertheless, the most intense vibrational bands of the elusive α,2- and α,4-didehydrotoluene diradicals were observed in the Ar matrices. In addition, high-energy and astrochemically relevant C7H6 isomers 1-, 2-, and 5-ethynylcyclopentadiene are observed at even higher pyrolysis temperatures along with fulvenallene. Complementary quantum chemical computations on the C7H6 potential energy surface predict a feasible reaction cascade at high temperatures from the diradicals to fulvenallene, supporting the experimental observations in both the gas phase and cryogenic matrices.

Gas-phase preparation of azulene (C10H8) and naphthalene (C10H8) via the reaction of the resonantly stabilized fulvenallenyl and propargyl radicals

Synthetic routes to the 10π Hückel aromatic azulene (C10H8) molecule, the simplest polycyclic aromatic hydrocarbon carrying an adjacent five- and seven-membered ring, have been of fundamental importance due to the role of azulene - a structural isomer of naphthalene - as an essential molecular building block of saddle-shaped carbonaceous nanostructures such as curved nanographenes and nanoribbons. Here, we report on the very first gas phase preparation of azulene by probing the gas-phase reaction between two resonantly stabilized radicals, fulvenallenyl and propargyl , in a molecular beam through isomer-resolved vacuum ultraviolet photoionization mass spectrometry. Augmented by electronic structure calculations, the novel Fulvenallenyl Addition Cyclization Aromatization (FACA) reaction mechanism affords a versatile concept for introducing the azulene moiety into polycyclic aromatic systems thus facilitating an understanding of barrierless molecular mass growth processes of saddle-shaped aromatics and eventually carbonaceous nanoparticles (soot, interstellar grains) in our universe.

Gas-Phase Infrared Spectra of the C7H5 Radical and Its Bimolecular Reaction Products

Resonance-stabilized radicals are considered as possible intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs) in interstellar space. Here, we investigate the fulvenallenyl radical, the most stable C₇H₅ isomer by IR/UV ion dip spectroscopy employing free electron laser radiation in the mid-infrared region between 550 and 1750 cm^-1. The radical is generated by pyrolysis from phthalide. Various jet-cooled reaction products are identified by their mass-selective IR spectra in the fingerprint region, based on a comparison with computed spectra. Interestingly, benzyl is present as a second resonance-stabilized radical. It is connected to fulvenallenyl by a sequence of two H atom losses or additions. Among the identified aromatic hydrocarbons are toluene and styrene, as well as polycyclic molecules, such as indene, naphthalene, fluorene and phenanthrene. Mechanisms for the formation of PAH from C₇H₅ have already been suggested in previous computational work. In particular, the radical/radical reaction of two fulvenallenyl radicals provides an efficient route to phenanthrene in one bimolecular step and might be relevant for PAH formation under astrochemical conditions.

Photoelectron spectroscopy in molecular physical chemistry

Photoelectron spectroscopy has long been a powerful method in the toolbox of experimental physical chemistry and molecular physics. Recent improvements in coincidence methods, charged-particle imaging, and electron energy resolution have greatly expanded the variety of environments in which photoelectron spectroscopy can be applied, as well as the range of questions that can now be addressed. In this Perspectives Article, we focus on selected recent studies that highlight these advances and research areas. The topics include reactive intermediates and new thermochemical data, high-resolution comparisons of experiment and theory using methods based on pulsed-field ionisation (PFI), and the application of photoelectron spectroscopy as an analytical tool to monitor chemical reactions in complex environments, like model flames, catalytic or high-temperature reactors.

Continuous Butadiyne Addition to Propargyl: A Radical-Efficient Pathway for Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) play a crucial role in soot inception, interstellar evolution, and nanomaterial synthesis. Although several mechanisms, such as hydrogen-abstraction acetylene/vinylacetylene addition, have previously been proposed, PAH formation and growth are not yet fully understood. We propose an alternate PAH growth mechanism wherein propargyl radical reacts with butadiyne to form larger radicals containing newly fused aromatic rings. Butadiyne is an important intermediate in hydrocarbon oxidation and carbon rich stars, while propargyl is one of the most important resonantly stabilized radicals that persists for long times. Our proposed mechanism is validated by quantum chemical calculations, elementary reaction experiments, laminar flame analysis, and kinetic modeling. Our findings challenge the conventional wisdom that radical site regeneration, being central to PAH growth, requires sequential hydrogen elimination and/or abstraction. In our proposed mechanism, PAH growth does not depend on abundant free radical consumption, and could, therefore, help explain carbonaceous nanoparticle coalescence in radical-deficient reaction environments.

Five Birds with One Stone: Photoelectron Photoion Coincidence Unveils Rich Phthalide Pyrolysis Chemistry

Phthalide pyrolysis has been assumed to be a clean fulvenallene source. We show that this is only true at low temperatures, and the C₇H₆ isomers 1-, 2-, and 5-ethynylcyclopentadiene are also formed at high pyrolysis temperatures. Photoion mass-selected threshold photoelectron spectra are analyzed with the help of (time-dependent) density functional theory, (TD-)DFT, and equation-of-motion ionization potential coupled cluster, EOM-IP-CCSD, calculations, as well as Franck-Condon simulations of partly overlapping bands, to determine ionization energies. The fulvenallene ionization energy is confirmed at 8.23 ± 0.01 eV, and the ionization energies of 1-, 2 and 5-ethynylcyclopentadiene are newly determined at 8.27 ± 0.01, 8.49 ± 0.01 and 8.76 ± 0.02 eV, respectively. Excited state features in the photoelectron spectrum, in particular the Ã^+ 2A' band of 1-ethynylcyclopentadiene, are shown to be practical to isomer-selectively detect species when the ground-state band is congested. At high pyrolysis temperatures, the C₇H₆ isomers may lose a hydrogen atom and yield the fulvenallenyl radical. Its ionization energy is confirmed at 8.20 ± 0.01 eV. The vibrational fingerprint of the first triplet fulvenallenyl cation state is also revealed and yields an ionization energy of 8.33 ± 0.02 eV. Further triplet cation states are identified and modeled in the 10-11 eV range. A reaction mechanism is proposed based on potential energy surface calculations. Based on a simplified reactor model, we show that the C₇H₆ isomer distribution is far from thermal equilibrium in the reactor, presumably because irreversible H loss competes efficiently with isomerization.

Gas-Phase Formation of Fulvenallene (C7H6) via the Jahn-Teller Distorted Tropyl (C7H7) Radical Intermediate under Single-Collision Conditions

The fulvenallene molecule (C₇H₆) has been synthesized via the elementary gas-phase reaction of the methylidyne radical (CH) with the benzene molecule (C₆H₆) on the doublet C₇H₇ surface under single collision conditions. The barrier-less route to the cyclic fulvenallene molecule involves the addition of the methylidyne radical to the π-electron density of benzene leading eventually to a Jahn-Teller distorted tropyl (C₇H₇) radical intermediate and exotic ring opening-ring contraction sequences terminated by atomic hydrogen elimination. The methylidyne-benzene system represents a benchmark to probe the outcome of the elementary reaction of the simplest hydrocarbon radical-methylidyne-with the prototype of a closed-shell aromatic molecule-benzene-yielding nonbenzenoid fulvenallene. Combined with electronic structure and statistical calculations, this bimolecular reaction sheds light on the unusual reaction dynamics of Hückel aromatic systems and remarkable (polycyclic) reaction intermediates, which cannot be studied via classical organic, synthetic methods, thus opening up a versatile path to access this previously largely obscure class of fulvenallenes.

Thermal Decomposition of Benzyl Radicals: Kinetics and Spectroscopy in a Shock Tube

Understanding the mechanism of high-temperature reactions of aromatic hydrocarbons and radicals is essential for the modeling of hydrocarbon growth processes in combustion environments. In this study, the thermal decomposition reaction of benzyl radicals was investigated using time-resolved broadband cavity-enhanced absorption spectroscopy behind reflected shock waves at a postshock pressure of 100 kPa and temperatures of 1530, 1630, and 1740 K. The transient absorption spectra during the decomposition were recorded over the spectral range of 282-410 nm. The spectra were contributed by the absorption of benzyl radicals and some transient and residual absorbing species. The temporal behavior of the absorption was analyzed using a kinetic model to determine the rate constant for benzyl decomposition. The obtained rate constants can be represented by the Arrhenius expression k₁ = 1.1 × 10¹² exp(-30 500 K/T) s^-1 with an estimated logarithmic uncertainty of Δlog₁₀ k = ±0.2. Kinetic simulation of the secondary reactions indicated that fulvenallenyl radicals are potentially responsible for the transient absorption that appeared around 400 nm. This assignment is consistent with the available spectroscopic information of this radical. Possible candidates for the residual absorbing species are presented, suggesting the potential importance of ortho-benzyne as a reactive intermediate.

Pentadiynylidene and Its Methyl-Substituted Derivates: Threshold Photoelectron Spectroscopy of R1-C5-R2 Triplet Carbon Chains

Mass-selective threshold photoelectron spectroscopy in the gas phase was employed to characterize the dialkynyl triplet carbenes pentadiynylidene (HC₅H), methylpentadiynylidene (MeC₅H), and dimethylpentadiynylidene (MeC₅Me). Diazo compounds were employed as precursors to generate the carbenes by flash pyrolysis. The R₁-C₅-R₂ carbon chains were photoionized by vacuum ultraviolet (VUV) synchrotron radiation in photoelectron photoion coincidence (PEPICO) experiments. High-level ab initio computations were carried out to support the interpretation of the experiments. For the unsubstituted pentadiynylidene (R₁ = R₂ = H) the recorded spectrum yields an adiabatic ionization energy (IE_ad) of 8.36 ± 0.03 eV. In addition, a second carbene isomer, 3-(didehydrovinylidene)cyclopropene, with a singlet electronic ground state, was identified in the spectrum based on the IE_ad of 8.60 ± 0.03 eV and Franck-Condon simulations. We found that multireference computations are required to reliably calculate the IE_ad for this molecule. CASPT2 computations predicted an IE_ad = 8.55 eV, while coupled-cluster computations significantly overestimate the IE. The cyclic isomer is most likely formed from another isomer of the precursor present in the sample. Stepwise methyl-substitution of the carbene leads to a reduction of the IE to 7.77 ± 0.04 eV for methylpentadiynylidene and 7.27 ± 0.06 eV for dimethylpentadiynylidene. The photoionization and dissociative photoionization of the precursors is investigated as well.

How the methyl group position influences the ultrafast deactivation in aromatic radicals

Excited xylyl (methyl–benzyl) radical isomers have been studied by femtosecond time-resolved photoelectron spectroscopy and mass spectrometry.

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.

Diborene: Generation and Photoelectron Spectroscopy of an Inorganic Biradical

Diborenes, R-BB-R', are of current interest in inorganic chemistry because they offer the opportunity to tune the properties of a biradical by modifying the substituents of the diborene parent, HBBH. Here we synthesize the elusive diborene by H atom abstraction from diborane, B₂H₆, using fluorine atoms and report a vibrationally resolved photoelectron spectrum of the HBBH biradical. The spectrum is interpreted by comparison with high-level ab initio computations, taking into account the Renner-Teller splitting in the X^+ 2Π ionic ground state, which show an excellent agreement with the experimental spectrum. An adiabatic ionization energy of 9.080 ± 0.015 eV was determined, and a vibrational progression in the boron-boron stretching vibration of 0.14 eV is visible. This is due to the reduction of bond order upon ionization, accompanied by an increase of the computed boron-boron bond length, R_BB, from 1.514 to 1.606 Å.

Kinetics of the a-C3H5 + O2 reaction, investigated by photoionization using synchrotron radiation

The kinetics of the combustion-relevant reaction of the allyl radical, a-C3H5, with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring, using the CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence Spectroscopy) setup. The ability to measure threshold photoelectron spectra enables a background-free detection of reactive species as well as an isomer-specific analysis of reaction products. Allyl was generated by direct photodissociation of allyl iodide at 266 nm and 213 nm and indirectly by the reaction of propene with Cl atoms, which were generated by photolysis from oxalyl chloride at 266 nm. Experiments were conducted at room temperature at low pressures between 0.8 and 3 mbar using Ar as the buffer gas and with excess O2 to maintain nearly pseudo-first-order reaction conditions. Whereas allyl was detected by photoionisation using synchrotron radiation, the main reaction product allyl peroxy was not observed due to dissociative ionisation of this weakly bound species. From the concentration-time profiles of the allyl signal, second-order rate constants between 1.35 × 1011 cm3 mol-1 s-1 at 0.8 mbar and 1.75 × 1011 cm3 mol-1 s-1 at 3 mbar were determined. The rates obtained for the different allyl radical generation schemes agree well with each other, but are about a factor of 2 higher than the ones reported previously using He as a buffer gas. The discrepancy is partly attributed to the higher collision efficiency of Ar causing a varying fall-off behavior. When allyl is produced by the reaction of propene with Cl atom, an unexpected product is observed at m/z = 68, which was identified as 1,3-butadienal in the threshold photoelectron spectrum. It is formed in a secondary reaction of allyl with the OCCl radical, which is generated in the 266 nm photolysis of oxalyl chloride.

Probing different spin states in xylyl radicals and ions

Resonant one-color two-photon ionization spectroscopy and mass-selected threshold photoelectron spectroscopy were applied to study the electronic doublet states of the three xylyl (methyl-benzyl) radicals above 3.9 eV as well as the singlet and triplet states of the cations up to 10.5 eV. The experiments are complemented by quantum chemical calculations and Franck-Condon simulations to characterize the transitions and to identify the origin bands, allowing a precise determination of singlet-triplet splittings in the cations. Torsional motions of the methyl group notably affect the D₀ → D₃ transition of m-xylyl. All other investigated transitions either lead to electronic states with very low rotational barriers or suffer from spectral broadening in excess of methyl torsional energy levels. The methyl internal rotational potential is faithfully reproduced with the most basic ab initio methods, yet hyperconjugation could not be identified as a significant force shaping them. Time-dependent density functional theory describes the excited electronic states better than wave function theory approaches, notably EOM-CCSD.

The ortho-benzyne cation is not planar

A recent review on the photoionisation of the C₆H₄ isomer ortho-benzyne suggests that bands reported in earlier photoelectron spectra might be due to side products or contaminations, while computations raise doubts, whether the cation has a planar geometry. We therefore reinvestigate the photoionisation of ortho-benzyne, generated by pyrolysis from benzocyclobutenedione, by photoion mass-selected threshold photoelectron (ms-TPE) spectroscopy using synchrotron radiation. The experiments are accompanied by a theoretical study that investigates the structure of the ortho-benzyne cation systematically as a function of the computational method, up to CASPT2(11,14) ab initio computations. Our study leads to a re-evaluation of the ionisation energy of ortho-benzyne. It reveals that the ortho-benzyne cation has indeed a twisted C₂ geometry rather than a C_2v structure. A vertical ionisation energy IE_vert of 9.77 eV and an adiabatic ionisation energy of IE_ad = 9.56 eV are computed for ortho-benzyne. A Franck-Condon simulation of the photoelectron spectrum based on the CASPT2 results and including three electronic states of the cation is in agreement with the experiment and yields IE_ad = 9.51 eV (+50 meV/-100 meV). Since this value is in contrast with previous work, the ionisation energy has to be revised based on our study. Computational methods based on density functional theory give a reasonable description of the cationic ground state, but fail for the corresponding excited electronic states that are indispensible for a proper assignment of the photoelectron spectrum.

Experimental and kinetic modeling investigation of rich premixed toluene flames doped with n-butanol

Blending of n-butanol for rich toluene combustion strongly suppresses the formation of PAHs.

Photodissociation dynamics of fulvenallene and the fulvenallenyl radical at 248 and 193 nm

Photofragment translational spectroscopy was used to study the photodissociation of fulvenallene, C₇H₆, and the fulvenallenyl radical, C₇H₅. Fulvenallene only loses H atoms to form fulvenallenyl. Fulvenallenyl exhibits both C₂H₂-loss and C₃H₃-loss pathways.

9-Ethynylfluoroenyl Radicals: Regioselective Dimerization and Post Ring-Cyclization Reactions

9-Ethynylfluorenyl radical derivatives were readily prepared in situ and underwent simultaneous intermolecular coupling reactions. Interestingly, the dimerization process took place in either a head-to-tail or a head-to-head mode between the acetylenic or the allenic resonance forms dependent on the terminal substituents, which could be well explained by their different spin distribution and steric hindrance effects. The structures of the products were confirmed by X-ray crystallographic and other spectroscopic analyses. It was also found that the newly generated dipropinyl dimers underwent a rearrangement and ring-cyclization reaction at room temperature, eventually giving unique difluorenylidene cyclobutene derivatives.

Gas Phase Detection of Benzocyclopropenyl

The gas phase detection of benzocyclopropenyl is reported. In this aromatic resonance stabilized radical, a large angular strain is present due to a three-membered ring annelated to a benzene. The resonant two-color two-photon ionization technique is used to record the D1((2)A2) ← D0((2)B1) electronic transition of this radical after the in situ synthesis in a discharge source. The spectrum features absorptions up to 3300 cm(-1) above the origin band at 19,305 cm(-1). Benzocyclopropenyl is possibly the major product of the bimolecular reaction of benzene and an atomic carbon at low temperatures.

Photoelectron-photoion coincidence spectroscopy for multiplexed detection of intermediate species in a flame

Complex reactive processes in the gas phase often proceed via numerous reaction steps and intermediate species that must be identified and quantified to develop an understanding of the reaction pathways and establish suitable reaction mechanisms. Here, photoelectron-photoion coincidence (PEPICO) spectroscopy has been applied to analyse combustion intermediates present in a premixed fuel-rich (ϕ = 1.7) ethene-oxygen flame diluted with 25% argon, burning at a reduced pressure of 40 mbar. For the first time, multiplexing fixed-photon-energy PEPICO measurements were demonstrated in a chemically complex reactive system such as a flame in comparison with the scanning "threshold" TPEPICO approach used in recent pioneering combustion investigations. The technique presented here is capable of detecting and identifying multiple species by their cations' vibronic fingerprints, including radicals and pairs or triplets of isomers, from a single time-efficient measurement at a selected fixed photon energy. Vibrational structures for these species have been obtained in very good agreement with scanning-mode threshold photoelectron spectra taken under the same conditions. From such spectra, the temperature in the ionisation volume was determined. Exemplary analysis of species profiles and mole fraction ratios for isomers shows favourable agreement with results obtained by more common electron ionisation and photoionisation mass spectrometry experiments. It is expected that the multiplexing fixed-photon-energy PEPICO technique can contribute effectively to the analysis of chemical reactivity and kinetics in and beyond combustion.

Assignment of high-lying bending mode levels in the threshold photoelectron spectrum of NH2: a comparison between pyrolysis and fluorine-atom abstraction radical sources

The bending mode progression in the photoelectron spectrum of NH₂ was observed and assigned up to υ₂⁺ ≤ 5 and K_a⁺ ≤ 3.

Photoionization and pyrolysis of a 1,4-azaborinine: retro-hydroboration in the cation and identification of novel organoboron ring systems

The photoionization and dissociative photoionization of 1,4-di-tert-butyl-1,4-azaborinine by means of synchrotron radiation and threshold photoelectron photoion coincidence spectroscopy is reported. The ionization energy of the compound was determined to be 7.89 eV. Several low-lying electronically excited states in the cation were identified. The various pathways for dissociative photoionization were modeled by statistical theory, and appearance energies AE0K were obtained. The loss of isobutene in a retro-hydroboration reaction is the dominant pathway, which proceeds with a reverse barrier. Pyrolysis of the parent compound in a chemical reactor leads to the generation of several yet unobserved boron compounds. The ionization energies of the C4 H6 BN isomers 1,2- and 1,4-dihydro-1,4-azaborinine and the C3 H6 BN isomer 1,2-dihydro-1,3-azaborole were determined from threshold photoelectron spectra.

Photoionisation of the tropyl radical

We present a study on the photoionisation of the cycloheptatrienyl (tropyl) radical, C₇H₇, using tunable vacuum ultraviolet synchrotron radiation. Tropyl is generated by flash pyrolysis from bitropyl. Ions and electrons are detected in coincidence, permitting us to record mass-selected photoelectron spectra. The threshold photoelectron spectrum of tropyl, corresponding to the X^{+ 1}A₁’ ← X²E₂” transition, reveals an ionisation energy of 6.23 ± 0.02 eV, in good agreement with Rydberg extrapolations, but slightly lower than the value derived from earlier photoelectron spectra. Several vibrations can be resolved and are reassigned to the C–C stretch mode ν₁₆⁺ and to a combination of ν₁₆⁺ with the ring breathing mode ν₂⁺. Above 10.55 eV dissociative photoionisation of tropyl is observed, leading to the formation of C₅H₅⁺ and C₂H₂.

Spectroscopic and thermochemical properties of the c-C6H7 radical: a high-level theoretical study

The electronic ground state (X(2)B(1)) of the cyclohexadienyl radical (c-C(6)H(7)) has been studied by explicitly correlated coupled cluster theory at the RCCSD(T)-F12x (x = a, b) level, partly in combination with the double-hybrid density functional method B2PLYP. An accurate equilibrium structure has been established and the ground-state rotational constants are predicted to be A(0) = 5347.3 MHz, B(0) = 5249.7 MHz, and C(0) = 2692.5 MHz. The calculated vibrational wavenumbers agree well with the recent p-H(2) matrix IR data [M. Bahou, Y.-J. Wu, and Y.-P. Lee, J. Chem. Phys. 136, 154304 (2012)] and several predictions have been made. A low value of 6.803 ± 0.005 eV is predicted for the adiabatic ionization energy of c-C(6)H(7). Owing to a moderately large change in the equilibrium structure upon ionization, the first band of the photoelectron spectrum is dominated by the adiabatic peak (100%) and only the peaks corresponding to excitation of the two lowest totally symmetric vibrations (ν(12) and ν(11)) by one vibrational quantum have relative intensities of more than 15%. The C(6)H(6)-H dissociation energy is calculated to be D(0) = 85.7 kJ mol(-1), with an estimated error of ~2 kJ mol(-1).

A new double imaging velocity focusing coincidence experiment: i2PEPICO

The vacuum ultraviolet (VUV) beamline of the Swiss Light Source has been upgraded after two years of operation. A new, turntable-type monochromator was constructed at the Paul Scherrer Institut, which allows for fast yaw-alignment as well as quick grating change and exchange. In addition to the original imaging photoelectron photoion coincidence endstation (iPEPICO), a second, complementary double imaging setup (i(2)PEPICO) has been built. Volatile samples can be introduced at room temperature or in a molecular beam, a pyrolysis source allows for radical production, and non-volatile solids can be evaporated in a heated cell. Monochromatic VUV radiation ionizes the sample and both photoelectrons and photoions are velocity map imaged onto two fast position sensitive detectors and detected in delayed coincidence. High intensity synchrotron radiation leads to ionization rates above 10(5) s(-1). New data acquisition and processing approaches are discussed for recording coincidence processes at high rates. The setup is capable of resolving pulsed molecular beam profiles and the synchrotron time structure temporally. The latter is shown by photoelectron autocorrelation, which displays both the 1.04 MHz ring clock frequency as well as resolving the micro-pulses with a separation of 2 ns. Kinetic energy release analysis on the dissociative photoionization of CF(4) indicates a dissociation mechanism change in the Franck-Condon allowed energy range of the first ion state.

Fulvenallenyl cation (C7H5(+)) and its complex with an argon atom: results of high-level quantum-chemical calculations

The fulvenallenyl cation (C(7)H(5)(+)) and its complex with an argon atom have been studied by explicitly correlated coupled cluster theory at the CCSD(T)-F12x(x = a, b) level and by the double-hybrid density functional B2PLYP-D. For the free cation, an accurate equilibrium structure has been established and ground-state rotational constants of A(0) = 8116.4 MHz, B(0) = 2004.3 MHz, and C(0) = 1606.9 MHz are predicted. The equilibrium dipole moment is calculated to be μ(e) = 1.305 D, with the positive end of the dipole at the acetylenic hydrogen site. Anharmonic wavenumbers of C(7)H(5)(+) were obtained by combination of harmonic CCSD(T*)-F12a values and B2PLYP-D anharmonic contributions. The most intense vibration is the pseudoantisymmetric CC stretching vibration at 2083 cm(-1). The potential energy surface of the complex C(7)H(5)(+)·Ar is characterized by two energy minima of C(s) symmetry which are separated by a very low energy barrier. The dissociation energy of the most stable structure is predicted to be D(0) = 530 ± 30 cm(-1).