Photoionization and ionic dissociation of the C3H3NS molecule induced by soft X‐ray near the C1s edge

Time of flight mass spectrometry, electron‐ion coincidence, and ion yield spectroscopy were employed to investigate for the first time the thiazole (C₃H₃NS) molecule in the gas phase excited by synchrotron radiation in the soft X‐ray domain. Total ion yield (TIY) and photoelectron‐photoion coincidence (PEPICO) spectra were recorded as a function of the photon energy in the vicinity of the carbon K edge (C1s). The C1s resonant transitions as well as the core ionization thresholds have been determined from the profile of TIY spectrum, and the features were discussed. The corresponding partial ion yields were determined from the PEPICO spectra for the cation species produced upon the molecular photodissociation. Additional ab initio calculations have also been performed from where relevant structural and electronic configuration parameters were obtained for this molecule.     

Unimolecular Reaction Mechanism of an Imidazolin‐2‐ylidene: An iPEPICO Study on the Complex Dissociation of an Arduengo‐Type Carbene

The photoionization and dissociative photoionization of Im(iPr)₂, 1,3‐diisopropylimidazolin‐2‐ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone‐pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IE_ad=7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate‐determining step as hydrogen‐atom migration from the isopropyl group to the carbene carbon center forming a resonance‐stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene‐loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance‐stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate‐sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.     

Dissociative photoionization of Al2(Ch3)6 and Al2(Ch3)3Cl3 in the range 40–100 eV

Dissociation processes of the organoaluminum compounds Al₂(CH₃)₆ and Al₂(CH₃)₃Cl₃ have been studied in the range of valence and Al:2p core-level ionization by means of photoelectron–photoion and photoion–photoion coincidence techniques. The double-ionization threshold and the Al:2p core-ionization threshold of Al₂(CH₃)₆ are estimated to be about 30 and 80 eV

1 1 eV = 96.4853 kJ mol^?1.

respectively. The relative yields of the H⁺?Al⁺ and H⁺?CH_m,⁺ (m′ = 0–3) ion pairs are enhanced around the Al:2p core-ionization threshold of Al₂(CH₃)₆. The photoion–photoion coincidence intensities of Al₂(CH₃)₃Cl₃ are negligibly small throughout the energy range studied. The ratio of the relative yield of AlC₂H₆⁺ to that of Al⁺ increases smoothly through the Al:2p core-ionization and/or excitation region of Al₂(CH₃)₃Cl₃. The variation of the fragmentation pattern with photon energy is discussed in conjunction with the relevant electronic states.     

Negative‐ion/positive‐ion coincidence spectroscopy as a tool to identify anionic fragments: The case of core‐excited CHF3

We have studied the dissociation of the trifluoromethane molecule, CHF₃, into negative ionic fragments at the C 1s and F 1s edges. The measurements were performed by detecting coincidences between negative and positive ions. We observed five different negative ions: F^?, H^?, C^?, CF^?, and F₂^?. Their production was confirmed by the analysis of triple coincidence events (negative‐ion/positive‐ion/positive‐ion or NIPIPI coincidences) that were recorded with cleaner signals than those of the negative‐ion/positive‐ion coincidences. The intensities of the most intense NIPIPI coincidence channels were recorded as a function of photon energy across the C 1s and F 1s excitations and ionization thresholds. We also observed dissociation channels involving the formation of one negative ion and three positive ions. Our results demonstrate that negative‐ion/positive‐ion coincidence spectroscopy is a very sensitive method to observe anions, which at inner‐shell edges are up to three orders of magnitude less probable dissociation products than cations.     

Unimolecular dissociations of excited C3H6O+: A photoelectron—photoion coincidence study of cyclopropanol and allyl alcohol

The photoelectron spectrum of cyclopropanol has been determined and the first few bands have been assigned to the respective ionization processes. The dissociative photoionization of cyclopropanol and allyl alcohol has been studied by means of HeIα photoelectron—photoion coincidence spectroscopy. The breakdown diagrams of the two corresponding radical cations are practically indistinguishable within the entire energy range investigated (= 5 eV). This implies that extensive isomerizations to a single or to a mixture of common precursor structure(s) precede the dissociative processes. The currently available information on the structure of this reactant and, in particular, the role of other C₃H₆O⁺ isomers is discussed. The kinetic and thermodynamic aspects of the present results are outlined. Although three generations of daughter ions are involved, the relative importance of the different fragmentation pathways could be determined. The RRKM analysis of the coincidence data suggests that the formation of the ethylene cation by loss of a CO molecule from the parent ion hardly competes with the other four primary fragmentation reactions in the sense of the statistical theory of unimolecular reactions.     

Electronic properties and dissociative photoionization of thiocyanates. Part II. Valence and shallow-core (sulfur and chlorine 2p) regions of chloromethyl thiocyanate, CH2ClSCN

A combination of photoelectron spectroscopy and synchrotron based photoelectron photoion coincidence (PEPICO) spectra has been applied to investigate the electronic structure and the dissociative ionization of the CH(2)ClSCN molecule in the valence region. The PES is assigned with the electronic structure calculations at the outer-valence Green's function and symmetry adapted cluster/configuration interaction (SAC-CI) levels offer an explanation of our experimental results. Upon vacuum ultraviolet irradiation the low-lying radical cation, located at 10.39 eV is formed. The molecular ion is observed in the time-of-flight mass spectra, together with the CH(2)SCN(+) and CH(2)Cl(+) daughter ions. The total ion yield spectra have been measured in the S 2p and Cl 2p regions and several channels have been determined in dissociative photoionization events for the core-excited species. Thus, by using time-of-flight mass spectrometry and synchrotron radiation the relative abundances of the ionic fragments and their kinetic energy release values were obtained from both PEPICO and photoelectron photoion photoion coincidence spectra. Possible fragmentation processes are discussed and compared with that found for the related CH(3)SCN species.     

Experimental and theoretical investigation of the parabanic acid molecule following VUV excitation and photodissociation

Photodissociation experiments have been performed for the parabanic acid (C₃H₂N₂O₃) molecule in vapor phase using time-of-flight mass spectrometry and synchrotron radiation in the VUV photon energy range. Electron ion coincidence (PEPICO) spectra and partial ion yields have been recorded as a function of the photon energy covering the 11–21 eV valence range region. The resulting photoionization products as well as proposed fragmentation pathways leading to those species are presented and discussed. Electronic structure computations for the neutral and ionic species were also carried out at the B3LYP/aug-cc-pVTZ level of theory.     

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 AE_0K 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 C₄H₆BN isomers 1,2‐ and 1,4‐dihydro‐1,4‐azaborinine and the C₃H₆BN isomer 1,2‐dihydro‐1,3‐azaborole were determined from threshold photoelectron spectra.     

VUV photoionization of acetamide studied by electron/ion coincidence spectroscopy in the 8-24 eV photon energy range

A VUV photoionization study of acetamide was carried out over the 8-24 eV photon energy range using synchrotron radiation and photoelectron/photoion coincidence (PEPICO) spectroscopy. Threshold photoelectron photoion coincidence (TPEPICO) measurements were also made. Photoion yield curves and branching ratios were measured for the parent ion and six fragment ions. The adiabatic ionization energy of acetamide was determined as I.E. (1²A′) = (9.71 ± 0.02) eV, in agreement with an earlier reported photoionization mass spectrometry (PIMS) value. The adiabatic energy of the first excited state of the ion, 1²A″, was determined to be ≈10.1 eV. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made. The neutral species lost in the principal dissociative photoionization processes are CH₃, NH₂, NH₃, CO, HCCO and NH₂CO. Heats of formation are derived for all ions detected and are compared with literature values. Some astrophysical implications of these results are discussed.     

Valence Photoionization of Thymine: Ionization Energies,Vibrational Structure,and Fragmentation Pathways from the Slow to the Ultrafast

The photoionization of thymine has been studied by using vacuum ultraviolet radiation and imaging photoelectron photoion coincidence spectroscopy after aerosol flash vaporization and bulk evaporation. The two evaporation techniques have been evaluated by comparison of the photoelectron spectra and breakdown diagrams. The adiabatic ionization energies for the first four electronic states were determined to be 8.922±0.008, 9.851±0.008, 10.30±0.02, and 10.82±0.01 eV. Vibrational features have been assigned for the first three electronic states with the help of Franck–Condon factor calculations based on density functional theory and wave function theory vibrational analysis within the harmonic approximation. The breakdown diagram of thymine, as supported by composite method ab initio calculations, suggests that the main fragment ions are formed in sequential HNCO-, CO-, and H-loss dissociation steps from the thymine parent ion, with the first step corresponding to a retro-Diels–Alder reaction. The dissociation rate constants were extracted from the photoion time-of-flight distributions and used together with the breakdown curves to construct a statistical model to determine 0 K appearance energies of 11.15±0.16 and 11.95±0.09 eV for the m/z 83 and 55 fragment ions, respectively. These results have allowed us to revise previously proposed fragmentation mechanisms and to propose a model for the final, nonstatistical H-loss step in the breakdown diagram, yielding the m/z 54 fragment ion at an appearance energy of 13.24 eV.     

Dissociative ionisation of CS2 and the formation of S2+

Photoelectron–photoion coincidence spectroscopy has been used to examine dissociative ionisation of CS₂ from electronic states of CS₂⁺ up to 27 eV, including the satellite states 3, 4, 6 and 10 whose decay has not been studied before. Branching ratios to the ions S⁺, CS⁺, S₂⁺ and C⁺ have been determined throughout the range and kinetic energy release distributions have been deduced from peak shapes, allowing inferences on the states of the fragments. The choice of product channel is not strongly dependent on initial parent ion state identity. The products are formed in many different final states, but kinetic energy releases less than 3 eV are favoured, corresponding to formation of highly excited states of the products. In confirmation, optical emission has been found in coincidence with photoelectrons from formation of several inner valence states of the ions. Formation of S₂⁺ occurs from several initial states of the parent ion and possible mechanisms are considered. It is concluded that a “quasi-statistical” model may best describe the dissociation of CS₂⁺ from the inner valence states.     

VUV photoionisation of free azabenzenes: Pyridine, pyrazine, pyrimidine, pyridazine and s-triazine

Photoionisation mass spectrometry was used to obtain the fragmentation pathways of pyridine, pyridazine, pyrimidine, pyrazine and s-triazine molecules upon absorption of 23.0, 15.7 and 13.8 eV synchrotron photons. The ionic fragments observed vary from molecule to molecule, however C₂H₂⁺, HCN⁺and HCNH⁺ are common to all five molecules at the three photon energies. Furthermore, the presence of C₂H₂N₂⁺, C₃H₃N⁺ and C₄H₄⁺ in the spectra of some of the molecules suggests dissociation pathways via loss of HCN moieties. The respective parent cations, m/q=79, 80 and 81 have a greater yield at low photon energies when compared to the most intense fragment peak in each spectra. We recorded two of the fragment cation yields, as well as the parent photoion yield curves of pyridine, pyridazine, and pyrimidine in the 8–30 eV range. The formation of abundant cation fragments show a strong propensity of the molecules for dissociation after the absorption of VUV photons higher than 14 eV. The differences in relative fragment yields from molecule to molecule, and when changing the excitation energy, suggest significant bond rearrangements and nuclear motion during the dissociation time. Thus, bond cleavage is dependent on the photon energy deposited in the molecule and on intramolecular reactivity. With the aid of photoion yield curves and energy estimations we have assigned major peaks in the spectra and discussed their fragmentation pathways.     

The fragmentation and isomerization of internal energy selected acetaldehyde molecular cations

The fixed wavelength photoelectron—photoion coincidence technique has been employed to study the fragmentation behaviour of excited acetaldehyde molecular cations with internal energies up to 7 eV. The recorded breakdown curves of the parent ion as well as the C₂H₃O⁺, CHO⁺ and CH₃⁺ fragment ions enable to reject state specific fragmentation behaviour of the title compound into the CHO⁺ and CH₃⁺ fragment ion channels. The present data give evidence of a fast isomerization of the CH₃CHO⁺ cation from its first electronically excited state ā(²A″) to the oxirane cation in its electronical ground state X?(²B₂).     

Gas‐phase intramolecular hydroxyl‐amino exchange of protonated arginine and verified by the synthetic intermediate compound

A new fragmentation process was proposed to interpret the characteristic product ion at m/z 130 of protonated arginine. The α‐amino group was dissociated from protonated arginine and then combined with the (M + H‐NH₃) fragment to form an ion‐neutral complex which further generated a hydroxyl‐amino exchange intermediate compound through an ion‐molecule reaction. This intermediate compound was synthesized from argininamide through a diazo reaction, and then the reaction mixture was analyzed using liquid chromatography combined with mass spectrometry (LC‐MS). The collision‐induced dissociation experiments under the same conditions revealed that this intermediate compound produced the characteristic product ion at m/z 130 as well as protonated arginine, and in addition, density functional theory calculations were performed to confirm simultaneous loss of NH₃ and CO from this intermediate to give the m/z 130 ion.     

On the photoionization and fragmentation of ammonia clusters using TPEPICO

Measurements of the appearance potentials of ammonia cluster ions (NH₃) _n ⁺ and (NH₃) _n ?2NH ₄ ⁺ using the threshold photoelectron photoion coincidence (TPEPICO) time-of-flight method are performed. The results agree well with other available data, however, solvation energies derived from these and from thermochemical data show significant discrepancies. The energetics for ammonia clusters are elucidated from various points of view.     

Zirconocene Catalysts: Ion‐pairs,Zwitterions, or Weakly Bound Molecules?

The bonding between electron deficient zirconocene complexes such as [(η⁵‐C₅Me₅)₂ZrR]⁺ and the counterions [(C₆F₅)₃BR]^– (R = CH₃, 1 , R = H, 2 ) has been investigated by DFT and extended Hückel calculations. The molecular bonding analysis has been carried out in the light of the structural information on the inter‐ion interactions available for these and other similar electron deficient coordinatively unsaturated zirconocene complexes. It has been shown that the “anion‐cation” interactions in 1 and 2 are different manifestations of the same need to provide electron density to the metal centre. In 1 this is achieved via the ‐C–H σ‐bonds and has been described as a pseudo‐agostic interaction, whereas in 2 electron donation occurs via the phenyl F‐atoms. The study of the covalent component of the bonding between the ionic components has shown that the weak electron donation, though not sufficient to saturate the metal centre, stabilises the unsaturated species and allows it to have a longer life time. It has been inferred that in the homogenous Ziegler‐Natta catalytic process the approach of a strongly coordinating molecule, such as an olefin in the polymerisation, destroys the weak interaction and allows the reaction to proceed whilst the ion‐pairing maintains the counterion in proximity of the complex.     

Probing the pyrolysis of ethyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O = C (H)– O – C (H₂)–CH₃ and trans (staggered) configuration of the O= C (H)– O – C (H₂)– C H₃ dihedral angles. We observed the following ethyl formate pyrolysis products: CH₃CH₂OH, CH₃CHO, C₂H₆, C₂H₄, HC(O)OH, CH₂O, CO₂, and CO, with HC(O)OH and C₂H₄ pyrolyzing further, forming CO + H₂O and C₂H₂ + H₂. The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH₃CH₂O + CHO and CH₃CH₂ + HC(O)O, were studied computationally at the M06-2X-GD3/aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.     

立方烷型2-羟甲基-N-邻羟基苯亚甲基苯胺四核镍配合物的合成与晶体结构

The tetranuclear cubane-like complex, [NiL（EtOH）]4·0.5EtOH （1） with tridentate Schiff base ligand （H2L= 2-Hydroxymethyl-N-salicylideneaniline） has been synthesized and its crystal structure and spectroscopic properties have been studied. The complex consists of a tetranuclear （NiO）4 cubane core, of which four nickel（Ⅱ） ions are bridged by μ3-alkoxide group and each nickel（Ⅱ） ion is coordinated to three μ3-alkoxide oxygen atoms, one imino nitrogen atom and one phenoxide oxygen atom from Schiff base ligand, and further ligated by one EtOH molecule, completing a distorted octahedral geometry.     

Electron-ion coincidence study of photofragmentation of the CdCl(2) molecule

In this work, the photofragmentation subsequent to valence and Cd4d photoionization of cadmium dichloride (CdCl₂) were studied using He I and synchrotron excitation. The measurements were performed with a photoelectron‐photoion coincidence (PEPICO) setup, and the connection between the singly ionized electronic states and cationic fragments was investigated. The valence‐ionized states were found to lead to , Cd⁺ and CdCl⁺. The Cd4d^{? 1} states were found to lead only to Cl⁺ ions. The observed charge transfer effect between Cd and Cl was concluded to take place due to internal conversion or fluorescence decay to dissociating valence states either directly or through consecutive fragmentation. The fragmentation energetics were investigated with molecular ab initio calculations, and the calculated energies were found to agree with the detected fragment appearances. Copyright © 2011 John Wiley & Sons, Ltd.