RESUMO
The dissociative photoionization dynamics of the chloroacetone molecule (C3H5OCl) in the gas phase, induced by vacuum ultraviolet (VUV) synchrotron radiation in the range from 10.85 to 21.50 eV, has been investigated by using time-of-flight mass spectrometry in the photoelectron-photoion coincidence mode. The appearance energies for the most relevant cation fragments produced in this energy range have been analyzed, and the fragmentation pathways leading to the formation of the cation species have been proposed and discussed. The mass spectra show that the most dominant VUV photodissociation cation product appears at m/z 43 and has been assigned to the C2H3O+ species. Enthalpies of formation (ΔfH°0K) for the neutral chloroacetone molecule and its molecular cation have been derived and correspond to -207.8 ± 5.8 kJ/mol and 755.1 ± 6.8 kJ/mol, respectively. In addition to the spectral analysis, the structural and energetic parameters for the cations produced have also been examined on the basis of high-level quantum chemical numerical calculations.
RESUMO
RATIONALE: The class of active components of the group of ß-lactam antibiotics is very important for several fields and applications, although their stability and radiation reactivity properties are not yet well understood. We have studied the interaction of an important building block species, the 2-azetidinone (C3 H5 NO) molecule, with monochromatic VUV (synchrotron radiation) photons in the 9.5-21.5 eV range, using time-of-flight mass spectrometry (TOF-MS), electron-ion coincidence (PEPICO), and high-level theory methods. METHODS: A 2-azetidinine sample was introduced into the UH-vacuum chamber, without purification, through an inlet system for the gas-phase experiments with monochromatic light in the VUV range from the TGM beamline at the Brazilian Synchrotron Facility. A Wiley-McLaren type mass spectrometer in the PEPICO mode was employed to detect and characterize the photoionization and photodissociation products of the 2-azetidinone. The analysis and discussion of the results were supported by high-level density functional theory (DFT) and ab initio methods. RESULTS: The adiabatic ionization energy was determined experimentally in this work as 9.745 ± 0.020 eV, and this was supported by the high level of theory result with good agreement. The heat of formation for the 2-azetininone cation has been derived for the first time as 844.2 ± 1.9 kJ/mol. The dominant ion dissociation channel in the VUV energy range up to 21.5 eV is associated with the cation species at m/z 28. CONCLUSIONS: The structural properties, VUV-induced photoionization, and photodissociation dynamics of the 2-azetidinone molecule in the gas phase have been successfully investigated in the energy range of 9.5-21.5 eV. PEPICO mass spectra have been determined for the first time for this molecule at several selected photon energies from which the partial ion yields were determined for all cation species produced from this molecule.
RESUMO
The unimolecular photofragmentation mechanisms of chlorosulfonyl isocyanate, ClSO2NCO, excited with tunable synchrotron radiation between 12 and 550 eV, were investigated by means of time-of-flight (TOF) coincidence techniques. The main fragmentation mechanism after single ionization, produced by irradiation of an effusive beam of the sample with synchrotron light in the valence electron region, occurs through the breaking of the Cl-S single bond, giving a chloride radical and a SO2NCO(+) fragment. This mechanism contrasts with the one observed for the related FSO2NCO, in which the rupture of the S-N bond originates the FSO2(+) fragment. The energies of the shallow- (S 2p, Cl 2p, and S 2s) and core-shell (C 1s, N 1s, and O 1s) electrons were determined by X-ray absorption. Transitions between these shallow and core electrons to unoccupied molecular orbitals were also observed in the total ion yield (TIY) spectra. Fourteen different fragmentation mechanisms of the doubly charged parent ion, ClSO2NCO(2+), were inferred from the bidimensional photoelectron-photoion-photoion-coincidence (PEPIPICO) spectra. The rupture of the S-N bond can evolve to form NCO(+)/SO2(â¢+), NCO(+)/SO(â¢+), or S(â¢+)/NCO(+) pairs of ions. The Cl-S bond breaking originates different mechanisms, Cl(+)/SO(â¢+), Cl(+)/S(â¢+), CO(â¢+)/S(â¢+), O(â¢+)/SO(â¢+), O(â¢+)/Cl(+), O(â¢+)/S(â¢+), C(â¢+)/S(â¢+), and C(â¢+)/O(â¢+) pairs being detected in coincidence as the final species. Another three coincidence islands can only be explained with an initial atomic rearrangement forming ClNCO(2+), ONCO(2+), and ClCO(2+), as precursors of CO(â¢+)/Cl(+), O(â¢+)/CO(â¢+), and C(â¢+)/Cl(+) pairs, respectively. The formation of Cl(â¢) radical is deduced from several mechanisms.
RESUMO
The simultaneous evaluation of the PES and valence synchrotron photoionization studies complemented by the results of quantum chemical calculations offers unusually detailed insights into the valence ionization processes of small covalent molecules. Thus, methyl thiochloroformate, ClC(O)SCH(3), has been investigated by using results from both photoelectron spectroscopy (PES) and synchrotron radiation in the valence energy range. In an additional series of experiments, total ion yield (TIY) and photoelectron-photoion coincidence (PEPICO) spectra have been recorded. Furthermore, the relative yields for ionic fragments have been determined as a function of the photon energy. Vibronic structure has been observed in the TIY spectrum recorded in the synchrotron experiments. The photodissociation behavior of ClC(O)SCH(3) can be divided into two well-defined energy regions.