Electron delocalisation in conjugated sulfur heterocycles probed by resonant Auger spectroscopy.

We propose a novel approach for an indirect probing of conjugation and hyperconjugation in core-excited molecules using resonant Auger spectroscopy. Our work demonstrates that the changes in the electronic structure of thiophene (C4H4S) and thiazole (C3H3NS), occurring in the process of resonant sulfur K-shell excitation and Auger decay, affect the stabilisation energy resulting from π-conjugation and hyperconjugation. The variations in the stabilisation energy manifest themselves in the resonant S KL2,3L2,3 Auger spectra of thiophene and thiazole. The comparison of the results obtained for the conjugated molecules and for thiolane (C4H8S), the saturated analogue of thiophene, has been performed. The experimental observations are interpreted using high-level quantum-mechanical calculations and the natural bond orbital analysis.

VUV-Induced Photodissociation of the Chloroacetone Molecule Studied by Photoelectron-Photoion Coincidence Spectroscopy.

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.

Experimental and Computational Studies on the Gas-Phase Acidity of 5,5-Dialkyl Barbituric Acids.

The gas phase acidities (GA) of 5,5-alkylbarbituric acids have been experimentally determined by electrospray ionization-triple quadrupole (ESI-TQ) mass spectrometry and by using the extended kinetic Cooks method (EKCM). The GAs of C-H (1330.9 ± 10.0 kJ mol-1) and N-H (1361.5 ± 10.5 kJ mol-1) deprotonated sites of bifunctional barbituric acid were determined from the selective production of their corresponding heterodimers. The GA value in the N-H site was confirmed by measuring the GAs of 5,5-dimethyl- and 5,5-diethyl barbituric acids (∼1368 kJ mol-1). The experimental results have been rationalized and discussed with the support of quantum chemical calculations with Gaussian-n (G3 and G4) composite methods, which confirmed the excellent consistency of the results.

Investigation of the molecular structure and VUV-induced ion dissociation dynamics of 2-azetidinone (C3 H5 NO).

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.

Si 1s-1, 2s-1 and 2p-1 lifetime broadening of SiX4 (X = F, Cl, Br, CH3) molecules: SiF4 anomalous behaviour reassessed.

The Si 1s-1, Si 2s-1, and Si 2p-1 photoelectron spectra of the SiX4 molecules with X = F, Cl, Br, CH3 were measured. From these spectra the Si 1s-1 and Si 2s-1 lifetime broadenings were determined, revealing a significantly larger value for the Si 2s-1 core hole of SiF4 than for the same core hole of the other molecules of the sequence. This finding is in line with the results of the Si 2p-1 core holes of a number of SiX4 molecules, with an exceptionally large broadening for SiF4. For the Si 2s-1 core hole of SiF4 the difference to the other SiX4 molecules can be explained in terms of Interatomic Coulomb Decay (ICD)-like processes. For the Si 2p-1 core hole of SiF4 the estimated values for the sum of the Intraatomic Auger Electron Decay (IAED) and ICD-like processes are too small to explain the observed linewidth. However, the results of the given discussion render for SiF4 significant contributions from Electron Transfer Mediated Decay (ETMD)-like processes at least plausible. On the grounds of our results, some more molecular systems in which similar processes can be observed are identified.

Neutral, ion gas-phase energetics and structural properties of hydroxybenzophenones.

We have carried out a study of the energetics, structural, and physical properties of o-, m-, and p-hydroxybenzophenone neutral molecules, C(13)H(10)O(2), and their corresponding anions. In particular, the standard enthalpies of formation in the gas phase at 298.15 K for all of these species were determined. A reliable experimental estimation of the enthalpy associated with intramolecular hydrogen bonding in chelated species was experimentally obtained. The gas-phase acidities (GA) of benzophenones, substituted phenols, and several aliphatic alcohols are compared with the corresponding aqueous acidities (pK(a)), covering a range of 278 kJ.mol(-1) in GA and 11.4 in pK(a). A computational study of the various species shed light on structural effects and further confirmed the self-consistency of the experimental results.