Dissociation Dynamics Of Ions Research Articles

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

RationaleThe class of active components of the group of β‐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 (C3H5NO) 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.MethodsA 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.ResultsThe 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.ConclusionsThe 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.

Ion Dissociation Dynamics of 1,2,3,4-Tetrahydronaphthalene: Tetralin as a Test Case For Hydrogenated Polycyclic Aromatic Hydrocarbons.

The unimolecular dissociation of ionized tetralin was probed by tandem mass spectrometry, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy, and theory. The major reactions observed were the loss of the hydrocarbons CH3•, C2H4, and C3H5• together with H•-atom loss. RRKM modeling of the iPEPICO data suggested a two-well potential energy surface. Ionized tetralin can lose all four neutrals via H-shift and ring-opening reactions or CH3• and C2H4 after interconversion to the 1-methylindane ion, a process similar to that found for ionized 1,2-dihydronaphthalene (isomerizing to form the 1-methylindene ion structure). This was confirmed at the B3LYP/6-31+G(d,p) level of theory, and potential mechanisms for all reactions are described. The ionization energy of tetralin was established from the threshold photoelectron spectrum to be 8.46 ± 0.01 eV.

Dissociation dynamics of energy-selected ions using threshold photoelectron-photoion coincidence velocity imaging

Threshold photoelectron-photoion coincidence (TPEPICO) is a powerful method to prepare and analyze internal energy- or state-selected ions. Here, we review the state-of-the-art TPEPICO imaging technique combining with tunable vacuum ultraviolet (VUV) synchrotron radiation and its recent applications at Hefei Light Source (HLS), especially on the fundamental data measurement and the dissociation dynamics of ions. By applying the double velocity map imaging for both electrons and ions in coincidence, the collection efficiency of the charged particles, the electron energy resolution and the resolving power of the released kinetic energy in dissociation have been greatly improved. The kinetic energy and the angular distributions of fragment ions dissociated from parent ions with definitive internal energy or state have been acquired directly from TPEPICO images. Some dissociation mechanisms involving non-adiabatic quantum effects, like conical intersection and internal conversion, have been revealed. Moreover, the mass-selected threshold photoelectron spectroscopy (MS-TPES) shows tremendous advantages in isomer-specific analysis of complex systems.

Ion dissociation dynamics of the chlorine azide cation (ClN3+) investigated by velocity map imaging

The velocity map imaging technique was applied to study the unimolecular dissociation dynamics of the chlorine azide cation (ClN3+) fragmenting into N2 and NCl+ and the results have been compared to quantum chemical calculations. The ClN3+ ion was produced in a molecular beam by two-photon ionization of chlorine azide (ClN3) with laser light at λ≈202 nm. Rotationally resolved REMPI spectra and velocity map images of state selected N2(X 1Σg+,v,J) quantify the energy deposition into N2 and NCl+ products. Photoelectron velocity map images show near zero-energy electron production. Angular distributions of the photo-fragments suggest we are witnessing the influence of the ions first excited (A′) electronic state on its decomposition. The N2(X 1Σg+) dissociation products are mainly in the vibrational ground state, but rotationally excited with J up to ∼60. An unusual vibrational distribution in the NCl+ (X 2Π,v=1–12) product was observed, which is evidence of nonstatistical ion decomposition. This work also provides an accurate determination of the energetics for the reaction, ClN3+→N2(X 1Σg+)+NCl+(X 2Π)(ΔE<−0.2 eV), from which one may use prior photoionization threshold data to derive the energetics of the ClN3→N2(X 1Σg+)+NCl(a 1Δ)(ΔE<0.3 eV) dissociation.

Ion dissociation dynamics and thermochemistry by photoelectron photoion coincidence (PEPICO) spectroscopy

Photoelectron photoion coincidence (PEPICO) is a technique that permits the mass spectrometric investigation of energy-selected ions. Dissociation rates and accurate dissociation onsets can be measured. Such data provide not only rigorous tests of the statistical theories for unimolecular reactions, but also provide benchmark heats of formation for anchoring the gas-phase proton affinity scale. When combined with other mass spectrometric methods and quantum calculations of ionic structures, PEPICO provides a means to place the field of ion structures on a firm quantitative basis. Ions are produced by dispersed vacuum ultraviolet radiation in a small electric field in which electrons and ions are extracted in opposite directions. By measuring only ions in coincidence with energy-selected electrons, the ion internal energy is established by conservation of energy [ E(ion) = hν − IE − E( el)]. Recent results as well as new developments in pulsed-field ionization and velocity map imaging are discussed.

Vacuum ultraviolet photoionization and photoelectron studies in the new millennium: recent developments and applications

Vacuum ultraviolet (VUV) photoionization mass spectrometry and photoelectron spectroscopy have played a central role in providing energetic and spectroscopic information for neutrals and cations. The most important data obtainable in a VUV photoionization and photoelectron experiment are ionization energies and 0 K ion dissociation thresholds or appearance energy (AE), from which 0 K bond dissociation energies for neutrals and cations can be deduced. The recent developments in VUV lasers and third-generation synchrotron sources, together with the introduction of the pulsed-field ionization (PFI), photoelectron (PFI-PE), and PFI-photoion (PFI-PI) methods, have revolutionized the field of photoelectron and ion spectroscopy by significantly improving the energy resolution to the range of 0.025–1.0 meV (full width at half maximum, FWHM). These resolutions, which make possible the measurement of photoelectron spectra for many simple molecules at the rotational-resolved level, are ≈100-fold better than those observed in traditional photoelectron studies, making the PFI-PE technique a true spectroscopic method. The recent introduction of the synchrotron-based PFI-PEPICO scheme has shown that AE values for a range of molecules can be determined with an unprecedented precision limited only by the PFI-PE measurement. The synchrotron-based PFI-PEPICO and PFI-PI schemes show great promises for future studies of state- or energy-selected ion-dissociation dynamics and energy-selected ion-molecule reaction dynamics. Further improvement in energy resolution for PFI-PE and PFI-PI measurements has been demonstrated using the two-color photo-induced Rydberg ionization (PIRI) spectroscopic scheme, which involves the photo-induced ionization of intermediate long-lived high- n ( n≥100) Rydberg states. The incorporation of this method by VUV photoexcitation to prepare intermediate high- n ( n≥100) Rydberg states is also expected to greatly increase the energy range of PFI studies. The availability of this array of laser- and synchrotron-based PFI methods, including PFI-PE, PFI-PEPICO, PFI-PI, PFI-ion-pair, and PIRI schemes, ensures an exciting and bright future for VUV photoionization and photoelectron studies in the new millennium.