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Projeto de investigação
Electron transfer processes in biologically relevant molecules
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Isotope Effect in D2O Negative Ion Formation in Electron Transfer Experiments
Publication . Kumar, Sarvesh; Hoshino, Masamitsu; Kerkeni, Boutheïna; García, Gustavo; Limão-Vieira, Paulo; DF – Departamento de Física; CeFITec – Centro de Física e Investigação Tecnológica; ACS - American Chemical Society
H2O/D2O negative ion time-of-flight mass spectra from electron transfer processes at different collision energies with neutral potassium yield OH-/OD-, O-, and H-/D-. The branching ratios show a relevant energy dependence with an important isotope effect in D2O. Electronic state spectroscopy of water has been further investigated by recording potassium cation energy loss spectra in the forward scattering direction at an impact energy of 205 eV (lab frame), with quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom supporting most of the experimental findings. The DO-D bond dissociation energy has been determined for the first time to be 5.41 ± 0.10 eV. The collision dynamics revealed the character of the singly excited (1b2-1) molecular orbital and doubly excited states in such K-H2O and K-D2O collisions.
Sensing the ortho Positions in C6Cl6 and C6H4Cl2 from Cl2− Formation upon Molecular Reduction
Publication . Kumar, Sarvesh; Romero, José; Probst, Michael; Maihom, Thana; García, Gustavo; Limão-Vieira, Paulo; DF – Departamento de Física; CeFITec – Centro de Física e Investigação Tecnológica; MDPI - Multidisciplinary Digital Publishing Institute
The geometrical effect of chlorine atom positions in polyatomic molecules after capturing a low-energy electron is shown to be a prevalent mechanism yielding Cl2−. In this work, we investigated hexachlorobenzene reduction in electron transfer experiments to determine the role of chlorine atom positions around the aromatic ring, and compared our results with those using ortho-, meta- and para-dichlorobenzene molecules. This was achieved by combining gas-phase experiments to determine the reaction threshold by means of mass spectrometry together with quantum chemical calculations. We also observed that Cl2− formation can only occur in 1,2-C6H4Cl2, where the two closest C–Cl bonds are cleaved while the chlorine atoms are brought together within the ring framework due to excess energy dissipation. These results show that a strong coupling between electronic and C–Cl bending motion is responsible for a positional isomeric effect, where molecular recognition is a determining factor in chlorine anion formation.
SF6 Negative Ion Formation in Charge Transfer Experiments
Publication . Kumar, Sarvesh; Hoshino, Masamitsu; Kerkeni, Boutheïna; Garcia, Gustavo; Ouerfelli, Ghofrane; Al-Mogren, Muneerah Mogren; Limão-Vieira, Paulo; CeFITec – Centro de Física e Investigação Tecnológica; DF – Departamento de Física; MDPI - Multidisciplinary Digital Publishing Institute
In the present work, we report an update and extension of the previous ion-pair formation study of Hubers, M.M.; Los, J. Chem. Phys. 1975, 10, 235–259, noting new fragment anions from time-of-flight mass spectrometry. The branching ratios obtained from the negative ions formed in K + SF6 collisions, in a wide energy range from 10.7 up to 213.1 eV in the centre-of-mass frame, show that the main anion is assigned to SF5− and contributing to more than 70% of the total ion yield, followed by the non-dissociated parent anion SF6− and F−. Other less intense anions amounting to <20% are assigned to SF3− and F2−, while a trace contribution at 32u is tentatively assigned to S− formation, although the rather complex intramolecular energy redistribution within the temporary negative ion is formed during the collision. An energy loss spectrum of potassium cation post-collision is recorded showing features that have been assigned with the help of theoretical calculations. Quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom are performed to support the experimental findings. Apart from the role of the different resonances participating in the formation of different anions, the role of higher-lying electronic-excited states of Rydberg character are noted.
Electron Transfer Processes in Biologically Relevant Molecules. An Experimental Study of Neutral Alkali Atom-Molecule Collisions
Publication . Kumar, Sarvesh; Limão-Vieira, Paulo
The work developed within the context of this thesis includes negative ion formation in charge transfer processes from collisions of neutral potassium atoms with key selected neutral mole-cules. The crossed molecular beam set up used to obtain the anion yields as a function of the collision energy as well as relevant information about the lowest-lying anionic states that are accessed in the temporary negative ion formation, is fully equipped with a time−of−flight mass spectrometer and an energy loss analyser. The spectrometric technique includes a linear and a reflectron type time-of-flight mass spectrometers to provide information about the neg-ative ions formed in such collisions. We have made use of a home−built Wiley McLaren type Linear Time-of-flight mass spectrometer (L−TOF−MS), to extract relevant information about the kinetic energy release distribution of selected fragment anions, as a function of the collision energy, while the reflectron time−of−flight mass-spectrometer (r−TOF−MS) was used due to its higher mass resolution serving as a proper tool to resolve close lying fragment anions dif-fering by just 1 amu in the fragmentation of nimorazole. In order to obtain relevant infor-mation about the most accessed negative ion states in the collision process, a post−collision potassium cation (K+) energy loss spectra in the forward scattering direction (θ ≈ 0°) with the beam's optical path have been recorded in a hemispherical energy loss analyser. The combi-nation of TOF−MS and these two techniques helped us to gain deep insight into electron trans-fer processes of molecules under investigation.
The set of chlorinated molecules investigated include a group (C6H5Cl,C6D5Cl, C6H11Cl,and C6Cl6) that has been properly chosen to explore the role of direct dissociation through electron transfer into a σCl∗ antibonding orbital and more importantly to determine the role of intramolecular electron transfer through π∗σCl∗⁄ coupling yielding anion formation. From the comprehensive investigation of these molecules in a wide energy range of collision energies, 10 − 103 eV in lab frame, we have obtained for the first time the relative cross−sec-tion for Cl− formation. The other group of molecules include biological relevant targets as the radiosensitizer nimorazole (NIMO) and water. Regarding the former, although the major signal is assigned to the non-dissociated parent anion (80% of the total anion yield), NO2− accounts for just 10−15% of the total anion yield a detailed trace fragmentation pattern indicates decomposition of NIMO's 4−nitroimidazole and morpholine rings. As far as the latter is concerned, H2O and its deuterated counterpart D2O were investigated and are presented in the third part of this thesis. The fragmentation pattern from H2O includes H−, O−, and OH− whereas from D2O in-cludes D−, O−, and OD−. From the different TOF mass spectra the fragment anions' thresholds of formation have been obtained. K+ energy loss spectra from NIMO, H2O and D2O were also recorded, revealing the experimental vertical electron affinities of the most accessed negative ion states.
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Fundação para a Ciência e a Tecnologia
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OE
Número da atribuição
COVID/BD/152673/2022