Selective Bond Excision in Nitroimidazoles by Electron Transfer Experiments

6 pags. 4 figs.We have performed comprehensive charge-transfer experiments yielding negative ion formation in collisions of fast neutral potassium atoms with nitroimidazole and methylated derivative molecules. The anionic pattern reveals that in the unimolecular decomposition of the precursor parent anion, single and multiple bond cleavages are attained. Selective excision of hydrogen atoms from the N1 position in 4-nitroimidazole (4NI) is completely blocked upon methylation in 1-methyl-4-nitroimidazole (1m4NI) and 1-methyl-5-nitroimidazole (1m5NI). Additionally, only 4NI and 2-nitroimidazole (2NI) are efficient in selectively producing neutral OH and NO radicals in contrast to 1m4NI and 1m5NI. These findings present a novel experimental evidence of selective chemical bond breaking by just tuning the proper collision energy in atom-molecule collision experiments. The present work contributes to the current need of pinpointing a class of charge-transfer collisions that exhibit selective reactivity of the kind demonstrated here, extending to tailored chemical control for different applications such as tumor radiation therapy through nitroimidazole-based radiosensitization.M.M. acknowledges the Portuguese National Funding Agency FCT-MCTES through PD/BD/106038/2015 and together with PLV the research grants UID/FIS/00068/2019 (CEFITEC) and PTDC/FIS-AQM/31281/2017. This work was also supported by the Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2012); UID/ Multi/04378/2013 (UCIBIO). G.G. acknowledges the partial financial support from the Spanish Ministerio de Ciencia, Innovacion y Universidades (project no. FIS2016-80440) M.P. acknowledges the partial support by the Austrian Ministry of Science BMWF as part of the Uni-Infrastruturprogramm of the platform Scientific Computing at LFU Innsbruck

Fragmentation of allylmethylsulfide by chemical ionization: dependence on humidity and inhibiting role of water

We report on a previously unknown reaction mechanism involving water in the fragmentation reaction following chemical ionization. This result stems from a study presented here on the humidity-dependent and energy-dependent endoergic fragmentation of allyl methyl sulfide (AMS) upon protonation in a proton transfer reaction-mass spectrometer (PTR-MS). The fragmentation pathways were studied with experimental (PTR-MS) and quantum chemical methods (polarizable continuum model (PCM), microhydration, studied at the MP2/6-311+G(3df,2p)//MP2/6-31G(d,p) level of theory). We report in detail on the energy profiles, reaction mechanisms, and proton affinities (G4MP2 calculations). In the discovered reaction mechanism, water reduces the fragmentation of protonated species in chemical ionization. It does so by direct interaction with the protonated species via covalent binding (C3H5+) or via association (AMS·H+). This stabilizes intermediate complexes and thus overall increases the activation energy for fragmentation. Water thereby acts as a reusable inhibitor (anticatalyst) in chemical ionization. Moreover, according to the quantum chemical (QC) results, when water is present in abundance it has the opposite effect and enhances fragmentation. The underlying reason is a concentration-dependent change in the reaction principle from active inhibition of fragmentation to solvation, which then enhances fragmentation. This amphoteric behavior of water is found for the fragmentation of C3H5+ to C3H3+, and similarly for the fragmentation of AMS·H+ to C3H5+. The results support humidity-dependent quantification efforts for PTR-MS and chemical ionization mass spectrometry (CIMS). Moreover, the results should allow for a better understanding of ion-chemistry in the presence of wate

Combinations of density functionals for accurate molecular properties of Be/W/H compounds

Beryllium and tungsten species can form by plasma-induced erosion of the walls of a fusion reactor. Accurate and fast evaluation of energies and geometries of Be/W/H compounds is needed for direct molecular dynamics of the plasma-wall interface or for generating training data for potential energy surfaces. Density functional calculations can serve this purpose but within the magnitude of suggested functionals no single one is the obvious choice. We investigate the performance of compact linear combinations of density functionals on some Be/W/H compounds by statistical machine learning.Equilibrium geometries and atomization energies of the neutral molecules Ben, BenHm, Wn, WnBem, and WnHm with m+n≤4 from 16 density functionals were compared with their counterparts from coupled cluster calculations. A statistical learning method was used to find combinations of these functionals that can accurately reproduce the results of the much more costly coupled cluster method. Linear models of two or three functionals predict the coupled cluster data quite well with an accuracy of 98.2% and 99.7%, respectively, much better than any of the functionals alone. This simple procedure is, for example, useful for the calculation of species concentrations in reaction networks of molecules close to plasma facing components in a fusion device. Accurate molecular energies are crucial for determining the species concentrations which depend exponentially on their differences

Sensing the ortho Positions in C6Cl6 and C6H4Cl2 from Cl2− Formation upon Molecular Reduction

13 pags., 6 figs., 2 tabs.The geometrical effect of chlorine atom positions in polyatomic molecules after capturing a low-energy electron is shown to be a prevalent mechanism yielding Cl. 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 Cl formation can only occur in 1,2-CHCl, 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.This project has received funding from the Portuguese National Funding Agency (FCT) through research grants CEFITEC (UIDB/00068/2020) and PTDC/FIS-AQM/31281/2017. The computations were enabled by resources provided by the University of Innsbruck, Austria