The predissociation spectrum of the Cl−35(H2) complex is measured between 450 and 800 cm−1 in a multipole radiofrequency ion trap at different temperatures using the FELIX infrared free electron laser. Above a certain temperature, the removal of the Cl−(p-H2) para nuclear spin isomer by ligand exchange to the Cl−(o-H2) ortho isomer is suppressed effectively, thereby making it possible to detect the spectrum of this more weakly bound complex. At trap temperatures of 30.5 and 41.5 K, we detect two vibrational bands of Cl−(p-H2) at 510(1) and 606(1) cm−1. Using accurate quantum calculations, these bands are assigned to transitions to the inter-monomer vibrational modes (v1,v2l2) = (0, 20) and (1, 20), respectively.

Metronidazole belongs to the class of nitroimidazole molecules and has been considered as a potential radiosensitizer for radiation therapy. During the irradiation of biological tissue, secondary electrons are released that may interact with molecules of the surrounding environment. Here, we present a study of electron attachment to metronidazole that aims to investigate possible reactions in the molecule upon anion formation. Another purpose is to elucidate the effect of microhydration on electron-induced reactions in metronidazole. We use two crossed electron/molecular beam devices with the mass-spectrometric analysis of formed anions. The experiments are supported by quantum chemical calculations on thermodynamic properties such as electron affinities and thresholds of anion formation. For the single molecule, as well as the microhydrated condition, we observe the parent radical anion as the most abundant product anion upon electron attachment. A variety of fragment anions are observed for the isolated molecule, with NO2− as the most abundant fragment species. NO2− and all other fragment anions except weakly abundant OH− are quenched upon microhydration. The relative abundances suggest the parent radical anion of metronidazole as a biologically relevant species after the physicochemical stage of radiation damage. We also conclude from the present results that metronidazole is highly susceptible to low-energy electrons.

Three-body reaction rates of Cl– with H2 to form the weakly bound complex Cl–(H2) are measured between 10 and 26 K in a linear radio-frequency wire trap. Formation of larger clusters of the form Cl–(H2)2 are also observed. The three-body (or termolecular) rate coefficients follow the form aT –1, with a = 1.12(2) × 10–29 cm6 K s–1. Reverse reactions to repopulate the Cl– parent ion were measured, even though the binding energy of the complex makes bimolecular dissociative collisions energetically inaccessible at low temperatures. The back-reaction was found to be proportional to the cube of the hydrogen density, suggesting that the dissociation mechanism depends on multiple collisions. Comparisons of the rate coefficients measured in a 16-pole wire trap and a 22-pole trap demonstrate significantly lower ion temperatures in the wire trap.

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