Reactive collisions between Li(+) ions and i-C(3)H(7)Cl molecules have been studied in the 0.20-12.00 eV center-of-mass energy range using an octopole radio frequency guided-ion beam apparatus recently developed in our laboratory. At low collision energies, dehydrohalogenation reactions giving rise to Li(C(3)H(6))(+) and Li(HCl)(+) are the main reaction channels, while at higher ones C(3)H(7)(+) and C(2)H(3)(+) become dominant, all their reactive cross sections having been measured as a function of the collision energy. To obtain information about the potential energy surfaces (PESs) on which the reactive processes take place, ab initio calculations at the MP2 level have been performed. For dehydrohalogenations, the reactive ground singlet PES shows ion-molecule adduct formation in both the reactant and product sides of the surface. Following the minimum energy path connecting both minima, an unstable intermediate and the corresponding barriers, both lying below the reactant's energy, have been characterized. The entrance channel ion-molecule adduct is also involved in the formation of C(3)H(7)(+), which then generates C(2)H(3)(+) via an CH(4) unimolecular elimination. A qualitative interpretation of the experimental results based on ab initio calculations is also included.
Total and partial cross sections for Penning ionization of methyl chloride and methyl bromide by metastable neon atoms have been measured as a function of collision energy in the 0.040-0.15 eV range. The partial cross sections for the formation of CH 3 X + , CH 3 + , and CH 2 X + (X ) Cl, Br) show a decreasing trend, with different slopes, when the collision energy increases. The branching ratios indicate that the production of fragment ions is favored at higher energies. Based on new correlation rules that allow to estimate pure van der Waals but also charge-transfer contributions to intermolecular potentials, the anisotropy of the Ne*-CH 3 Cl interaction has been semiempirically estimated. Within the electron exchange model of Penning ionization, it is shown that the anisotropy of interaction, together with the anisotropy of electron distribution of the orbitals involved in ionization, is correlated with the behavior of the ionization cross sections and branching ratios as a function of collision energy. In particular, the presence of the attractive interaction at the two ends of the molecule is responsible for the decreasing energy dependence of the total ionization cross sections, while a softer repulsive wall around the methyl group is responsible for the increase of fragment ions when collision energy increases.
The association reactions of benzene molecules with alkali ions M(+) (Li(+), Na(+) and K(+)) under single collision conditions have been studied using a radiofrequency-guided-ion-beam apparatus and mass spectrometry characterization of the different adducts. Cross-section energy dependences for [M-C(6)H(6)](+) adduct formation have been measured at collision energies up to 1.20 eV in the center of mass frame. All excitation functions decrease when collision energy increases, showing the expected behaviour for barrierless reactions. From ab initio chemical structure calculations at the MP2(full) level, the formation of the adducts makes evident the alkali ion-benzene non-covalent chemical bond. The cross-section energy dependence and the role of radiative cooling rates and unimolecular decomposition on the stabilization of the energized collision complex are also discussed.
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