Collision-induced dissociation mechanisms of [Li(uracil)]+

Abstract

The collision-induced dissociation (CID) of the [Li(uracil)]⁺ complex with Xe is studied by means of quasi-classical trajectory calculations. The potential energy surface is obtained “on the fly” from AM1 semiempirical calculations, supplemented with two-body analytical potentials to model the intermolecular interactions. The simulations show that Li⁺ production is the primary channel, in agreement with a previous experimental study [M. T. Rodgers and P. B. Armentrout, J. Am. Chem. Soc., 2000, 122, 8548]. Collision-induced isomerization of [Li(uracil)]⁺ was found to be very important as well in the 2.5−10 eV collision energy range. Three minor channels are also identified: complex formation between Xe and [Li(uracil)]⁺, ligand exchange to form LiXe⁺, and fragmentations of the uracil ring, which are strongly nonstatistical. Additional quasi-classical trajectory calculations carried out to investigate in more detail the fragmentations of the uracil ring reveal the presence of bifurcations in the potential energy surface, as trajectories starting from a transition state give rise to four different product channels. The integral cross sections for Li⁺ production calculated in this work agree well with those obtained in the experiments only for the lowest collision energies, being ∼20 times greater than the experimental values for a collision energy of 10 eV. Finally, the initial translational energy is transferred preferentially to the [Li(uracil)]⁺ vibrational degrees of freedom, with energy transfer to rotation being modest. The amount of energy transfer to the different degrees of freedom as a function of the collision energy follows quite nicely a model recently proposed by our group.