Direct versus indirect microscopic mechanisms in the Li + HF reaction

Abstract

Quasiclassical trajectory calculations on the chemical reactions Li + XF(v, J = 0)--* LiF + X (X = Mu, IH, 3H, l~ at selected initial atom-diatom orientations, v = 0, 1 and 2 and 15kcalmol-1 of collision energy have been carried out. A potential-induced rotation of the target molecule, favoured by the great angular forces of the system, is, as a rule, necessary for the reactive success of the collision if v = 0 or 1, but not if v = 2. The ease or difficulty of rotation according to whether the molecule is light (MuF or tHF) or heavy (3HF or I~ explains most of the observed orientational dependence of reactivity.

Correlation of attack and recoil angles for the Li+HF reaction: An exact quantum mechanical study at low and high total angular momentum

“…A classical picture leading to this effect is that of the HF molecule rotating as Li approaches in order to achieve the bent configuration of the saddle point, with the H atom continuing its flight as the LiF and H products separate. This picture is also in accord with the conclusions of Alvariño et al [8] where rotation of the HF target for v = 0 initial states of the reaction is crucial in order to reach the saddle point. However, since this effect is not seen in other cases certainly means that more factors are to be taken into account.…”

“…This caveat holds especially for the lower energy, not only because of the reduced number of entrance states but also for the considerable resonance contribution to reactivity at this energy. Nevertheless, it makes sense to say (as found also in trajectory calculations [8]) that, at low energy, the system displays a more resonant behaviour that implies some internal rearrangement and a higher reactivity for attacks on the side opposite to the direct one. On the contrary, at the higher energy, where the resonance contribution to reactivity is substantially diminished, a higher contribution to the reaction tends to come from more direct approaches to the F end of the diatomic target.…”

Impact of the Long-Range Interaction on the Efficiency of the Li + ClH → LiCl + H Reaction

“…Thanks to the already mentioned capability of the QCT method of both properly selecting the initial conditions of the individual trajectories and of following their time evolution, in Figure 4 it has been possible to provide a graphical illustration of typical trajectories associated with the different attractive or repulsive nature of the long-range interaction. Such mechanisms, already discussed in the past 25,26 for the Li + FH system, leverage the long-range interaction to gradually reorient the collision partners and smoothly convert energy while approaching the saddle from long-range in the entrance channel. This allows collisions occurring on the HYLC (and TZYGL) PES to better overcome the saddle energetic barrier and to show a mild and monotonous increase with collision energy (with virtually no threshold).…”

Attack and Recoil Angle Dependence of the Li + HF → LiF + H Reaction at J = 0

“…The stereodynamics of the Li + HF reaction has been the subject of both experimental − and theoretical ,,− studies, and it is now known that the reaction probability can be strongly dependent on the attack angle. Different aspects of such dependence have been observed experimentally, − in quasiclassical trajectory calculations 16,17 and in quantum studies either at zero total angular momentum 9,10 or within the framework of the centrifugal sudden approximation .…”