Effect of vibrational excitation on the molecular beam reactions of Ca and Sr with HF and DF

Abstract

The reactions of Ca and Sr with HF and DF are studied using a beam-gas configuration as a function of reagent and product vibration to obtain state-to-state reaction rates. A beam of alkaline earth metal atoms enters a scattering chamber containing hydrogen fluoride. The HF and DF reagents are pumped from v = 0 to v = I by a pulsed HF or DF laser; the alkaline earth monofluoride products are detected by a pulsed tunable probe laser. The product internal state distribution is determined from the laser excitation spectrum. While no products could be detected for reactions with DF(v = I) or DF(v = 0), the rate constant for the Ca and Sr reactions with HF(v = I) are both at least four orders of magnitude larger than the reactions with HF(v = 0). The implications of these results on bond energies and the fate of the excess energy of reaction are discussed.

Quantum stereodynamics of the Li+HF(v,j) reactive collision for different initial states of the reagent

“…This factor in the cross section is increased with respect to the equivalent one between the reaction probabilities because the number of partial waves is considerably increased for v ϭ1. This large enhancement of the reaction with initial vibrational excitation of the reagents has been observed experimentally for some related systems, like KϩHCl, 1 BaϩHF, 2 CaϩHF, 3,10 SrϩHF, 3,4,10 and NaϩHF. 4 Most of these systems are endothermic for v, while LiϩHF is nearly thermoneutral, but this difference does not seem to make important differences.…”

Transition State Spectroscopy of the Photoinduced Ca + CH3F Reaction. 2. Experimental and Ab Initio Studies of the Free Ca···FCH3 Complex

“…The squares (fitted by the dashed curve) in the figure were provided by a CCSD(T) calculation where all the coordinate but the C-x -Ca angle were adjusted to minimize the potential energy. Then a BSSE correction is performed using the counterpoise method of Boys and Bernardi for the fragments 2 In preliminary calculations, we have identified a much deeper well on the ground state PES, which corresponds to an inserted structure FCaCH 3 connected without any barrier to the CaF + CH 3 asymptote. We can provide an estimate of the dissociation energy, 2.4 eV with respect to CaF + CH 3 , i.e.…”

“…This optimum geometry is stabilized by 850 cm −1 with respect to the Ca + CH 3 F asymptote. 2 Of course when the CH 3 group is let free to rotate about the C-F axis, there is an almost isoergic path corresponding to a circular groove about the axis. In the planar cut to the ground state PES that is defined in Figure 1, the groove appears simply as two quasi degenerate minima at C-x -Ca angles of 140 and 220 o .…”

“…[2,3]. Since the bonding in CaF is ionic, the mechanism of the reaction could be considered as a simple harpoon reaction where a low ionization energy atom, Ca, transfers one of its valence electron to an electron accepting molecule, CH 3 F. However, a complication arises due to the internal degrees of freedom of CH 3 F that has motivated our interest to this system.…”

Transition State Spectroscopy of the Photoinduced Ca + CH3F Reaction. 1. A Cluster Isolated Chemical Reaction Study

“…[9,10]. Its mechanism can be viewed as a harpoon reaction going through an electron transfer from Ca to CH 3 F. However, no bound state for an additional electron exists at the equilibrium geometry of the molecular reactant and the halogen bond has to be stretched for the molecule to attach efficiently the extra electron [11].…”