On the dynamics of chemical reactions of negative ions

Abstract

This review discusses the dynamics of negative ion reactions with neutral molecules in the gas phase. Most anion-molecule reactions proceed via a qualitatively different interaction potential than cationic or neutral reactions. It has been and still is the goal of many experiments to understand these reaction dynamics and the different reaction mechanisms they lead to. We will show how rate coefficients and cross sections for anion-molecule reactions are measured and interpreted to yield information on the underlying dynamics. We will also present more detailed approaches that study either the transient reaction complex or the energy-and angle-resolved scattering of negative ions with neutral molecules. With the help of these different techniques many aspects of anion-molecule reaction dynamics could be unravelled in the last years. However, we are still far from a complete understanding of the complex molecular interplay that is at work during a negative ion reaction.

Theoretical Studies on F– + NH2Cl Reaction: Nucleophilic Substitution at Neutral Nitrogen

“…Moreover, a Cs hydrogen-bonded intermediate F − ---HCH 2 Cl was also located as a minimum in the F − + CH 3 Cl entrance channel, and its transformation to the ion-dipole complex F − ---CH 3 Cl results in only a small change of ∼1.3 kcal/mol in the potential energy. The F − + CH 3 Cl → CH 3 F + Cl − back-side S N 2 reaction is of large reaction exothermicity and small F − ---CH 3 Cl → [F---CH 3 ---Cl] − central barrier, which are −31.9 and 3.4 kcal/mol, respectively, at the FPA level.…”

“…There exist back-side and front-side S N 2 and proton transfer pathways for the F − + NH 2 Cl reaction, and all pathways involve a hydrogenbonded F − ---NH 2 Cl complex formed by the initial association of the reactants. The double well potential model for the backside S N 2 pathway of F − + CH 3 Cl system, which is well-known for the S N 2 reactions at carbon, is also presented by its nitrogen analogue F − + NH 2 Cl. However, in contrast to the C 3 V iondipole complexes involving carbon, the corresponding nitrogen species lose all symmetry and are characterized by a NH---X (X = F and Cl) hydrogen bond, in line with the previous predictions.…”

Cold hybrid ion-atom systems

“…Multichannel quantum defect theory (MQDT) allows us to make use of the simple form of the long-range potential and to describe the properties of ion-atom systems with minimal numerical effort. While originally developed to describe the electronic structure of atoms, it found widespread applications in atomic and molecular physics (Gao, 1998;Gao et al, 2005;Greene et al, 1979;Greene and Jungen, 1985;Mies, 1984;Seaton, 1983). The basic idea of MQDT is to make use of the fact that the interaction potential approaches a simple form at large interparticle distances.…”

“…We will now briefly review the MQDT treatment of low-energy ion-atom collisions following (Idziaszek et al, 2009Mies, 1984). An equivalent treatment based on a slightly different formulation has been developed by Gao and coworkers (Gao, 2010(Gao, , 2013Li and Gao, 2012;Li et al, 2014), allowing for additional insights e.g.…”

Dissociation energies of Ag–RG (RG = Ar, Kr, Xe) and AgO molecules from velocity map imaging studies

“…[19] Alternatively VMI can be used for detecting the product state distributions of chemical reactions of neutral [19,27,28] or ionmolecule reactions. [29][30][31] In our experiments, however, by virtue of the laser ablation technique, a range of species is often present within our molecular beam and much of the analysis involves identifying the co-fragment of a photodissociation event, and hence the identity of the parent molecule. The situation is further complicated by multiple-photon processes and the possibility of dissociation on both neutral and ionic potential energy surfaces.…”