Reactions with Halogen-containing Molecules

The study of alkali atom reactions with halogen-containing molecules comprises much of the history of reactive scattering in molecular beams. The broad features of the reaction dynamics and their relation to the electronic structure of the potential energy surface are well understood.2 The reaction is initiated by an electron jump transition in which the valence electron of the alkali atom M is transferred to the halogen-containing molecule RX. Subsequent interaction of the alkali ion and the molecule anion, in the exit valley of the potential surface, leads to an alkali halide product molecule MX. 

Energy disposal in the reactions of electronically excited inert gas atoms with halogen-containing molecules has been studied by observing the ultraviolet or visible emission spectra of the inert gas halide exciplex products under flow or molecular beam conditions. The experimental information consists of branching ratios for the formation of different electronic states of the inert gas halide, vibrational population distributions (obtained by computer simulation of the bound-free spectrum) and the degree of polarisation of the chemiluminescence emission. The metastable inert gases have ionisation potentials that are very similar... 

This effect is best viewed in single harpoon reactions such as those of alkali metal atoms with halogen-containing molecules discussed in Section 2.3.1. A series of studies conducted in a crossed-beam experiment by Lee s group at Berkeley have demonstrated how the electronic excitation of sodium affects the dynamics of these reactions. 

The other route applies when the reactivity of laser-excited species is to be studied. It is then possible to polarize the excited orbital and observe how this polarization affects the dynamics of the reaction. This was first demonstrated by observing the alignment-dependent chemiluminescence in reactions of aligned Ca(4s4p P) with halogen-containing molecules [224, 225]. This work will serve to rationalize the branching to chemiluminscence observed in reactions induced in van der Waals complexes (see Section 2.6.1). It has been extended very recently to other molecular reactants [196]. 

C. Reaction with Other Halogen-Containing Molecules. 261... 

When a heteroatom, such as N, O, or a halogen, is present in a molecule containing an aromatic ring or a double bond, lithiation is usually quite regio-selective. The lithium usually bonds with the sp carbon closest to the hetero atom, probably because the attacking species coordinates with the hetero atom. Such reactions with compounds such as anisole are often called directed metala-tions. In the case of aromatic rings, this means attack at the ortho position.Two examples are... 

Such a molecule, containing alternating single and double bonds, would be expected to be quite reactive. Actually, benzene is quite unreactive, and its chemical properties resemble those of the alkanes much more than those of the unsaturated hydrocarbons. For example, the characteristic reaction of benzene with halogens resembles that of the reaction of the alkanes ... 

The atmospheric fate of a halocarbon molecule depends upon whether or not it contains a hydrogen atom. Hydrohalomethanes are oxidized by a series of reactions with radicals prominant in the troposphere, predominantly hydroxyl OH. Fully halogenated methanes are unreactive towards these radicals and consequently are transported up through the troposphere into the stratosphere, where their oxidation is initiated by UV photolysis of a carbon-halogen bond. 

In the last reaction, the pi electrons of an alkene attack the bromine molecule, expelling bromide ion. A bromonium ion results, containing a three-membered ring with a positive charge on the bromine atom. This bromonium ion is similar in structure to the mercurinium ion discussed in Section 8-5. Similar reactions with other halogens form other halonium ions. The structures of a chloronium ion, a bromonium ion, and an iodonium ion are shown next. 

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