Halogen-containing molecules

The formation of (CF3)+ and of (C2FS)+ following FI of hexafluoro-ethane has been examined using a tip emitter and a single-focussing mass spectrometer [446]. The molecular ion was not detected. The translational energy distribution of the fragment ions contained structure, nevertheless it was possible to conclude that both the ions were formed within 500 fs of FI. The reliability of this time depends upon how well the electric field was known. 

Loss of (C4H9)-from the molecular ion of rans-4- -butylcyclohexyl bromide at 10ps—Ins has been shown to proceed at higher rates than the corresponding loss from the molecular ion of the cis isomer [336]. The difference was interpreted in terms of a concerted reaction forming a cyclic bromonium ion. The comparison of rates is intermol-ecular, however, so that the differences could stem from differences in internal energy, P(E). 

Loss of (C2Hs)- from the molecular ion of n-hexylbromide has been observed at short times following FI [520]. 

Isotopic substitution affects rates of ionic decompositions and isomerisations in essentially the same ways as isotopic substitution affects rates of thermal reactions [360, 608, 654, 764, 905, 925]. Mass spectrometry does, however, own a few idiosyncracies in this area and it is important to distinguish clearly the different sorts of isotope effects involved. The term kinetic isotope effects in this review will be restricted to effects of isotopic substitution on the values of rate coefficients, k(E). Kinetic isotope effects on unimolecular gas-phase 

With thermal systems either in the gas phase or in solution, it is in ter -molecular isotope effects which are more commonly studied. Intramolecular isotope effects involve distinguishing and measuring two, or more, chemically identical but isotopically different products produced in the same reaction vessel from the same reactant. The situation is different in mass spectrometry. Intramolecular isotope effects are conveniently studied, because the chemically identical products are naturally separated according to their masses. Intermolecular isotope effects on ion abundances are also easily measured, but, as regards kinetics and mechanism of reaction, their value is limited. Whereas an intramolecular isotope effect (on ion abundances) reflects kinetic isotope effects, an intermolecular isotope effect (on ion abundances) reflects kinetic isotope effects, isotope effects on the internal energy distribution, P(E), and other factors as well and the effects cannot be easily separated (vide infra). 

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

Among the processes leading to annihilation of free electrons, the most efficient is the dissociative recombination of an electron with a molecular ion. At small electron energies the cross section of such processes exceeds 10 13 cm2. 240 The cross sections for other types of recombination are much smaller. The cross section of dissociative attachment of an electron to a neutral molecule can vary within broad limits from 10 23 to 10 14 cm2, 223 243-244 and is the largest for halogen-containing molecules. 

We shall discuss some examples of reactions of excited van der Waals complexes. Up to now, only a few examples of a atom-diatom reactions have been studied—the atom being Hg, Ca, or Xe, and the diatom being H2 and halogen-containing molecules. These examples show clearly the new features in the reaction dynamics, such as orbital specificity, selectivity in the products, products state distribution, and observation of the intermediate states. 

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... 

Compound 192 is a useful precursor for the transfer of the ligand with halogen-containing molecules by precipitation of HgX2 (241). 

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]. 

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. 

G3 theory was designed to correct some of the dehciencies of G2 theory for systems such as halogen-containing molecules, unsaturated hydrocarbons, etc. It also contains... 

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