Atom-Transfer Reactions Internal Excitation

For reasons given in Section 1.2.2, there have been few direct experiments on bimolecular reactions involving molecules with more than one quantum of vibrational excitation. However, the energies associated with single quanta are comparable with the activation energies of many elementary atom-transfer reactions, so the resultant rate enhancement can be considerable and revealing. In this section, data on the reactions (and parallel relaxations) of diatomic hydrides such as Hg, HX (X = F, Cl, Br, I), and OH, are reviewed first, and then some examples are provided of measurements on reagents excited by CO2 laser photons. 

There have been three studies comparing the reactions of F atoms with 

Experiments involving vibrationally excited H2 are difficult. In the low-pressure flow systems which have been used to study reactions of with radicals, the stimulated Raman effect generates insufficient concentrations of =1) thermal or electrical methods, each capable of producing 17o excitation, are used. Neither of the methods used to observe H2(i = 1), vacuum uv spectrophotometry and sensitized infrared fluorescence, are easy to apply accurately. 

The results of experiments on vibrationally excited H2 are summarized in Table 1.2. Those on the removal of H2(i = 1) by and of D2(t = 1) by are of special fundamental importance. Heidner and Kasper  

Recently, the dynamics of collisions between H and H atoms were examined in QCL trajectory calculations on a potentiaP which is a parameterized best fit to an accurate ab initio surface for collinear The 

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Reaction (25) represents collisional relaxation of the excited oxygen atom. While halogenated hydrocarbons form too small a fraction of the available collision partners to be of atmospheric consequence for the relaxation channel, this pathway must be considered for laboratory experiments relying on 0( D) or 0( P) vs. time profiles to deduce the rate constant of reaction (24). In reaction (26), a sizable fraction of the 190 kJ mol excess energy of 0( D) is transferred to internal excitation of the HCFC, which then can dissociate to products. Between reactions (24) and (26) a range of products is possible, including OH + R, CIO -I- R, and 0( P) -I- HCl -f chlorofluoroalkene. 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

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