Thermochemical excitation energy distribution

Figure 3. Approximate lower-bound thermochemical excitation energy distribution for F-for-F activated CFs F (49)...

Significance of Thermochemical Excitation Energy Distributions. Two fundamentally different approaches have been used for the elucidation of primary product excitation distributions for hot atom activated species. Many workers have utilized the RRKM unimolecular rate theory to invert pressure falloff data measured for the products from energetic H-for-H (19,64,65,74,83,84,85) or F-for-X (19,27) substitution reactions. More complete citations to the early literature are given elsewhere (57,58,59,77,80). This inversion procedure is computationally straightforward. However, it involves several a priori untested and... 

Figure 8. Approximate thermochemical excitation energy distributions for the F-for-k alkyl replacement channels in CHfiHF (28, 77)...

The fractional product yields and decomposition threshold energies corresponding to the mechanism shown in Reactions 77-84 have been shown in Table XIII. On simple thermochemical grounds the detection of CClg F product shows that the nascent CFs F excitation energy distribution extends to at least 15 eV. 

The search for a phenomenological alternative to RRKM inversion distribution mapping does not represent a novel idea. The first step in the RRKM modeling procedure for a chemically activated species involves the a priori characterization of its initial excitation energy distribution (70,89,90). For species produced from exoergic reactions this information is normally obtained from thermochemical data. A correspondingly simple direct method has not yet emerged for hot atom activation processes, because the associated dynamics are incompletely imderstood. 

Striking similarities exist between the product decomposition phenomena and the derived thermochemical excitation distributions for particular classes of hot H or F reactions occurring in closely related reactants (16,25). Although further measurements would be useful, this evidence suggests that hot reactions related in this fashion may occur upon topologically similar potential energy hypersurfaces (77,81,82). 

The latter process is not competitive with the former above the thermodynamic threshold. Furthermore, as long as the available energy was kept below the thermochemical threshold for the production of vibrationally excited CN radicals, it was possible to fit the observed rotational distributions with phase space theory. The upper electronic state that is involved in the two-photon dissociation was shown to originate below 22,000 cm l and is thought to be repulsive. It could be the same state that has its absorption maximum at 270 nm. 

On a more positive note, our recently derived detailed thermochemical CF3 F excitation distribution from Reaction 78 (47,48,49) exhibits excellent qualitative consistency with RRKM inversion results obtained for H-for-H activated cyclo-C4H7 H (66), suggesting that the general form of the derived excitation distributions may be characteristic of many hot atom systems. Significant quantitative discrepancies, however, have been noted between the RRKM and thermochemical (25) excitation distribution threshold energies for H-for-H activated molecules, suggesting the need for further research. It would be particularly useful to have both methods applied to the same recoil activated molecule. 

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