Theories of translational energy release

Perhaps the point to emphasise in discussing theories of translational energy release is that the quasiequilibrium theory (QET) neither predicts nor seeks to describe energy release [576, 720], Neither does the Rice— Ramspergei Kassel—Marcus (RRKM) theory, which for the purposes of this discussion is equivalent to QET. Additional assumptions are necessary before QET can provide a basis for prediction of energy release (see Sect. 8.1.1) and the nature of these assumptions is as fundamental as the assumption of energy randomisation (ergodic hypothesis) or that of separability of the transition state reaction coordinate (Sect. 2.1). The only exception arises, in a sense by definition, with the case of the loose transition state [Sect. 8.1.1(a)]. 

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Theories of translational energy release in unimolecular decompositions are discussed in Sect. 8.1. Qualitative lines of explanation are discussed, in conjunction with the experimental results to which they relate, in Sects. 8.2—8.4. The extensive data on translational energy releases in source reactions, including PIPECO, and in metastable ion decompositions are collected together in tables (Sect. 8.5). The emphasis is on decompositions of polyatomic ions, although many triatomics are included in the tables. The coverage includes both fundamental and mechanistic studies. 

Phase space theory has been able to reproduce experimental distributions of translational energy releases closely for a number of decompositions which do not have energy barriers to the reverse reactions [165, 485] (see Sect. 8). Phase space theory does focus attention on a very late stage of reaction since the degrees of freedom of the loose transition state can be identified as vibrations, rotations and translations. 

It has been mentioned that phase space theory, i.e. assuming a loose transition state, has been able to explain the translational energy releases in the decomposition of certain ion—molecule collision complexes [485] and in some unimolecular decompositions measured by PIPECO (see Sect. 8.2). There is a larger number of translational energy releases from PIPECO and a body of data as to translational energy releases in source reactions of positive ions formed by El [162, 310] (Sect. 8.3.1) with which the predictions of phase space theory are in poor agreement. The predicted energy releases are too low. 

Because dissociation on So is barrierless, the product state distributions should be well approximated by statistical theories, especially when the excess energy is small, as in the Valachovic study. Product state distributions arising from the So pathway should be characterized by small translational energy release, but significant rovibrational excitation of HCO. This signature is demonstrated in the top panel of Fig. 17, which shows a HRTOF spectrum with... 

The tetrafluoromethane ion has also been found to decay before electronic randomisation has occurred [129, 769] (see Sect. 5.3 and 5.9 for other perfluorinated molecules). The breakdown diagrams for CF3X molecules (X = a halogen atom other than F) have been reported [690]. Translational energy release distributions have also been measured for these molecules and shown to be in agreement with the predictions of statistical theory (phase space theory) [691]. Carbonyl chloride and fluoride have been studied [451] (see Sect. 8). 

The loss of ketene from the molecular ion of acetanilide is the largest system studied to date [176]. The theory predicted that almost none of the reverse critical energy should appear as translational energy release, which was what had been found experimentally. 

The mean translational energy releases in the loss of C2H2 from the molecular ion of thiophene exceed the values predicted by phase space theory by about a factor of 2 [152]. The suggestion [152] that the disagreement stems from vibrational energy not being randomised prior to dissociation seems unjustified. The loose model assumed for the transition state would seem to be the prime suspect. 

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