Thermoneutral reactions, dynamics

Figure 35 shows the energy dependence of the cross section for a model thermoneutral reaction with an activation barrier located in the product vaUey at Rbc == 1-2 A and Rab = 0.8 A (as in Fig. 34b). For this type of the potential energy surface the effectivity of vibrational energy in overcoming the potential barrier is seen to be higher than that of translational energy. The reverse is true of a potential surface of the type shown in Fig. 34a. For non-thermoneutral reactions the situation becomes more complicated because the dynamics depends here both on the barrier and the valley depth.

Fig. 10.13. Effect of noncollinearity on CM reaction dynamics for a thermoneutral reaction for particles A, B, and C of mass 86, 1, and 2, respectively—a model for Kr+ + HD dynamics. Length of vectors indicates velocity. Particle A is so heavy that it is nearly stationary direction of motion is indicated. Case I (i) initial trajectory (ii) instant after AB collision (iii) instant after AC collision (iv) instant after AC distance reaches its maximum. Note that an AC diatom has formed even though initial collision was with B. The AC diatom has vibrational and rotational energy. Case II (i) initial trajectory (ii) instant after AB collision (iii) BC remains bound— no reaction occurs. The BC diatom has vibrational and rotational energy.

This result is consistent with / = 0 as both the electrical and the thmno-dynamic reference state, but it is not consistent with the existing experimental evidence on which indicates that electrode reactions are not thermoneutral reactions. 

B. A. Blackwell, J. C. Polanyi, and J. J. Sloan, Effect of changing reagent energy on reaction dynamics. VIII. Highly vibrationally-excited product from the thermoneutral reaction Cl + 0H(v<9) > HCKv lll) + 0, Chem. Phys. 24 25 (1977). 

These features, together with the reversible, thermoneutral nature of the cyanohydrin formation, make the reaction potentially useful for applications in dynamic chemistry. This was also recently shown in a DCR study [4],... 

Displacement reactions observed in the gas phase are generally exothermic or thermoneutral as in the case of simple isotope exchange. This requirement is consistent with the limited dynamic range of the experimental techniques which precludes the observation of reactions with sizable activation energies. The relevant thermochemical data for negative ions have become available in recent years through the determination of electron affinities (Janousek and Brauman, 1979), and indirectly from gas-phase acidity scales (Bartmess and Mclver, 1979). Relative gas-phase acidities available at present (Bartmess et al., 1979 Cumming and Kebarle, 1978) are an important consideration in... 

Returning to the forward reaction, we note that, due primarily to its small probability and nearly thermoneutral character, detailed dynamical studies (i.e., state-sensitive results or cross sections rather than thermal rate constants) have become available only very recently, in the crossed molecular beam (CMB) experiments of Casavecchia and coworkers [48]. At a collision energy of 6.4 kcal, they found that the DCl produced in the Cl + T>2 reaction is mainly scattered more than 80° backwards from the incident D direction (in the center-of-mass frame) and that about 80% of the total available energy is disposed into relative translation of the products. 

When the dynamic parameter A is, in absolute terms, much greater than the reaction energy, A IA I, then d is independent of the reaction energy. Under such conditions d has a constant value that is the sum of bond extensions of reactants and products at thermoneutrality, d(fi)... 

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