Energy barrier for reaction

Figure A3.12.1. Schematic potential energy profiles for tluee types of iinimolecular reactions, (a) Isomerization, (b) Dissociation where there is an energy barrier for reaction in both the forward and reverse directions, (c) Dissociation where the potential energy rises monotonically as for rotational gronnd-state species, so that there is no barrier to the reverse association reaction. (Adapted from [5].)...

Ab initio [515, 516] and semi-empirical calculations [517] of the reaction potential-energy surface show that the potential-energy barrier for reaction depends on the angle of the H—H—F transition state and is lowest for the collinear configuration, having a value 4 kJ mole-1. Thus, collisions involving a nearly collinear approach of F to H2 make the major contribution to reaction and give backward-scattered products. All the surfaces are of a repulsive type. 

In the first and second equation, E is the energy of activation. In the first equation A is the so-called frequency factor. In the second equation AS is the entropy of activation, the interatomic distance between diffusion sites, k Boltzmann s constant, and h Planck s constant. In the second equation the frequency factor A is expressed by means of the universal constants X2 and the temperature independent factor eAS /R. For our purposes AS determines which fraction of ions or atoms with a definite energy pass over the energy barrier for reaction. 

If only the thermal energy of the bombarding olefin molecules was available to overcome the activation energy barrier for reaction then there should be a marked dependence of the conversion on the jet temperature. The results (Fig. 15) show clearly that this is not so and in fact the points for different temperatures all fall on the same curve. Therefore the thermal energy of the alighting molecules is unimportant. Either the reactions must occur at 77°K after the molecules have lost their thermal energy, or if they do occur immediately after bombardment... 

Not every collision will result in reaction only those collisions that have sufficient kinetic energy to surmount the energy barrier for reaction will lead to reaction. For a Maxwell distribution the fraction of encounters that have energy greater than a barrier E (kJ mol-1) is exp(-E/RT). The rate of reaction is then... 

FIGURE 14.9 Ifbenzene is destabilized, the energy barrier for reaction should be reduced. 

Polyatomic molecules have large zero-point energies and in classical mechanics simulations of bimolecular reactions this energy may be accessible for surmounting the potential energy barrier for reaction. " " In the absence of quantum-mechanical tunneling, the threshold for a bimolecular reaction is the vibrationally adiabatic barrier " " with zero-point energy in the vibrational... 

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