Transition State Analysis A Summary

On the other hand, in the transition state, the interaction with the metal will also weaken the interaction between C-H in the dissociating CH bond. This is not accounted for in the comparison made within the strain activation model. Hence, the gas-phase strain energy estimate of the bond energy of CH in the transition state provides an upper bound to it and the interaction with the surface metal atom will be overestimated. 

This follows well from a bond order analysis (Eq. (10.7)) of the changing interactions when the molecule moves form its initial state through transition state to final state. This is illustrated in Table 10.7. 

We observe the initial bond order of the CH bond that is cleaved to be close to one. It decreases to 0.29 in the transition state. This implies a more substantial weakening in contrast to the assumption of the activation strain model. The M-H and M-C bond orders are, respectively, 0.38 and 0.43, to be compared with the final state values 0.42 and 0.54, respectively. Compared to the initial CH bond interaction in the transition state, the CH bond interaction has been substantially weakened. One also finds that the M-C and M-H interactions are already close to those of the final state. The bonding interactions are distributed in a close to even distribution over the three bonds of H-C, C-Rh, and H-Rh. In the final state, the dissociating H atom moves to a twofold adsorption site. The closeness of the bondorders of the C-Rh bond and one of H-Rh bonds to those in the final state illustrates the late transition state nature of CH bond activation. 

Bond Reactant state Transition state Product state 

As long as the structures of ttansition state and dissociated state are close, changes in metal-atom interactions will lead to the Bronsted-Evans-Polanyi relation between activation energy and reaction energy of a surface elementary reaction. Interestingly, microscopic reversibility imphes that the Bronsted-Evans-Polanyi proportionality constant for recombination is typically 0.1. This implies that the ratio of the energy of the surface fragments in the transition state compared to the dissociated state is a constant and on the order of 90%. 

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