Estimation of Rate Parameters by Quantum Mechanics

Two likely paths exist for the thermal unimolecular decomposition of silane. The first reaction path involves the scission of the SiHs-H bond, with a stretched or loose transition state  

For this reaction, the reaction coordinate is simply the Si-H bond distance. Since the reverse of this reaction is radical-radical recombination and should have no activation energy barrier, the E for forward reaction would be expected to be AH,. 

The other SiH4 decomposition pathway involves Hj elimination via a three-center cyclic transition state (Roenigk et ai, 1987)  

The reaction coordinate again is simple it is the H-Si-H bond angle. In this case, however, the reverse reaction is a radical-molecule reaction, and we cannot make the a priori assumption that its activation energy would be zero. In fact, the literature is full of examples of radical-molecule reactions with large activation energies (Benson, 1976). As a result, we cannot also make the assumption that for the forward reaction E = AH as we did in the case of the Si-H bond fission reaction. At this point, we must resort to either quantum chemical calculations or experiments to resolve this issue. 

As seen from Fig. 12, quantum chemical calculations indeed support the fact that the activation energy for the SiHj-H bond scission reaction corresponds to the bond dissociation energy or the heat of reaction, i.e., AH, % s 90 kcal/mol. In addition, at the transition state, the Si-H bond distance is calculated to be about 2.5 A, a clear indication of the loosness and the increase in entropy associated with the formation of the transition state. 

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