Energy Barriers in the Isomerisation of Cyclopropane

In the early days of the transition-state theory, there was great hope that this theory would allow quantitative predictions of rate constants, although this requires adequate preexponential factors and energy barriers for the reactions. The theories previously discussed are essentially concerned with the pre-exponential factors of Arrhenius-type equations. Currently energy barriers for unimolecular reactions are estimated by ab initio methods or by density functional theory (DFT). Such quantum-mechanical methods are particularly useful for small systems, but the numerical solution they offer are not easily related to processes of a similar kind. 

ISM can also be employed to provide some insight for unimolecular reactions. We will consider the case of the isomerisation of cyclopropane to propene. 

In the high-pressure region (P 10 Torr) for the temperature range of 470-520 °C, the rate constant obeys the following expression  

The isomerisation process also requires changes in the bending modes of CCC and CCH. The minimum energy required for harmonic bending motions is 

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