Computational Elucidation of the Transition State Shape Selectivity Phenomenon

The most commonly cited example of a transition state shape selective reaction, m-xylene
disproportionation in zeolites, is examined to determine if the local spatial environment of a reaction can
significantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136
kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediated
pathway. Both pathways are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trimethylbenzene), but relative selectivity to the other two isomers varies with pore geometry, mechanistic
pathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different common
zeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration of
pore shape. Variation of the environment shape at the critical transition states is thus shown to affect the
course of reaction. Barrier height shifts on the order of 10−20 kJ/mol are achievable. Observed selectivities
do not agree with the transition state characteristics calculated here and, hence, are most likely due to
product shape selectivity. Further examination of the pathways highlights the importance of mechanistic
steps that do not result in isomer-defining bonds and leads to a more robust definition of transition state
shape selectivity.