When a reactive mixture is confined within a material with pore sizes comparable to the molecular dimensions, molecular interactions with the pore walls can cause dramatic changes in the potential energy surface of the reaction. In such cases, the porous material enhances or hinders reactions depending on the degree to which the pore shape corresponds to that of the transition state. We illustrate the magnitude of these effects by reporting density functional theory calculations for the rotational isomerization of 4-membered hydrocarbons in narrow carbon pores. The structure of the stable states and their equilibrium distributions are drastically modified by confinement. We show that in all cases the reaction rate varies approximately as the double exponential of the pore size in the molecular sieving limit, leading to very high sensitivity of the kinetics to pore width. We explain such high sensitivity by considering the intermolecular forces involved, and expect this to be a general phenomenon for reactions where transition state theory is a reasonable approximation. The results suggest that, with sufficient control over the synthesis of nano-porous materials, greatly improved catalytic effects could be achieved.

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