Confinement and Diffusion of Small Molecules in a Molecular-Scale Tunnel

Abstract

Multi-step reaction cascades can be designed to include channeling mechanisms, which provide electrostatic or steric control over intermediate transport such that intermediates do not escape to the bulk between active sites. Physical confinement of the intermediate pathway between sites retains intermediate from bulk access and thus provides high transport efficiency. In this work, we use molecular dynamics to study the transport of intermediates (charged oxalate and neutral ethanol) inside a nanochannel represented by a single-walled carbon nanotube (SWCNT). This approach reveals that solvent orientation highly impacts intermediate transport. At small nanochannel diameter near 1 nm, highly structured solvent water and Knudsen diffusion decreases effective intermediate diffusivity. Finally, modified SWCNT termini with electrostatically-charged carboxylate groups are shown to increase intermediate retention for both charged and uncharged intermediates by up to five-fold. When catalyst sites are located within the nanochannel, decreased diffusion rate and increased retention time will enhance cascade efficiency.