A Remarkable Nano-Confinement Effect on Chemical Equilibrium: From Nucleotide Dimer Formation in Molecular Cages to Deuterium Exchange Reactions on Interstellar Dust Grain Surfaces

With current developments in nanoscience and nanotechnology, experimental studies of reactions in confined nanospaces have been emerged. Thus, a variety of tailor-made “nanoreactors” have been fabricated: (i) molecular capsules, held together either by covalent bonding or by self-assembly via weaker interactions such as metal coordination and hydrogen bonds, for organic and metal catalyzed synthesis; (ii) carbon nanotubes for the synthesis of nanowires, nanobeads, or polymeric chains, and more. In a related field, considerable research has been devoted to explain and/or reproduce observations of deuterium fractionation in interstellar molecules originally adsorbed on surfaces of tiny dust grains. Yet, while focusing mainly on reaction kinetics, the possible role of equilibration has not been explored in these studies. Since classical thermodynamics of macroscopically large systems is not fully applicable under such conditions, we recently developed a pertinent statistical-mechanical formulation for chemical equilibria, based on the canonical-ensemble and the ideal lattice gas model [1]. As shown, unique properties characterize a closed small system of reaction mixture molecules at equilibrium, including significant extra stabilization of exothermic reaction products, namely, the enhancement of the reaction extent or equilibrium constant. The origin of this remarkable deviation from the macroscopic thermodynamic limit (TL), which we name "The nano-confinement effect on chemical equilibrium" (NCCE), is elucidated. In a first implementation of the theory, the work focuses on experimentally explored artificial and natural confined reaction systems, representing widely divergent branches of chemistry.