Kinetics of stepwise nitrogen adsorption by size-selected iron cluster cations: Evidence for size-dependent nitrogen phobia

We present a study of stepwise cryogenic N-2 adsorption on size-selected Fe-n(+) (n = 8-20) clusters within a hexapole collision cell held at T = 21-28 K. The stoichiometries of the observed adsorption limits and the kinetic fits of stepwise N-2 uptake reveal cluster size-dependent variations that characterize four structural regions. Exploratory density functional theory studies support tentative structural assignment in terms of icosahedral, hexagonal antiprismatic, and closely packed structural motifs. There are three particularly noteworthy cases, Fe-13(+) with a peculiar metastable adsorption limit, Fe-17(+) with unprecedented nitrogen phobia (inefficient N-2 adsorption), and Fe-18(+) with an isomeric mixture that undergoes relaxation upon considerable N-2 uptake.