Thermodynamics of ammonia activation by iron cluster cations: Guided ion beam studies of the reactions of Fen+ (n=2–10,14) with ND3

Abstract

The kinetic energy dependences of the reactions of Fen+ (n=2–10,14) with ND3 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0–10 eV. Dehydrogenation of ammonia to form FenND+ is found to be efficient and exothermic for n=4 in agreement with previous FT-ICR studies. In contrast to the ICR studies, we also observe exothermic dehydrogenation for n=3 and 5, although these processes are much less efficient than for n=4. Other clusters also undergo this process but exhibit an energy threshold. A multitude of other primary products are observed including Fen−1ND3+ (n=2–4,9,10), FenND2+ (n=1,4–8), and Fen−1ND2+ (n=2–5), which all have reaction efficiencies that depend on cluster size. At high energies, FenN+ and FenD+ are observed along with products corresponding to Fe atom loss from the primary products. Thresholds for the various primary and secondary reactions are analyzed and bond energies for iron cluster cations bound to N, ND, ND2, and ND3 are determined. Comparisons of this thermochemistry to that for isoelectronic CD, CD2, and CD3 show that the binding of ND and ND2 is enhanced by dative interactions of the lone pair of electrons with the cluster. In contrast, the N atom is found to have a weaker bond than CD for most cluster sizes. On the basis of this thermochemistry, we find that there are barriers to the dehydrogenation reactions of about 1 eV for all the clusters, except n=3–5 exhibit an additional low energy pathway. Bond energies for larger clusters are used to estimate the bond energies of the ND and ND2 molecular fragments to iron surfaces.