The interaction of ammonia with small iron clusters: infrared spectra and density functional calculations of Fe n(NH 3) m and Fe n(ND 3) m complexes

Abstract

Bare iron clusters react in the gas phase with ammonia to form Fe n (NH 3) m complexes. In the present study, the iron cluster–ammonia interaction within Fe n (NH 3) m and Fe n (ND 3) m complexes ( n=7–16) are investigated by molecular beam infrared depletion spectroscopy and density functional theory (DFT) calculations. Experimentally, we observe an absorption band within Fe n (ND 3) m complexes in the 880–890 cm −1 range, which is attributed to the ν 2 inversion mode of ND 3. DFT calculations performed for Fe n (NH 3) m and Fe n (ND 3) m model complexes ( n=1, 4, 7, and 13) predict that three of the four vibrational fundamentals of ammonia are only slightly shifted from their gas phase values, but that the symmetric ν 2 fundamental shifts substantially upward in the complex. For ND 3, ν 2 is predicted to shift from 748 cm −1 to the 850–900 cm −1 range when adsorbed to Fe n , as is observed experimentally in both the present cluster study and in iron single-crystal surface studies. DFT calculations of vertical ionization potentials of Fe n /Fe n (NH 3) m species and of Fe n –NH 3 binding energies are found to be in near-quantitative agreement with previously measured values.