The interaction of ammonia with small iron clusters: infrared spectra and density functional calculations of Fen(NH3)m and Fen(ND3)m complexes Original Research Article

Bare iron clusters react in the gas phase with ammonia to form Fen(NH3)m complexes. In the present study, the iron cluster–ammonia interaction within Fen(NH3)m and Fen(ND3)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 Fen(ND3)m complexes in the 880–890 cm−1 range, which is attributed to the ν2 inversion mode of ND3. DFT calculations performed for Fen(NH3)m and Fen(ND3)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 ν2 fundamental shifts substantially upward in the complex. For ND3, ν2 is predicted to shift from 748 cm−1 to the 850–900 cm−1 range when adsorbed to Fen, as is observed experimentally in both the present cluster study and in iron single-crystal surface studies. DFT calculations of vertical ionization potentials of Fen/Fen(NH3)m species and of Fen–NH3 binding energies are found to be in near-quantitative agreement with previously measured values.