The interaction of ammonia with small iron clusters: Infrared spectra and density functional calculations of Fe(n)(NH₃)(m) and Fe(n)(ND₃)(m) complexes

Bare iron clusters react in the gas phase with ammonia to form Fe(n)(NH₃)(m) complexes. In the present study, the iron cluster-ammonia interaction within Fe(n)(NH₃)(m) and Fe(n)(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 Fe(n)(ND₃)(m) complexes in the 880-890 cm^-1 range, which is attributed to the v₂ inversion mode of ND₃. DFT calculations performed for Fe(n)(NH₃)(m) and Fe(n)(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 v₂ fundamental shifts substantially upward in the complex. For ND₃, v₂ 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)(NH₃)(m) species and of Fe(n)-NH₃ binding energies are found to be in near-quantitative agreement with previously measured values.