Hydrogenated and deuterated iron clusters: Infrared spectra and density functional calculations

Iron clusters react sequentially with hydrogen molecules to form multiply h ydrogenated products. The increases in cluster ionization potential upon re action verify that hydrogen chemisorbs dissociatively to form iron cluster- hydride complexes, FenHm. At low source temperatures, the cluster-hydride c omplexes take up additional hydrogen molecules which are shown to be physis orbed onto the underlying FenHm complexes to form FenHm(H-2)(p) species. Th e infrared spectra of FenHm and FenDm (n=9-20) were obtained by the photodi ssociation action spectroscopic method in which depletion of the FenHm(H-2) (p) and FenDm(D-2)(p) species was the signature of absorption. The spectra, recorded in the 885-1090 cm(-1) region, consist of several overlapping ban ds, each approximately 20 cm(-1) in width. The dissimilarity of each FenHm( H-2)(p) spectrum with the corresponding FenDm(D-2)(p) spectrum indicates th at the carrier involves hydrogen and is not merely due to absorption by the underlying iron cluster. Density functional calculations were performed on model complexes, Fe13H14 and Fe13D14, the iron portion of which was assume d to have T-h symmetry. The infrared-active vibrational frequencies involvi ng hydrogen bending and deuterium stretching are predicted to lie within th e experimental frequency range of the experiment, well removed from the ske letal modes of the underlying iron cluster. The complexity of the observed spectra as compared to simulations based on the assumed thigh-symmetry) mod el imply that the experimentally produced complexes possess low symmetry. ( C) 1998 American Institute of Physics. [S0021-9606(98)01148-9].