Infrared Spectra of Aluminum Hydrides in Solid Hydrogen: AI2H4 and AI2H6

摘要

The reaction of laser-ablated Al atoms and normal-H(sub 2) during co- deposition at 3.5 K produces AlM, AlH(sub 2), and AlH(sub 3) based on infrared spectra, and the results of isotopic substitution (D(sub 2), H(sub 2) + D(sub 2) mixtures, HD). Four new bands are assigned to Al(sub 2)H(sub 4) from annealing, photochemistry, and agreement with frequencies calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH(sub 3) and seven new absorptions for Al(sub 2)H(sub 6) in the infrared spectrum of the solid hydrogen sample. These seven vibrational frequencies include terminal Al-H(sub 2) and bridge Al-H-Al stretching and AlH(sub 2) bending modes, which are accurately predicted by quantum chemical calculations for dibridged Al(sub 2)H(sub 6), a molecule isostructural with diborane. Annealing these samples to remove the H(sub 2) matrix decreases the sharp AlM(sub 3) and Al(sub 2)H(sub 6) absorptions and forms broad 1720 plus or minus 20 and 720 plus or minus 20 /cm bands, which are due to solid (AlH(sub 3))n formed on the CsI window. Complementary experiments with thermal Al atoms and para-H(sub 2) at 2.4 K give similar spectra and most product frequencies within 2 /cm. Although many volatile binary boron hydride compounds are known, binary aluminum hydride chemistry is limited to the polymeric (AlH(sub 3))n solid. Our experimental characterization of the dibridged Al(sub 2)H(sub 6) molecule provides an important link between the chemistries of boron and aluminum.