Infrared spectra of aluminum hydrides in solid hydrogen: Al2H4 and Al2H6

The reaction of laser-ablated Al atoms and normal-H2 during co-deposition at 3.5 K produces AlH, AlH2, and AlH3 based on infrared spectra and the results of isotopic substitution (D2, H2 + D2 mixtures, HD). Four new bands are assigned to Al2H4 from annealing, photochemistry, and agreement with frequencies calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH3 and seven new absorptions for Al2H6 in the infrared spectrum of the solid hydrogen sample. These frequencies include terminal Al-H2 and bridge Al-H-Al stretching and AlH2 bending modes, which are accurately predicted by quantum chemical calculations for dibridged Al2H6, a molecule isostructural with diborane. Annealing these samples to remove the H2 matrix decreases the sharp AlH3 and Al2H6 absorptions and forms broad 1720 ± 20 and 720 ± 20 cm-1 bands, which are due to solid (AlH3)n. Complementary experiments with thermal Al atoms and para-H2 at 2.4 K give similar spectra and most product frequencies within 2 cm-1. Although many volatile binary boron hydride compounds are known, binary aluminum hydride chemistry is limited to the polymeric (AlH3)n solid. Our experimental characterization of the dibridged Al2H6 molecule provides an important link between the chemistries of boron and aluminum.