Infrared Spectra of Aluminum Hydrides in Solid Hydrogen:  Al₂H₄ and Al₂H₆

Abstract

The reaction of laser-ablated Al atoms and normal-H(2) during co-deposition at 3.5 K produces AlH, AlH(2), and AlH(3) based on infrared spectra and the results of isotopic substitution (D(2), H(2) + D(2) mixtures, HD). Four new bands are assigned to Al(2)H(4) from annealing, photochemistry, and agreement with frequencies calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH(3) and seven new absorptions for Al(2)H(6) in the infrared spectrum of the solid hydrogen sample. These frequencies include terminal Al-H(2) and bridge Al-H-Al stretching and AlH(2) bending modes, which are accurately predicted by quantum chemical calculations for dibridged Al(2)H(6), a molecule isostructural with diborane. Annealing these samples to remove the H(2) matrix decreases the sharp AlH(3) and Al(2)H(6) absorptions and forms broad 1720 +/- 20 and 720 +/- 20 cm(-1) bands, which are due to solid (AlH(3))(n). Complementary experiments with thermal Al atoms and para-H(2) 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 (AlH(3))( solid. Our experimental characterization of the dibridged Al(2)H(6) molecule provides an important link between the chemistries of boron and aluminum.

Infrared Spectra of Indium Hydrides in Solid Hydrogen and of Solid Indane

“…[8] The new 1820, 1803, 1297, 1059, 718, 535, and 526 cm À1 absorptions are appropriate for dibridged In 2 H 6 based on comparisons with recent MP2 calculations [8] ( Table 2) and the spectrum of Al 2 H 6 in solid hydrogen. [16,17] The two terminal In-H 2 modes at 1820 and 1803 cm À1 are higher than for InH 3 , as calculated and observed, and as found for the aluminum analogs. Similar experiments with laser-ablated thallium and hydrogen (deuterium) give weak bands at 1311 (940) cm À1 , which increased on ultraviolet irradiation and are slightly lower than gas-phase TlH (TlD) fundamentals [10] Table 1).…”

“…[8] Our TlH 3 band absorbance is much less intense than that observed for InH 3 , which in turn is expected to be much less than that observed for GaH 3 and AlH 3 . [16,17,24,25] Evaporation of the H 2 (D 2 ) matrix gave no evidence of broad absorption bands in the 1500-700 cm À1 region that might be due to solid thallium hydride. Our failure to observe (TlH 3 ) n at low temperature is due to the inherent instability of the Tl III hydride, which is also manifested in a low yield of TlH 3 .…”

“…This film exhibited broad IR absorptions at 1720 and 720 cm À1 , which are almost identical to the 1760 and 680 cm À1 bands reported for solid (AlH 3 ) n samples containing only bridging AlÀHÀAl linkages. [16][17][18][19] We report herein the first successful preparation of solid (InH 3 ) n using this new cryogenic method, and its spectroscopic characterization as being isostructural to solid (AlH 3 ) n . This work refutes the Wiberg claim [4] for the synthesis of (InH 3 ) n .…”

“…The laser-ablation experiment in conjunction with pure hydrogen as a matrix at 3.5 K has been described elsewhere. [16,17,20] Clearly the ablation laser energy must be limited or the heat load from material and radiation directed to the cold window will prevent the deposition of solid hydrogen.…”

Matrix isolation spectroscopy of H2O, D2O, and HDO in solid parahydrogen

“…Clearly, apriori knowledge of the spectroscopic features attributable to the pH 2 host, and to water molecules and other likely contaminants (29), is required to avoid confusion while assigning spectra of new species in solid pH 2 , e.g. dopant-H 2 reaction products (30,31).…”