Infrared Spectra of Magnesium Hydride Molecules, Complexes, and Solid Magnesium Dihydride

Laser-ablated Mg atoms react with molecular hydrogen to give magnesium hydrides during condensation in excess hydrogen and neon for characterization by matrix infrared spectroscopy. The observed absorptions are identified by isotopic substitution and theoretical calculations of vibrational fundamentals and assigned to the MgH, MgH 2, Mg 2 H, Mg 2 H 2, Mg 2 H 3, and Mg 2 H 4 molecules. Subsequent UV irradiation excites Mg to the 1 P state, which inserts into H 2 and markedly increases the yield of MgH 2 and Mg 2 H 4 . Interesting MgH 2 (H 2 ) n complexes are formed with high yield in solid hydrogen. Mg 2 H 4 is a dibridged molecule analogous to dialane. Sample annealing gives MgH 2 polymers, which favor single bridge-bond linkages. Warming above 8 K allows the H 2 matrix to evaporate and solid MgH 2 to form with broad 1160 and 560 cm -1 absorptions that remain on the window at 300 K. Thus, we have reacted the elements to give the stable MgH 2 molecule, formed its dimer and tetramer, and finally obtained the ultimate polymer, solid MgH 2, which is of interest for hydrogen storage.