Pulsed-laser ablated B atoms react with Hz upon condensation with excess argon to give BH, a (Hz)(BH) complex, and B2H6 as the major products. The initial reaction to form BH requires activation energy, and BH reacts with Hz to give additional products. Sharp new bands at 2587.3 and 1129.2 cm-I exhibit natural isotopic 1:4 doublets for vibrations involving a single boron atom and disappear on annealing to 25 K. Substitution of 1OB and D gives shifts that are matched by MBPT(2) calculations of vibrational spectra for planar BH3. Broader bands at 2475.2 and 1134.3 cm-1 exhibit similar isotopic shifts for vibrations of a BH3 submolecule and decrease on annealing to 25 K. The displacements from isolated BH3 frequencies suggest a (Hz)(BHs) complex and are in general agreement with recent quantumchemical calculations for BHs. A sharp 2679.9-cm-1 band gives the IOB shift predicted by MBPT(2) calculations for linear HBBH. A weak 2212.8-cm-l band exhibits the 1OB shift and frequency calculated for the strongest band of BH4-. Additional broad absorptions that remain on annealing are attributed to higher boranes.
Pulsed laser ablated magnesium atoms have been codeposited with mixtures of hydrogen and argon onto a 10 K substrate. Assignments to product molecules have been made on the basis of broad-band photolysis and annealing behavior, deuterium and 26Mg isotopic substitution, and ab initio calculations. The predominant reaction products are MgH (infrared absorption a t 1422.1 cm-l) and linear MgH2 (vj = 1571.9 cm-l and v2 = 439.8 cm-l). Absorptions attributed to the two lowest energy MgH dimers HMgMgH (1491.8 cm-l) and the rhombic dimer (MgH)2 (1022.8 and 605.4 cm-l) are evident as well as the bridged MgH2 dimer HMg(H)2- MgH (1531MgH ( .0, 1164MgH ( .2, 1013. An increase of HMgMgH and (MgH)2 dimer formation on photolysis indicates photoactivation of Mg2, allowing reaction with H2. Annealing of the matrix yielded little increase in dimer, which is indicative of limited mobility of magnesium hydrides in the matrix. However, H2 complexes with both MgH and MgH2 increased on annealing.
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