REACTIONS OF PULSED-LASER EVAPORATED BORON ATOMS WITH HYDROGEN - INFRARED-SPECTRA OF BORON HYDRIDE INTERMEDIATE SPECIES IN SOLID ARGON

Pulsed-laser ablated B atoms react with H-2 upon condensation with excess argon to give BH, a (H-2)(BH) complex, and B2H6 as the major products. The initial reaction to form BH requires activation energy, and BH reacts with H-2 to give additional products. Sharp new bands at 2587.3 and 1129.2 cm(-1) exhibit natural isotopic 1:4 doublets for vibrations involving a single boron atom and disappear on annealing to 25 K. Substitution of B-10 and D gives shifts that are matched by MBPT(2) calculations of vibrational spectra for planar BH,. 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 (H-2)(BH3) complex and are in general agreement with recent quantum chemical calculations for BH5. A sharp 2679.9-cm(-1) band gives the B-10 shift predicted by MBPT(2) calculations for linear HBBH. A weak 2212.8-cm(-1) band exhibits the B-10 shift and frequency calculated for the strongest band of BH4-. Additional broad absorptions that remain on annealing are attributed to higher boranes.