Ab Initio Calculations of Equilibrium Geometries and Vibrational Frequencies of Alkali Metal Tetrahydroborates MBH_4(M=Li, Na and K) and some Boron Hydrides (BH_3, B_2H_6 and BH_4^-)

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Alkali metal tetrahydroborates (MBH_4, M=Li, Na and K) and boron hydrides (BH_3,B_2H_6 and BH_4^-) have been studied by ab initio MO calculations. Geometry optimizations and frequency calculations have been performed at HF/3-21G^<**> level for MBH_4 in monodentate, bidentate and tridentate configurations. Calculated dipole moments in the tridentate configuration indicate that B-M bonds in MBH_4 are highly ionic, and the ionicity of B-M bond is related to the variation of geometrical parameters during the alkali metal substitution. Experimental B-M bond lengths and ν_4(B-M stretching) frequencies for LiBH_4 and NaBH_4 are well reproduced by MP2/6-31G^<**> calulation. Differences of B-H bond distances, B-H stretching and H-B-H bending force constants in each MBH_4 are discussed in relation to the difference of bonding environment between bridging and terminal hydrogen atoms.
1995-01-31