The Microwave Spectra, Structure, and Dipole Moment of Stable Pentaborane

The microwave spectra of several of the naturally occuring boron isotopic species of B5H9 and B5D9 have been observed. These data show that stable pentaborane consists of a tetragonal pyramid of boron atoms and has either C4 or C4v symmetry. The boron-boron distances are B1–B2=1.687±0.005 and B2–B2=1.800±0.003A. If C4v symmetry and B1–H1 and B2–H2 distances equal to 1.22A are assumed, the following boron-hydrogen parameters are obtained: B2–H3=1.35±0.02A, ∠B1–B2–H2=136°10′±30′ and ∠B1B2B2–B2B2H3=196±2°. Dipole moment values of 2.13±0.04 and 2.16±0.04 Debye units were measured for B5H9 and B5D9, respectively.