Microwave spectrum of alkali metal tetrahydroborate. I. Rotational transitions and molecular structure of NaBH4 in the ground vibrational state

Abstract

The rotational spectrum of NaBH4 was observed in the millimeter-wave region using a high temperature absorption cell. The observed spectrum of NaBH4 showed the pattern of a symmetric top molecule: Strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C3v symmetry. The rotational and centrifugal distortion constants for the 11B and 10B species were determined. The observed rotational constants of Na11BH4 and Na10BH4, combined with the assumption that r(B–Hb)−r(B−Ht)=0.04 Å and θ (Hb–B–Ht)=111°, gave estimates for r(Na–B) and r(B–Hb) to be 2.308±0.006 Å and 1.28±0.10 Å, respectively, where the uncertainties are mainly due to those of the assumed values. This bond length obtained for Na–B is much shorter than the reported value in crystal: 3.08 Å. The bond lengths derived indicate that NaBH4 has a tridentate molecular structure with three bridging hydrogens. This result agrees with those of ab initio calculations.