MICROWAVE-SPECTRUM OF ALKALI-METAL TETRAHYDROBORATE .2. ROTATIONAL TRANSITIONS IN THE GROUND AND EXCITED VIBRATIONAL-STATES AND MOLECULAR-STRUCTURE OF LIBH4

The rotational spectrum of LiBH4 was observed in the millimeter-wave region using a high-temperature absorption cell. The observed spectrum of LiBH4 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 C3-upsilon symmetry. The rotational constants and centrifugal distortion constants for (LiBH4)-Li-7-B-11, (LiBH4)-Li-7-B-10, (LiBH4)-Li-6-B-11, and (LiBH4)-Li-6-B-10 species were determined. Four observed rotational constants gave r(s) (Li-B) to be 1.939 38 +/- 0.000 10 angstrom, which, combined with the assumption that theta(H(b)-B-H(t)) = 113-degrees, led to r(B-H(b)) and r(B-H(t)) to be 1.256 +/- 0.015 angstrom and 1.216 +/- 0.015 angstrom, respectively, where the signs should be taken in the stated order. The uncertainties of r(B-H(b)) and r(B-H(t)) are mainly due to that of theta (H(b)-B-H(t)), which is estimated to be +/- 1.5-degrees. This bond length obtained for Li-B is much shorter than the reported value in crystal; 2.47 angstrom. The bond lengths derived indicate that LiBH4 has a tridentate molecular structure with three bridging hydrogen atoms as in the case of NaBH4, in agreement with predictions by ab initio calculations. Two sets of vibrational satellites were observed and analyzed for both (LiBH4)-Li-7-B-11 and (LiBH4)-Li-7-B-10. These satellites were assigned to the nondegenerate Li-B stretching and the doubly-degenerate Li-B-H bending mode.