Knowledge Bank

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 as in the case of $NaBH4$: strong and weak for $K=3n$ and $3n\pm 1$, respectively. The rotational and centrifugal distortion constants for the $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}Li11BH4$, $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}Li10BH4$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}Li11BH4$ species were determined. Three observed rotational constants of $LiBH4$ gave the $r5$ bond distance between the boron and lithium atoms to be 1.9394 {\AA}. This bond lengths obtained for Li-B is much shorter than the reported value in crystal; 2.47 {\AA}. The bond lengths derived indicate that $LiBH4$ has a tridentate molecular structure with three bridging hydrogen atoms. Furthermore, assuming that $\theta (Hb-B-Ht)=1130$, two bond distances $r(B-Hb)$ and $r(B-Ht)$ were obtained to be 1.256 and 1.216 {\AA}, respectively. This result agrees with those of ab initio calculations.