Microwave spectrum of alkali metal tetrahydroborate. II. Rotational transitions in the ground and excited vibrational states and molecular structure of LiBH4

摘要

The rotational spectrum of LiBH4was observed in the millimeterhyphen;wave region using a highhyphen; temperature absorption cell. The observed spectrum of LiBH4showed the pattern of a symmetric top molecule: strong and weak forK=3nand 3nplusmn;1, respectively, because of the nuclear spin statistical weight forC3vsymmetry. The rotational constants and centrifugal distortion constants for7Li11BH4,7Li10BH4,6Li11BH4, and6Li10BH4species were determined. Four observed rotational constants gavers(Lindash;B) to be 1.939thinsp;38plusmn;0.000thinsp;10 Aring;, which, combined with the assumption that thgr;(Hbndash;Bndash;Ht)=113thinsp;deg;, led tor(Bndash;Hb) andr(Bndash;Ht) to be 1.256plusmn;0.015 Aring; and 1.216mnplus;0.015 Aring;, respectively, where the signs should be taken in the stated order. The uncertainties ofr(Bndash;Hb) andr(Bndash;Ht) are mainly due to that of thgr; (Hbndash;Bndash;Ht), which is estimated to be mnplus;1.5thinsp;deg;. This bond length obtained for Lindash;B is much shorter than the reported value in crystal; 2.47 Aring;. The bond lengths derived indicate that LiBH4has a tridentate molecular structure with three bridging hydrogen atoms as in the case of NaBH4, in agreement with predictions byabinitiocalculations. Two sets of vibrational satellites were observed and analyzed for both7Li11BH4and7Li10BH4. These satellites were assigned to the nondegenerate Lindash;B stretching and the doublyhyphen;degenerate Lindash;Bndash;H bending mode.