Internal Motion in Lithium Tetrahydroborate LiBH4

Abstract

A vibrational satellite of the rotational spectrum of lithium tetrahydroborate LiBH4, which was assigned to the first excited torsional state of the BH4group, showed two types of anomalies, and they were explained by the interaction of the torsional motion with the overall rotation. The first anomaly, a “2,−1” interaction constantrtlarge compared with the “2,2”, i.e.,l-type doubling, constantqt, was ascribed to the cubic anharmonic potential constantkttt, which is anomalously large because of the unique shape of the potential function for the BH4torsion, and to the large dependence of thexzandyzcross products of inertia on the torsional coordinate. The second anomaly, a sizable increase in theDJKcentrifugal distortion constant with excitation of the torsional mode, was accounted for in a similar way by anomalously large cubicktttand quarticfttttanharmonic terms in the torsional potential function. The harmonic and anharmonic potential constants chosen so as to explain the anomalies were favorably compared with those estimated from a multipole expansion of the potential function.