Knowledge Bank

$HClO4$ is a nearly spherical rotor with a moderately low 3-fold barrier to internal rotation, $(s\approx 5.15)$. The a molecular axis is nearly coincident with the internal rotation axis of the OH and $ClO3$ groups. Only A torsional states are allowed. The rotational energies are dominated by the K dependent solutions of the Mathieu equation and have little resemblance to those of a normal asymmetric rotor. For the $HClO4$ ground torsional state, more than 700 features between 52 and 645 GHz with $J\le 61$ and $K\le 47$ of the two major isotopic species have been assigned and fitted. Because of the unusual distribution of energy levels, these include many perturbation enhanced transitions involving large changes in the K quantum number. Low lying excited torsional state spectra also have been assigned. Details of the fitting will be shown. The molecular parameters, which include rotational, centrifugal distortion, quadrupole coupling, torsion-rotation interaction constants and the barrier to internal rotation, will be presented. Structural implications will be discussed. [FIGURE]