Knowledge Bank

The general asymmetric rotor programs by Pickett have been modified to allow expansion of the molecular parameters as Fourier series in $\rho K/3$. The method takes advantage of the fact that the expectation values of the operators in the Mathieu equation may be expanded in such a way, but avoids the problems of making a particular selection of torsion-rotation interaction terms. Several molecules with methyl groups and three-fold barriers have been fit to within experimental error in this manner for the purpose of cataloging their spectra. Perchloric acid, $HClO4$, which has a heavy internal top, moderately low barrier $(s\approx 5.15)$, and several avoided crossings in the torsional ground state will be used as an illustration of the application of the program. Approximately 400 features covering the range have been well fitted. The $HClO4$ spectrum will be reviewed and the derived parameters will be discussed.