Microwave spectra have been investigated for GeH₄, AsD₃, AsD₂H, AsH₂D, PD₂H and PH₂D (and PD₃). Forbidden distortion spectra in the frequency range 8-26 GHz have been measured and analyzed for GeH₄, and AsD₃. Allowed rotational transitions, in the centimeter and millimeter wave regions, of AsD₂H, AsH₂D, PD₂H and PH₂D have been measured and analyzed. Harmonic force fields have been produced for ammonia, phosphine and arsine from various empirical data including the quartic centrifugal distortion constants obtained in this work as well as Coriolis coupling constants and vibrational frequencies obtained from other studies. A discussion of the standard "diatomic" anharmonic isty correction techniques to "harmonize" vibrational frequencies shows these techniques to be unsatisfactory and so corrections of this sort were not made.

From the harmonic force field analysis average "r_z" structures have been obtained for various isotopic derivatives of ammonia, phosphine and arsine. Using a linear extrapolation, equilibrium structures have been estimated.

A comparison of many of the symmetric top reduction schemes is also presented in order to make clear a distinction between similarly labeled parameters of different reductions;​ of special importance is the parameter τ_xxxz.

To "fit" spectra of light easily distorted molecules it has been suggested by previous authors that rational fraction Pade approximants might be useful. A motivation for why these approximants could prove beneficial is given along with a review of earlier attempts to incorporate them into general fitting schemes. In this review we find that indeterminacy relations exist between various parameters in these earlier schemes and so a new method, a method of successive separation, is proposed that circumvents these indeterminacies. This method should prove useful for any general study where power series in N variables are slow to converge.