Many molecules with two internal C3v rotors have been shown to have "rich" far infrared spectra. Barriers calculated in early studies used approximation methods derived from theoretical work done in the field of microwave spectroscopy. At most, only two Fourier coefficients of the potential function could be determined which allowed interpretation and assignment of only a few of the usually large number of spectral features found in the far infrared region. It is apparent that this approach of treating the semi-rigid model, based on the use of tabulated perturbation sums, is not at all sufficient to explain the commonly very "rich" torsional far infrared spectra. These early methods were not capable of extracting all of the information about the potential functions of these molecules. In a recent series of papers, we have demonstrated how far infrared interferometry and laser Raman spectroscopy may be combined with modern computing techniques to determine three or more Fourier coefficients of the potential function in two variables and to interpret most or all of the features observed in the far infrared spectral region of such two-top molecules. We have recorded the far infrared spectra of dimethylsulfide, dimethylselenide, 1-chloro-2-methylpropene, cis-dimethyloxirane, and trans-dimethyloxirane on a far infrared interferometer at a resolution of 0.25 cm-1. A large amount of torsional data have been obtained and the observed transitions have been assigned on the basis of the semi-rigid model. The barriers to internal rotation have been calculated and, for several of the molecules, both the cosine-cosine and sine-sine coupling terms have been obtained.