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The rotational spectrum of dimethylsulfide has been measured in the millimetre-wave range between 140 and 300 GHz. A new computer program based on the internal-axis method has been used to analyze the rotational spectrum in its torsional ground state. It has allowed us to fit the spectrum satisfactorily and to determine the rotational, internal-rotation, and centrifugal-distortion constants accurately. The influence of the approximations made during the internal-rotation analysis on the moment of inertia (Ia, Ib, Ic, and Iα) is pointed out. It shows the difficulty in determining accurate geometrical structures of two-top molecules from microwave data. For the AA substate, effective rotational parameters are given that allow the calculation of transition frequencies of possible astrophysical interest.

The ground state rotational spectrum of propane has been investigated between 6.4 and 26.5 GHz with a microwave Fourier transform spectrometer and between 140 and 300 GHz with a millimeter-wave spectrometer. High J transitions have been measured and fitted to a centrifugally distorted Hamiltonian including some sextic coefficients. The results of the analysis are sufficiently accurate for the prediction of all strong transitions of astrophysical interest.