In the course of this thesis, two different molecular species have been studied. Both are connected to different aspects of rotational spectroscopy regarding the challenges in the investigation as well as the scientific questions. Oxatrisulfane (HSSOH) is a molecule of chemical interest with the focus on obtaining the geometric structure and the stability of possible configurations of oxasulfanes. Since it is a very reactive molecular species, which is not stable even under laboratory conditions, the production of oxatrisulfane is the bottleneck to a spectroscopic investigation. The 13C-isotopologues of dimethyl ether (13CH3O12CH3 and (13CH3)2O) are stable molecules, but the analysis of their rotational spectrum is challenging due to large amplitude motions of the two methyl groups. The development of spectroscopic models for molecules undergoing internal rotation is still ongoing, involving combined theoretical and experimental efforts. In addition, isotopologues of dimethyl ether are of strong astronomical interest. Precise prediction of their rotational spectrum is essential to overcome the line confusion in astronomical observations of star-forming regions and to enable the detection of more exotic species. Furthermore, the relative isotopic abundance ratios of molecular species in the interstellar medium can contain information on their formation processes. These ratios are important pieces in the big puzzle of astrochemical networks which aim towards the understanding of the evolution of star-forming regions.