The knowledge of conformational behaviors and geometries is essential to understand, from the molecular level, the change of their physicochemical characteristics and biological activity upon halogenation. High resolution rotational spectra of certain series of flexible molecules are proposed to be investigated using pulse-jet Fourier transform microwave spectroscopic technique combining with theoretical calculation. Precise information on their shapes, geometries, conformational equilibria and internal dynamics will be obtained. Furthermore, it would shed light on the nature and discipline of the factors impacting molecular conformational behaviours, which will be very helpful to tune, more rationally, the organic compounds.