Abstract Microwave spectra of trans and gauche propyl fluoride and its isotopically substituted species have been measured. The rs structures of the trans and gauche isomers of this molecule are determined from the observed moments of inertia. It is found that the CCC angle values are largely different between two isomers, while the CCF angle values stay unchanged. The rs structures of ethyl fluorosilane and ethylmethlyl sulfide are re- ...