Evaluation of the substitution structures of two partially fluorinated cyclopentanes C₅H₃F₇ and C₅H₂F₈

The pure rotational spectra of the title compounds have been recorded and analyzed. For both species the spectra from all of the singly substituted carbon-13 isotopologues were observed in natural abundance. Ground state rotational constants together with the results of quantum chemical calculations have been used to investigate the ground state structures of each molecule. The substitution structures show carbon-carbon bond lengths which are clearly wrong. As a rationale for this finding it is proposed that the presence of very low frequency normal modes cause an isotopologue-dependent rovibrational interaction and, hence, differences between the ground state geometries of the parent isotopologues and those of the singly substituted carbon-13 rings. These differences preclude satisfactory substitution structural analyses.