Broadband microwave spectroscopy of cyclopentylsilane and 1,1,1-trifluorocyclopentylsilane

•Broadband microwave spectroscopy confirms global minimum equatorial structure.•Large amplitude motion arising from ring torsion leads to discrepancies between equilibrium and ground state structure.•Direct structure determination reveals silicon’s influence on aliphatic ring structure.•An excited vibrational state was observed spectroscopically and identified as a lowest-lying ring twisting mode.