Ring Puckering in Five-Membered Rings. III. The Microwave Spectrum, Dipole Moment, and Structure of Cyclopentanone

Microwave spectra of cyclopentanone and 2-d-cyclopentanone are reported. Eight vibrational satellites have been assigned for cyclopentanone, and two conformers have been observed for 2-d-cyclopentanone. The rotational constants of the ground states are: 6621, 3351.54, and 2410.40 MHz for the normal isotopic species of cyclopentanone; 6309, 3322.55, and 2382.82 MHz; and 6258, 3344.54, and 2361.96 MHz for the two conformers of the 2-deutero isotopic species. It was found that the nature of the ring-puckering potential is such that the ring is permanently twisted and undergoing bending and twisting vibration quite independently. The lower limit to the height of barrier-hindering pseudorotation was set about 400 cm−1. The structural parameters, r;(C1–C2), r;(C2–C3), ∠(C2–C1–C5), and the twist angle τ, were calculated by the iterative procedure of the weighed least-squares analysis. They are 1.504 ± 0.01 Å, 1.557 ± 0.007 Å, 110.5 ± 0.7°, and 23.6 ± 0.4°. The components of dipole moments were determined by the Stark effect: μa = 3.25 ± 0.02 D and μc = 0.00 ± 0.15 D.