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

Abstract

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, $\mathrm{r;(}{\mathrm{C}}_1–{\mathrm{C}}_2)$ , $\mathrm{r;(}{\mathrm{C}}_2–{\mathrm{C}}_3)$ , ∠(C₂–C₁–C₅), and the twist angle $\tau$ , 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: ${\mu }_a\text{\hspace{0.17em}=\hspace{0.17em}3.25\hspace{0.17em}±\hspace{0.17em}0.02}\mathrm{D}$ and ${\mu }_c\text{\hspace{0.17em}=\hspace{0.17em}0.00\hspace{0.17em}±\hspace{0.17em}0.15}\mathrm{D}$ .