Microwave Spectrum, Vibration–Rotation Interaction, and Ring Puckering Vibration in Silacyclobutane and Silacyclobutane-1,1-d2

The microwave spectrum of the molecules silacyclobutane and silacyclobutane‐1,1‐d₂ has been observed between 8 and 55 GHz and the first four vibrational states assigned. The ground state rotational constants of the former molecule are 8815.75, 6289.00, and 4245.32 MHz. The first two inversionlike intervals of the ring puckering mode were determined to be 75.75 and 7790 MHz in the normal isotopic species; the corresponding intervals in the deuterated species are 43.06 and 4430 MHz. The effect of these frequencies on the determination of the potential energy for the ring puckering vibration was investigated. The ring configuration was shown to be significantly puckered in the ground state. The $a$ component of the dipole moment was found to be nearly constant in the four vibrational states of the normal molecule, ${\mu }_a\text{\hspace{0.17em}=\hspace{0.17em}0.4396}\mathrm{D}$ , while the out‐of‐plane transition moment between inversion doublets dropped rapidly as the inversion levels approached the top of the barrier.