Microwave Spectrum, Ring-Puckering Potential Function, and Dipole Moment of Methylenecyclobutane

The microwave spectra of methylenecyclobutane in the ground state and in the first six excited states of the ring‐puckering vibration have been assigned. The $\text{υ\hspace{0.17em}=\hspace{0.17em}0}$ state and $\text{υ\hspace{0.17em}=\hspace{0.17em}1}$ state are very close in energy and are strongly coupled giving rise to nonrigid‐rotor spectra. Analysis shows the energy difference to be 33 592 MHz. This energy difference together with the vibrational dependence of the rotational constants gives the ring‐puckering potential function to be ${\text{7.65(X}}^4{\text{\hspace{0.17em}−\hspace{0.17em}9.15X}}^2){\mathrm{cm}}^{\text{−1}}$ where $X$ is a reduced ring‐puckering coordinate. This potential function contains a barrier to ring inversion of 160 ± 40 cm⁻¹. The $a$ component of the dipole moment is found to be 0.514 D in the ground state and to have the ring‐puckering vibrational dependence ${\mu }_a{\text{\hspace{0.17em}=\hspace{0.17em}0.496\hspace{0.17em}+\hspace{0.17em}0.0051 〈X}}^2{〉}_{\mathrm{\upsilon \upsilon }}$ (debye).