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 υ = 0 state and υ = 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 7.65(X4 − 9.15X2) cm−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 μa = 0.496 + 0.0051 〈X2〉υυ (debye).