The microwave spectrum,dipole moment and ring planarity of 1,2-dimethylenecyclobutane

The microwave spectrum of 1,2-dimethylenecyclobutane has been studied in the range 26.5–40 GHz using a Hewlett-Packard 8400C Stark-modulated spectrometer. The rigid rotor constants have been derived for the ground state (in MHz: A = 4925.22, B = 4089.88, C = 2301.67) and four excited states of the ring-puckering vibration. That the ring skeleton is planar is indicated by the smooth variation of the rotational constants with vibrational state and by the value of 1/2(I_a + I_b ? I_c) which is consistent with only 4 hydrogen atoms out of the plane of the remaining atoms.Analysis of the Stark effect yields a dipole moment lying along the b-axis; μ_b = 0.457 ± 0.002D. A physically reasonable set of structural parameters which reproduce the ground state rotational constants has been derived by adjustment of the carbon skeleton parameters by a diagnostic least-squares procedure.