Knowledge Bank

Rotational transitions of methylene cyclobutane in the first seven levels of the low-frequency ring-puckering vibration have been observed. The transitions for the $v=0$ and $v=1$ states are strongly perturbed by a coupling between rotation and vibration and could not be fit with a rigid rotor model. A similar, but much smaller, perturbations can be seen for the $v=2$ and $v=3$ states. The perturbation is adequately explained by assuming that the first two vibrational levels are nearly degenerate. The Hamiltonian describing this perturbation was first discussed by $Lide1$ for cyanamide and later used by Butcher and $Costain2$ to explain the spectrum of cyclopentene. Calculations show that the inversion splitting in methylene cyclobutane is about 33600 Mc.