Vibrational spectra of cyclobutane and monodeuterocyclobutane

Equilibrium conformations of four- and five-membered cyclic molecules in the gas phase: determination and classification

“…a. Two Endocyclic Methylene Groups Cyclopent-3-enone (78) has a planar ring at equilibrium and the puckering mode is governed by an essentially harmonic potential function. 48,52,86,291 Since there are no adjacent CH2 groups in 78, Dx = 0 and only the positive term E, contributes to 8, in eq 21, so that 8 = 8d + 8, will be large and positive-hence the quadratic nature of the function.…”