An infrared and Raman study of some selected cyclic and bicyclic molecules

Vita.The conformations of several cyclic and bicyclic molecules were investigated through interpretation of their infrared and Raman spectra. The molecular systems included cyclopentene and three deuterated derivatives, a series of bicycle [3.2.0.] heptenes, methylcyclopropane, propylene oxide and propylene sulfide. As an extension of the bicycle-heptene series, tricycle [3.2.1.0. (superscript 1,5)] octane, a strained propellane, was synthesized and studied spectroscopically. The far infrared and the low-frequency Raman spectra of the three deuterated cyclopentenes were recorded, and the ring-puckering mode of each molecule was analyzed with a mixed quartic/quadratic potential energy function. The study revealed that, like cyclopentene (non planar, inversion barrier 232 cm⁻¹) the three deuterated derivatives have non-planar rings with inversion barriers of 231 cm⁻¹ (d₁), 224 cm⁻¹ (d₄) and 215 cm⁻¹ (d₈). In addition, the d₁ and d₄ molecules showed infrared active ring-twisting vibrations in the form of a series of closely spaced Q-branches for each molecule. Various models (with and without CH₂, CD₂ rocking) for the ring-puckering motion were considered in the one-dimensional calculations performed. The addition of methylene rocking into the puckering model slightly improved the observed/calculated frequency fit, but did not account for the decrease in barrier height with increasing deuteration. The dihedral angle obtained for each molecule was determined to be 26°.