Theoretical Investigations of the Structures and the Ring-Puckering Vibrations for Several Bicyclic Molecules*

Ab initio calculations using the MP2/cc-pVTZ method have been carried out to calculate the structures and relative energies of the different conformations of five bicyclic molecules including bicyclo[3.1.0]hexane, 3-oxabicyclo[3.1.0]hexane, 6-oxabicyclo[3.1.0]hexane, 3,6-oxabicyclo[3.1.0]hexane, and bicyclo[3.1.0]hexan-3-one. Theoretical ring-puckering potential energy functions in terms of the ring-puckering coordinate have been calculated for each of the molecules and these were compared to those previously determined experimentally from spectroscopic data. Each potential function is asymmetric and has a single energy minimum corresponding to where the five-membered ring is puckered in the same direction as the attached three-membered ring. The calculations show that the ring-puckering motion is somewhat coupled to the ring-twisting and ring-flapping motions. Natural bond orbital analysis has also been performed and studied for these molecules and the wavefunctions for the lowest ring-puckering energy levels have been computed.