The Anomeric Effect in Five-Membered Ring Molecules. Comparison of Theoretical Computations and Experimental Spectroscopic Results

As demonstrated in spectroscopic studies of 1,3-dioxole (1,3-DO) and 1,3-benzodioxole (1,3-BDO), analysis of the ring-puckering potential energy function (PEF) of a “pseudo-four-membered ring” molecule can provide insight into understanding the magnitude of the anomeric effect. In this study ab initio computations have been utilized to calculate the PEFs for 1,3-BO and 1,3-BDO and ten related molecules containing sulfur and selenium atoms and possessing the anomeric effect. The potential energy parameters derived for the PEFs directly provide a comparison of the relative magnitudes of the anomeric effect for molecules possessing OCO, OCS, OCSe, SCS, SCSe, and SeCSe linkages. The torsional potential energies produced by the anomeric effect for these linkages were estimated to range from 5.97 down to 1.91 kcal/mole. The ab initio calculations also yielded the structural parameters, barriers to planarity, and ring-puckering angles for each of the twelve molecules studied. Improved PEFs for 1,3-DO and 1,3-BDO were also calculated. The calculations also support the conclusion that the relatively low barrier to planarity of 1,3-BDO results from competitive interactions between its benzene ring and the oxygen atom p orbitals.