A STUDY OF THE ANOMERIC EFFECT ON AN ELECTRONIC SCALE - THE ELECTRON-DENSITY OF 1,4-DIOXANE AND TRANS-2,5-DICHLORO-1,4-DIOXANE

The low-temperature molecular structure and electron density (ED) distribution of dioxane (DIOX) and trans-2,5-dichloro-1,4-dioxane (DICLOX) were determined near 100 K from X-ray data. The geometry of the chlorinated molecule is in accord with the predictions of the anomeric effect. Electron deformation density (EDD) maps were calculated by Fourier and multipole expansion techniques and compared to those obtained by ab initio Hartree-Fock methods. The multipole model charge distribution of DIOX determined from experimental data was taken as a reference state, relative to which the ED of the substituted ring was analyzed. These fragment.deformation density (FDD) maps show quadrupolar deformations which are strictly localized on the O, Cl, and anomeric C atoms. The highly correlated orientation of the lobes at these atomic sites may indicate that the observed charge rearrangement is due to three-center interactions, although these local features of the FDD cannot be interpreted on the basis of a specific molecular orbital coupling such as the n-pi(O)-sigma*(C-Cl) overlap. The quadrupoles at the O and at the Cl atoms are situated in the C-O-C plane and parallel to it, respectively. This may imply that the sigma-lone pair of the oxygen and the pi-lone pairs of the Cl atom are also involved in the bonding.