LOW-FREQUENCY VIBRATIONAL-SPECTRA AND RING-PUCKERING POTENTIAL-ENERGY FUNCTION OF 1,3-DIOXOLE - A CONVINCING DEMONSTRATION OF THE ANOMERIC EFFECT

1,3-Dioxole, OCH2OCH=CH, has been synthesized and its far-infrared and low-frequency Raman spectra have been analyzed. The gas-phase far-infrared spectrum shows a series of eleven single-quantum-jump and three triple-quantum-jump transitions in the 40-330 cm-1 region. The low-frequency Raman spectrum exhibits eight ring-puckering transitions corresponding to DELTAupsilon = 2 or 4 transitions in the 160-300-cm-1 region. The ring-puckering potential energy function was determined to be V(cm-1) = (1.59 x 10(6))x4 - (4.18 x 10(4))x2, where x is the ring puckering coordinate in angstroms. This function indicates that the molecule is puckered with a barrier to planarity of 275 cm-1 and a bending angle of 24-degrees. The unexpected nonplanarity of 1,3-dioxole is attributed to the anomeric effect which can be present in molecules with O-C-O linkages. Molecular mechanics calculations utilizing the MM3 parametrization predict a planar structure for this molecule. However, the anomeric effect dictates that each of the =C-O-C-O torsional angles should have a strong desire to increase from 0-degrees toward 90-degrees in order to optimize n-sigma* overlap. When the MM3 force field is modified to reflect this by increasing the magnitude of the 2-fold torsional potential energy term V2 to -5.965 kcal/mol, a reasonably good agreement between the experimental and molecular mechanics potential functions can be obtained.