The microwave spectrum of the sesamol (1,3-benzodioxol-5-ol) monomer and an analysis of its internal motion

Abstract

The microwave spectrum of the sesamol (CH2O2C6H3OH) monomer has been collected using CP-FTMW spectroscopy in the 5.5–18.75 GHz region of the electromagnetic spectrum. Density functional theory calculations of the monomer have been performed. B3LYP|cc-pVTZ predicts the global minimum geometry to be s-trans-sesamol where the hydroxyl group is oriented 180° away from the dioxole ring. A second, local minimum, geometry for s-cis-sesamol was calculated to have the hydroxyl group oriented 0° towards the dioxole ring, and the zero-point energy difference between the two geometries is predicted to be 1.4 kJ/mol. Both unique conformers have been identified in the experimental spectrum. In accordance with the predicted dipole moments, only b-type transitions have been observed for s-trans-sesamol, while a- and b-type transitions have been assigned to s-cis-sesamol. The relative intensities in the observed spectrum indicate that the molecular beam contains a higher population of the s-trans-sesamol monomer. Satellite transitions were observed for both sesamol conformers. Treating the parent and satellite transitions as coupled vibrational states, the energy differences and Coriolis coupling constants were determined for each conformer. The magnitude of the coupling constants, the energy difference between the states, and a comparison to a similar state observed in 1,3-benzodioxole indicate that the excited states correspond with CH2 puckering of the dioxiole ring. The second moments of inertia and their contributions from the ring puckering have been analyzed for a series of functionalized molecules including piperonal, 1,3-benzodioxole and sesamol.