Thermodynamic study of sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid: A combined experimental and theoretical investigation

The standard (p° = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of four 1,3-benzodioxole derivatives (sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid) were measured by static bomb calorimetry. The values of the standard molar enthalpies of sublimation, at T = 298.15 K, were derived from vapour pressure-temperature measurements using the Knudsen effusion technique. Combining these results the standard molar enthalpies of formation of the compounds, in the gas phase, at T = 298.15 K, have been calculated: sesamol (-325.7 ± 1.9) kJ mol^-1; piperonyl alcohol (-329.0 ± 2.0) kJ mol^-1; piperonylic acid (-528.9 ± 2.6) kJ mol^-1 and homopiperonylic acid (-544.5 ± 2.9) kJ mol^-1. The most stable geometries of all the compounds were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G** and 6-311G**. The nonplanariry of the molecules was analyzed in terms of the anomeric effect, which is believed to arise from the interaction between a nonbonded oxygen p orbital and the empty orbital σ*_co involving the other oxygen atom. Calculations were performed to obtain estimates of the enthalpies of formation of all the benzodioxoles using appropriate isodesmic reactions. There is a perfect agreement between theoretical and experimental results.