Energetics of coumarin and chromone.

Condensed phase standard (p degrees = 0.1 MPa) molar enthalpies of formation for coumarin and chromone were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. Combining these values, the following enthalpies of formation in the gas phase, at T = 298.15 K, were then derived: coumarin, -(163.4 +/- 3.3) kJ x mol(-1), and chromone, -(126.1 +/- 2.5) kJ x mol(-1). The temperatures of fusion, T(fusion), and fusion enthalpies, at T = T(fusion), were also reported. Additionally, theoretical calculations were done using different methods: DFT/B3LYP, MCCM (MC-UT/3 and MC-QCISD/3), and also the more accurate G3MP2 method. Good agreement between experimental and theoretical data was achieved. Some correlations between structure and energy were also made, and the aromaticity of the compounds was evaluated by the nucleus independent chemical shifts (NICS).

Experimental and computational thermochemistry of 1,4-benzodioxan and its 6-R derivatives.

The derivatives of 1,4-benzodioxan are found widely spread in nature and have great biomedical importance. The present work reports an experimental and computational study on the thermochemistry of 1,4-benzodioxan and several of its 6-R derivatives in the gaseous phase, at T = 298.15 K. Our current results were obtained from measurements of combustion energies, at T = 298.15 K, using a static bomb calorimeter. The standard molar enthalpies of vaporization/sublimation were measured by Calvet microcalorimetry and corrected to T = 298.15 K. Additionally, estimates were performed of the enthalpies of formation of all the studied compounds in the gas phase, using DFT and other more accurate correlated calculations, together with appropriate isodesmic or homodesmic reactions. There is a reasonable agreement between computational and experimental results.

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

The standard (p(o)= 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 nonplanarity 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 sigma*(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.