Experimental and computational thermochemistry of 1,3-benzodioxole derivatives

The standard (p degrees = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of four 1,3-benzodioxole derivatives (5-oxoethyl-1,3-benzodioxole, 5-cyano-1,3-benzodioxole, 5-nitro-1,3-benzodioxole, and 5-methyl-1,3-benzodioxole) were measured by static bomb calorimetry. The standard molar enthalpies of vaporization or of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. Combining these results, the standard molar enthalpies of formation of the compounds in the gas phase, at T = 298.15 K, have been calculated: 5-oxoethyl-1,3-benzodioxole (-319.3 +/- 2.8) kJ.mol(-1); 5-cyano-1,3-benzodioxole (-23.2 +/- 2.2) kJ.mol(-1); 5-nitro-1,3-benzodioxole (-176.0 +/- 3.2) kJ.mol(-1) and 5-methyl-1,3-benzodioxole (-184.7 +/- 3.5) 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(pi) 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 substituted benzodioxoles using appropriate isodesmic reactions. There is good agreement between theoretical and experimental results. This agreement allows us to estimate the enthalpies of formation of some 1,3-benzodioxole derivatives whose experimental study was impossible.