Polyacene Dianion Crystal Lattice Energies and Thermodynamic Stabilities: The Quantitative Effect of Aromaticity

Article abstract of DOI:10.1021/ja00410a012

A calorimeter interfaced with a data acquisition system has been used to measure the heats evolved from the reactions of solid sodium polyacene dianion salts with water.These heats were utilized in a thermochemical cycle to obtain the thermodynamic stabilities (heats of formation) of the salts.For all of the polyacene dianion salts including those from anthracene, tetracene, pentacene, etc., the heats of formation from the polycene and sodium metal were found to be negative.Electron-electron repulsion energies (E_(rep)) were calculated for each of the dianions.The calculated results could then be used with the experimental heats of formation to obtain the actual crystal lattice energies for the salts.For all of the systems studied the crystal lattice energies were found to be between 400 and 440 kcal/mol.Assuming a similar crystal lattice energy for the sodium benzene dianion (Na⁺2Bz^2-), its heat of formation from benzene and sodium metal was found to be very endothermic (+96 kcal/mol).Much of this endothermicity is due to the antiaromatic nature of the benzene dianion.The enthalpy of transferring two electrons from the benzene dianion to the antiaromatic (planar) cyclooctatetraene neutral molecule to form benzene and the cyclooctatetraene dianion is about -177 kcal/mol.