Thermochemistry of Bithiophenes and Thienyl Radicals. A Calorimetric and Computational Study.

The relative stabilities of 2,2′- and 3,3′-bithiophenes were evaluated by experimental thermochemistry and the results compared with data obtained from state of the art calculations, which were also extended to 2,3′-bithiophene. The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline 2,2′-bithiophene and 3,3′-bithiophene were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g) and H2SO4·115H2O, measured by rotating-bomb combustion calorimetry at T= 298.15 K. The vapor pressures of these two compounds were measured as a function of temperature by Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation, at T= 298.15 K, were derived from the Clausius−Clapeyron equation. The experimental values were used to calculate the standard (p° = 0.1 MPa) enthalpies of formation of the title compounds in the gaseous phase; the results were analyzed and interpreted in terms of enthalpic increments and molecular structure. Standard ab initio molecular orbital calculations at the G3(MP2)//B3LYP level were performed. Enthalpies of formation, using homodesmotic reactions, were calculated and compared with experimental data. The computational study was also extended to the isomeric compound 2,3′-bithiophene. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out. Finally, bond dissociation enthalpies (BDE) and enthalpies of formation of thienyl radicals were also computed. [ABSTRACT FROM AUTHOR]