Prediction of gas-phase thermodynamic properties for polychlorinated naphthalenes using G3X model chemistry and density functional theory

The standard gas-phase enthalpies of formation of polychlorinated naphthalenes (PCNs) have been predicted using G3X model chemistry, density functional theory (DFT), and second-order Muller-Plesset (MP2) theory. Two isodesmic reactions are used for better prediction of formation enthalpies. The first (IR1) employs chlorobenzene as a reference species and the second (IR2) employs polychlorinated benzenes as reference species. Among congeners, PCNs with simultaneous Cl-substitutions at positions 1 and 8 or 4 and 5 are the least stable, where the strong repulsion between Cl-atoms leads to non-planar structures for a few PCNs. The potential energy curves for ring-wagging motions in 1,8- or 4,5-PCNs are also extremely flat in the vicinity of equilibrium conformations, leading to extremely low harmonic frequencies for the ring-wagging modes. The contributions of these ring-wagging modes to entropy, heat capacity, and thermal corrections have been calculated using the numerically evaluated energy levels. The PCN isomer patterns are discussed based on the calculated Gibbs free energies.