Bond Dissociation Energies in Heavy Element Chalcogen and Halogen Small Molecules.

Thermodynamic properties including bond dissociation energies (BDEs), heats of formation, and gas-phase acidities for the hydrides and dimers of chalcogens and halogens, HY, HX, Y, and X for Y = Se, Te, and At and X = Br, I, and At, have been predicted using the Feller-Peterson-Dixon composite-correlated molecular orbital theory approach. A full four-component CCSD(T) approach was used to calculate the spin-orbit effects on thermodynamic properties, except for Se, where the AoC-DHF value was used due to strong multireference effects in Se for the SO calculations. The calculated results show that the At BDE is quite small, 19.5 kcal/mol, with much of the low bond energy due to spin-orbit effects. HPo is not predicted to be stable to dehydrogenation to Po + H in terms of the free energy at 298 K. In the gas phase, HAt is predicted to be a stronger acid than HSO. The current results provide insights into potential difficulties in the actual experimental observation of such species for heavy elements.