Energetics and Mechanism of the Hydrogenation of XHn for Group IV to Group VII Elements X

High-level ab initio calculations of the NESC/SOC/CCSD(T)/cc-pV5Z type (NESC, Normalized Elimination of the Small Component; SOC, spin-orbit coupling corrections using the Breit-Pauli Hamiltonian) are employed to determine the energetics of the 18 hydrogenation reactions XHn + H-2 -> XHn + H with X = F, Cl, Br, I, O, S, Se, Te, N, P, As, Sb, Bi, C, Si, Ge, Sn, and Pb. Accurate reaction and activation enthalpies as well as the corresponding free energies are obtained by calculating vibrational, thermochemical, and entropy corrections with a cc-pVTZ basis set. Also calculated are accurate X-H bond dissociation ent:nalpies at 298 K. The reaction mechanism of all 18 reactions is analyzed using the unified reaction valley approach (URVA) and UMP2/6-31G(d,p) to determine each reaction valley with high accuracy (step size 0.005 to 0.03 amu(1/2) bohr). By analyzing the reaction path curvature, the mechanism can be partitioned into four to six reaction phases, in which the reaction complex XHn -> H-2 undergoes specific chemical transformations. The fate of the reaction complex is determined at an early stage in the entrance channel. Curvature peaks reflect the strength of the bonds being broken or formed and provide the basis for a quantitative justification of the Hammond-Leffler postulate.