Kinetic and thermodynamic stability of the group 13 trihydrides

The kinetic and thermodynamic stabilities of the group 13 hydrides EH(3) (E = B, Al, Ga, In, Tl, E113) are investigated by relativistic density functional and wave function based theories. The unimolecular decomposition of EH(3) --> EH + H(2) becomes energetically more favorable going down the Group 13 elements, with the H(2)-abstraction of InH(3), TlH(3), and (E113)H(3) (E113: element with nuclear charge 113) being exothermic. In accordance with the Hammond-Leffler postulate, the activation barrier for the dissociation process decreases accordingly going down the group 13 elements in the periodic table shifting to an early transition state, with activation energies ranging from 88.4 kcal/mol for BH(3) to 41.3 kcal/mol for TlH(3) and only 21.6 kcal/mol for (E113)H(3) at the scalar relativistic coupled cluster level of theory. For both TlH(3) and (E113)H(3) we investigated spin-orbit effects using Dirac-Hartree-Fock and second-order Møller-Plesset theory to account for electron correlation. For (E113)H, spin-orbit coupling results in a chemically inert closed 7p(1/2)-shell, thus reducing the stability of the higher oxidation state even further. We also investigated the known organothallium compound Tl(CH(3))(3), which is thermodynamically unstable similar to TlH(3), but kinetically very stable with an activation barrier of 57.1 kcal/mol.