Exploring the Actinide-Actinide Bond: Theoretical Studies of the Chemical Bond in Ac_2,Th_2,Pa_2,and U_2

摘要

Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemical bond in the actinide diatoms Ac_2,Th_2,Pa_2,and U_2.Scalar relativistic effects and spin-orbit coupling have been included in the calculations.In the Ac_2 and Th_2 diatoms the atomic 6d,7s,and 7p orbitals are the significant contributors to the bond,while for the two heavier diatoms,the 5f orbitals become increasingly important.Ac_2 is characterized by a double bond with a ~3SIGMA_g~-(0_g~+) ground state,a bond distance of 3.64.A,and a bond energy of 1.19 eV.Th_2 has quadruple bond character with a ~3D_g(1_g) ground state.The bond distance is 2.76 A and the bond energy (D_0) 3.28 eV.Pa_2 is characterized by a quintuple bond with a ~3SIGMA_g~-(0_g~+) ground state.The bond distance is 2.37 A and the bond energy 4.00 eV.The uranium diatom has also a quintuple bond with a ~7O_g (8_g) ground state,a bond distance of 2.43 A,and a bond energy of 1.15 eV.It is concluded that the strongest bound actinide diatom is Pa_2,characterized by a well-developed quintuple bond.