Are the Ground States of the Later Actinocenes Multiconfigurational? All-Electron Spin−Orbit Coupled CASPT2 Calculations on An(η⁸-C₈H₈)₂ (An = Th, U, Pu, Cm)

Spin−orbit free and spin−orbit coupled CASPT2 wave functions and energies are presented for the ground and low-lying excited states of four actinide element sandwich molecules; thorocene (ThCOT₂), uranocene (UCOT₂), plutonocene (PuCOT₂), and curocene (CmCOT₂). Spin−orbit coupling is found to make little difference to the equilibrium geometry of uranocene and plutonocene but has a significant effect on the energy spectrum of all the systems considered here other than thorocene. In all cases, however, the spin−orbit free ground states make the dominant contribution to their spin−orbit coupled counterparts. Following work presented in J. Phys. Chem. A 2009, 113, 2896, the variation in the multiconfigurational character of the ground-state wave functions as the 5f series is crossed is quantified via the occupation of the a_u/b_1u (e_2u) metal-ring bonding and antibonding natural orbitals. The ground state of plutonocene is found to be nondegenerate with |M_J| = 0, in agreement with its temperature-independent paramagnetism.