Analyses of the excited 5f1 optical spectra of Th3+ compounds

Highlights: • Explains why the observed 6d¹ to 5f¹ optical spectrum in in Th(Cp″)₃ is truncated. • Shows the 6d¹ ground state in Th(Cp″)₃ (Cp″ = η⁵-C₅H₃(SiMe₃-1,3) has an orbitally quenched ground state. The electronic structure of Th(Cp″)₃ (Cp″ = η⁵-C₅H₃(SiMe₃-1,3), a molecule which has a 6d¹ ground configuration and an excited 5f¹ configuration beginning approximately 14,000 cm⁻¹ higher, is reexamined. The ground state of the 6d¹ configuration in a crystal field of D_3h symmetry is shown to be an orbitally quenched ²A₁ state, best described as a pure ²D_1/2 state. Electric dipole selection rules from this state to the states for the 5f¹ configuration in a D_3h crystal field result in the 5f¹ truncated optical spectrum observed. The optical spectrum of the square planar Th³⁺ compound [Li(THF)₄][Th(OAr′)₄], where (OAr′ = OC₆ H₂^tBu₂-2,6-Me-4), is discussed.