RELATIVISTIC DENSITY-FUNCTIONAL STUDY OF THE GEOMETRY, ELECTRONIC-TRANSITIONS, IONIZATION ENERGIES, AND VIBRATIONAL FREQUENCIES OF PROTACTINOCENE, PA(ETA(8)-C8H8)(2)

The geometric structure and electronic properties of the 5f(1) complex protactinocene, Pa(COT)(2) (COT = eta(8)-C8H8), have been investigate d using gradient-corrected density functional methods with the inclusi on of spin-orbit coupling. The calculated structure of Pa(COT)(2) with scalar relativistic corrections is intermediate between those of Th(C OT)(2) and U(COT)(2). Spin-orbit effects are essential for the calcula tion of state energies of Pa(COT)(2). Under D-8h double-group symmetr y, the ground state is found to be an E-5/2u state that corresponds to an (f phi)(1) electron configuration. The first excited state (E-1/2u ) lies only about 0.05 eV above the ground state. The low-lying states follow the ordering of E-5/2u (f phi) < E-1/2u (f sigma+f pi) much le ss than E-3/2u (f pi) similar to E-7/2u (f phi) < 2E(1/2u) (f pi+f sig ma) < E-1/2g (d sigma) much less than 2E(3/2u) (f delta) < 2E(5/2u) (f delta). The lowest-energy electric-dipole-allowed f --> d electronic transition is calculated to occur at 368 nm, which is in excellent agr eement with the experimental estimation of 365 nm. The first 20 vertic al ionization energies and the magnetic moment of Pa(COT)(2) have been predicted as based on the spin-orbit calculations. A comparison of th e calculated infrared vibrational frequencies and absorption intensiti es of Pa(COT)(2) with the available experimental data is presented, an d the vibrational spectra are assigned.