An ab initio study of PuO2 and of PuN2

Ab initio electronic structure calculations using correlated wavefunctions have been performed to investigate the structures, vibrational frequencies and energetics of PuO2 and PuN2 molecules in their low-lying electronic sta tes. For PuO2, the (5)Sigma(g)(+)- [(1 delta(u))(2)(1 phi(u))(2)] and the ( 5)Phi(u)-[(1 delta(u))(2)(1 phi(u))(1)(4 sigma(g))(1)] states are found to be nearly degenerate energetically. However, frequency considerations indic ate that the (5)Sigma(g)(+) State is the preferred candidate for the ground state. The CCSD(T) computed vertical ionization energy of PuO2 of 9.92 eV compare favorably with the experimentally determined ionization energy of 9 .43 eV. For PuN2, the (3)Pi(g)-[(1 delta(u))(1)(1 phi(u))(1)] state is pred icted to be the ground state. The omega(3) mode frequency computed for the (3)Pi(g) ground state at the CCSD and CCSD(T) levels, when scaled by 0.93 a nd 0.92, respectively, agrees perfectly with the nu(3) frequency of 1030 cm (-1) reported from infrared matrix isolation studies. (C) 2000 Elsevier Sci ence B.V. All rights reserved.