Magnetic Properties and Electronic Structure of Neptunyl(VI) Complexes: Wavefunctions, Orbitals, and Crystal‐Field Models

Abstract

The electronic structure and magnetic properties of neptunyl(VI), NpO2(2+), and two neptunyl complexes, [NpO2(NO3)3](-) and [NpO2Cl4](2-), were studied with a combination of theoretical methods: ab initio relativistic wavefunction methods and density functional theory (DFT), as well as crystal-field (CF) models with parameters extracted from the ab initio calculations. Natural orbitals for electron density and spin magnetization from wavefunctions including spin-orbit coupling were employed to analyze the connection between the electronic structure and magnetic properties, and to link the results from CF models to the ab initio data. Free complex ions and systems embedded in a crystal environment were studied. Of prime interest were the electron paramagnetic resonance g-factors and their relation to the complex geometry, ligand coordination, and nature of the nonbonding 5f orbitals. The g-factors were calculated for the ground and excited states. For [NpO2Cl4](2-), a strong influence of the environment of the complex on its magnetic behavior was demonstrated. Kohn-Sham DFT with standard functionals can produce reasonable g-factors as long as the calculation converges to a solution resembling the electronic state of interest. However, this is not always straightforward.

Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2

“…23,24,26 The electronic spectra of the solutions were first assigned using semi-empirical ligand-field theory (NpO 2 +27 and NpO 2 2+ 28 ) and were revised later by quantum chemical calculations. 19,25,[29][30][31][32][33][34][35] Matsika et al 19,25,29 used configuration interaction methods in conjunction with relativistic effective core potentials that incorporate also spin-orbit coupling 36 (SO-MRCI). In one of these studies, 19 coordination models with five explicit coordinating water molecules and chloride ions, respectively, were tested.…”

“…The hitherto most complete assessment of the electronic spectrum of Cs 2 NpO 2 Cl 4 at an adequate level of theory was performed by Su et al 33 who calculated both the vertical and adiabatic transitions of NpO 2 2+ and NpO 2 Cl 4 2− up to 21 000 cm −1 using the SO-RASPT2 method. The electronic structure and magnetic properties of NpO 2 2+ and a few complexes: NpO 2 Cl 4 2− , [NpO 2 (NO 3 ) 3 ] − , and [NpO 2 (CO 3 ) 3 ] 4− were studied by Gendron et al 34,35 using a combination of theoretical (among them SO-CASPT2) methods. The studies included a detailed analysis of SO coupling involving the first four electronic states of the given molecules.…”

“…12,19,[23][24][25][26][29][30][31][32][33][34][35]38,39 We computed the excited electronic states up to ca. 35 000 cm −1 using the SO-CASPT2 method.…”

“…[16][17][18][19][20][23][24][25][30][31][32][33][34][35] Due to the environment influencing considerably the spectral intensities in solutions and solids, our Einstein coefficients computed for the isolated molecules may not be suited for comparison. In contrast, the transition energies suffer less from the environment.…”

“…Selected experimental 20,23,24 and theoretical [29][30][31]33,34 results of NpO 2 2+ are compared in Table V. In the experimental spectra, five bands (two in solution 20 and five in the Cs 2 UO 2 Cl 4 crystal 23 ) have been identified as 5f-5f electronic transitions from the ground state to low-lying excited states.…”