A two-component relativistic density functional method based on the Douglas-Kroll-Hess transformation has been applied to the actinyls and hexafluorides of U and Np. All-electron scalar relativistic calculations as well as calculations including spin-orbit interaction have been compared to results obtained with a pseudopotential approach. In addition, several exchange-correlation potentials have been applied to examine their performance for the bond lengths and vibrational frequencies of the title compounds. The calculations confirm the well-known accuracy of the LDA approach for geometries and frequencies, which is corroborated for the hexafluorides where gas phase experimental data are available. Comparison with results of accurate wave function based methods provides further confirmation of this finding. Gradient-corrected functionals tend to overestimate bond lengths and underestimate frequencies also for actinide compounds. The results obtained with Stoll-Preuss (small core) effective core potentials agree very well with those of all-electron calculations, while calculations with Hay-Martin large core pseudopotentials are somewhat less accurate. For all molecules and properties considered, spin-orbit effects have been found negligible concomitant with the closed-shell electronic structure of the U(VI) compounds and the open-shell situation of the Np(VI) compounds with a single valence f electron.

No abstract

The six-valent uranyl and neptunyl complexes [An(VI)O2X4]n (An = U, Np; X = F, Cl, OH, n = -2; X = H2O, n = +2) have been studied within the framework of density functional theory. The relative stabilities of the cis and trans isomers, structural properties, charge distribution, and ligand binding energies have been determined using the modified Perdew-Burke-Ernzerhof functional at the all-electron scalar relativistic level. Uranyl and neptunyl complexes with different ligands have been compared in a systematic fashion, demonstrating close similarity of these actinides in oxidation state VI. In addition, the effect of an aqueous solution has been taken into account with the polarizable continuum model COSMO. Computed averaged ligand binding energies permit one to rationalize the observed different stabilities of the title species in aqueous media.

The Stark tuning rate, STR, of free and chemisorbed CO and OH− on Pt(111) is theoretically studied by means of a cluster model density functional theory, DFT, approach with the hybrid B3LYP potential for the exchange-correlation functional. The STR is obtained by explicit calculation of the corresponding vibrational frequencies and of their variation with respect to the intensity of a uniform external electric field. A point of special concern is the influence of the size of the model employed which has been tested by progressively increasing the number of metal atoms employed in the surface model. For both CO and OH− on Pt(111) results show that, except for extremely small cluster models, STR values do not appreciably vary with respect to cluster size. Therefore, the use of a cluster model does not introduce artefacts that may mislead the physical description of this property.

The interaction of carbon trioxide and carbonate on Pt(111) has been studied through the ab initio cluster model approach within the Hartree−Fock and hybrid density functional theory based methods. The equilibrium geometry of adsorbed CO3 and CO3 2- are found to be very similar, suggesting that, once on the surface, both molecules lead to the same adsorbed species but having different adsorption energies. This behavior is rationalized in terms of cost−benefit arguments. The analysis of the vibrational frequencies leads to several important conclusions. The most important one is that vibrational frequencies of the adsorbed species are not always comparable to those of carbonato complexes. However, it is shown that the vibrational frequency for the highest A1 normal internal mode can be transferred from the complex to the surface. The theoretical analysis is next used to interpret the “in situ” infrared spectra of adsorbed carbonate in an electrochemical environment.