George Schoendorff

Density functional studies on the complexation and spectroscopy of uranyl ligated with acetonitrile and acetone derivatives.

The coordination of nitrile (acetonitrile, propionitrile, and benzonitrile) and carbonyl (formaldehyde, acetaldehyde, and acetone) ligands to the uranyl dication (UO(2)(2+)) has been examined using density functional theory (DFT) utilizing relativistic effective core potentials (RECPs). Complexes containing up to six ligands have been modeled in the gas phase for all ligands except formaldehyde, for which no minimum could be found. A comparison of relative binding energies indicates that 5-coordinate complexes are predominant, while 6-coordinate complexes involving propionitrile and acetone ligands might be possible. Additionally, the relative binding energy and the weakening of the uranyl bond is related to the size of the ligand, and in general, nitriles bind more strongly to uranyl than carbonyls.

On the Formation of “Hypercoordinated” Uranyl Complexes

Recent gas-phase experimental studies suggest the presence of hypercoordinated uranyl complexes. Coordination of acetone (Ace) to uranyl to form hypercoordinated species is examined using density functional theory (DFT) with a range of functionals and second-order perturbation theory (MP2). Complexes with up to eight acetones were studied. It is shown that no more than six acetones can bind directly to uranium and that the observed uranyl complexes are not hypercoordinated. In addition, other more exotic species involving proton transfer between acetones and species involving enol tautomers of acetone are high-energy species that are unlikely to form.

Low valency in lanthanides: A theoretical study of NdF and LuF

The ground and low-lying excited state potential energy curves of neodymium monofluoride were calculated using multireference (CASSCF) and single reference (EOM-CR-CCSD(T)) methods. Optimized bond lengths were obtained and accurate bond dissociation energies were computed. The EOM-CR-CCSD(T) method was used to determine the bond dissociation energy of lutetium monofluoride, and it is shown that core correlation is required to produce bond dissociation energies in agreement with experiment.

A neoteric neodymium model: ground and excited electronic state analysis of NdF2+.

Neodymium monofluoride dication was studied as a model of the Nd-F bond in NdFx. Multiconfigurational self-consistent field (MCSCF) and second order multireference quasi-degenerate perturbation theory (MCQDPT2) methods were used with a variety of active spaces to elucidate the roles of the Nd 4f, 5d, and 6s orbitals. Spin-orbit coupling calculations were performed at the SO-MCQDPT2 level, and potential energy curves were obtained for the four lowest energy quartet states as well as for the four lowest doublet states and the lowest sextet state. Inclusion of spin-orbit coupling splits these states into 30 levels. Equilibrium bond lengths, dissociation energies, transition energies, and crossing points were determined.

Ground and Excited Electronic State Analysis of PrF2+ and PmF2+

The ground state and excited state manifolds are computed for PrF(2+) and PmF(2+) at the CASSCF (n,8) level of theory where the active space spans the Ln 4f orbitals as well as the F 2pz orbital. Dynamical correlation is included using second-order multireference quasidegenerate perturbation theory (MCQDPT2). The spin-orbit multiplets for each of the excited states are resolved, and spin-orbit coupling constants are computed using the Breit-Pauli spin-orbit operator. Equilibrium geometries for each of the ground and excited states are computed, and the nature of the Ln-F bond is examined. Potential energy curves for the lowest four triplet states and lowest two quintet states are computed for PrF(2+), which split into 14 levels upon application of the spin-orbit Hamiltonian. Likewise, the lowest six quintet states are computed for PmF(2+) as well as the lowest triplet state and the lowest two septet states. These nine states split into 43 terms upon application of the spin-orbit Hamiltonian.

Gas phase computational studies on the competition between nitrile and water ligands in uranyl complexes.

The gas phase formation of uranyl dicationic complexes containing water and nitrile (acetonitrile, propionitrile, and benzonitrile) ligands, [UO(2)(H(2)O)(m)(RCN)(n)](2+), has been studied using density functional theory with a relativistic effective core potential to account for scalar relativistic effects on uranium. It is shown that nitrile addition is favored over the addition of water ligands. Decomposition of these complexes to [UO(2)OH(H(2)O)(m)(RCN)(n)](+) by the loss of either H(3)O(+) or (RCN + H)(+) is also examined. It is found that this reaction is competitive with the ligand addition when the coordination sphere of uranyl is unsaturated. Additionally, this reaction is influenced by the size of the nitrile ligand with reactions involving acetonitrile being the most prevalent. Finally, ligand addition to the monocation shows trends similar to that of the dication with energetic differences being smaller for the addition to the monocation.

A Computational Study on the Ground and Excited States of Nickel Silicide.

Nickel silicide has been studied with a range of computational methods to determine the nature of the Ni-Si bond. Additionally, the physical effects that need to be addressed within calculations to predict the equilibrium bond length and bond dissociation energy within experimental error have been determined. The ground state is predicted to be a (1)Σ(+) state with a bond order of 2.41 corresponding to a triple bond with weak π bonds. It is shown that calculation of the ground state equilibrium geometry requires a polarized basis set and treatment of dynamic correlation including up to triple excitations with CR-CCSD(T)L resulting in an equilibrium bond length of only 0.012 Å shorter than the experimental bond length. Previous calculations of the bond dissociation energy resulted in energies that were only 34.8% to 76.5% of the experimental bond dissociation energy. It is shown here that use of polarized basis sets, treatment of triple excitations, correlation of the valence and subvalence electrons, and a Λ coupled cluster approach is required to obtain a bond dissociation energy that deviates as little as 1% from experiment.

The Importance of Orbital Analysis

It has long been known that there are multiple solutions to the self-consistent Hartree-Fock equations. This can be problematic if careful attention is not given to the orbital occupation and electronic state in the converged wave function. The issues with convergence have been demonstrated through the calculation of potential energy curves for O2, F2, Cl2, Br2, LiF, NaCl, CaO, MgO, ScO, FeO, TiO, YO, and ZrO. Hartree-Fock (HF) calculations were used to compute the points on the potential energy surface, with dynamic electron correlation included through the use of the completely renormalized coupled cluster, including singles, doubles, and perturbative triples [CR-CC(2,3)]. Even in regions with little to no multireference character, as determined by the T1/D1 diagnostics, HF does not always converge to the ground electronic state. As HF provides the reference wave function for CR-CC(2,3), and other post-Hartree-Fock ab initio methods, treatment of electron correlation does not necessarily result in a smooth potential energy curve, especially if HF is unable to produce a smooth curve. Even the convergence rate of multireference methods can be affected as the initial orbitals that form the basis for multireference calculations are frequently obtained from HF calculations.

Spectroscopic properties of Arx-Zn and Arx-Ag + (x = 1,2) van der Waals complexes

Potential energy curves have been constructed using coupled cluster with singles, doubles, and perturbative triple excitations (CCSD(T)) in combination with all-electron and pseudopotential-based multiply augmented correlation consistent basis sets [m-aug-cc-pV(n + d)Z; m = singly, doubly, triply, n = D,T,Q,5]. The effect of basis set superposition error on the spectroscopic properties of Ar-Zn, Ar2-Zn, Ar-Ag(+), and Ar2-Ag(+) van der Waals complexes was examined. The diffuse functions of the doubly and triply augmented basis sets have been constructed using the even-tempered expansion. The a posteriori counterpoise scheme of Boys and Bernardi and its generalized variant by Valiron and Mayer has been utilized to correct for basis set superposition error (BSSE) in the calculated spectroscopic properties for diatomic and triatomic species. It is found that even at the extrapolated complete basis set limit for the energetic properties, the pseudopotential-based calculations still suffer from significant BSSE effects unlike the all-electron basis sets. This indicates that the quality of the approximations used in the design of pseudopotentials could have major impact on a seemingly valence-exclusive effect like BSSE. We confirm the experimentally determined equilibrium internuclear distance (re), binding energy (De), harmonic vibrational frequency (ωe), and C(1)Π ← X(1)Σ transition energy for ArZn and also predict the spectroscopic properties for the low-lying excited states of linear Ar2-Zn (X(1)Σg, (3)Πg, (1)Πg), Ar-Ag(+) (X(1)Σ, (3)Σ, (3)Π, (3)Δ, (1)Σ, (1)Π, (1)Δ), and Ar2-Ag(+) (X(1)Σg, (3)Σg, (3)Πg, (3)Δg, (1)Σg, (1)Πg, (1)Δg) complexes, using the CCSD(T) and MR-CISD + Q methods, to aid in their experimental characterizations.