Ab initio spin-orbit calculations on the lowest states of the nickel dimer

Exploring structures of small anionic nickel-ethanol clusters with infrared spectroscopy

Small anionic nickel clusters with ethanol are investigated with a combination of mass-selective infrared photodissociation spectroscopy in a molecular beam and density functional theory simulations at the BLYP/6-311g(d,p) and TPSSh/def2-TZVPP level. In this context, the O-H stretching vibration of the ethanol is analyzed to obtain information about the structural motif, the geometry of the metal core, and the spin state of the clusters. For the [Ni2(EtOH)]- and [Ni3(EtOH)]- clusters, we assign quartet states of motifs with a hydrogen bond from the ethanol to the linear nickel core. The aggregation of a further ethanol molecule, yielding the [Ni3(EtOH)2]- cluster, results in the formation of a cooperative hydrogen bond network between the nickel core and the two ethanol molecules.

Low-Lying Electronic States of the Nickel Dimer

The generalized Van Vleck second order multireference perturbation theory (GVVPT2) method was used to investigate the low-lying electronic states of Ni₂. Because the nickel atom has an excitation energy of only 0.025 eV to its first excited state (the least in the first row of transition elements), Ni₂ has a particularly large number of low-lying states. Full potential energy curves (PECs) of more than a dozen low-lying electronic states of Ni₂, resulting from the atomic combinations ³F₄ + ³F₄ and ³D₃ + ³D₃, were computed. In agreement with previous theoretical studies, we found the lowest lying states of Ni₂ to correlate with the ³D₃ + ³D₃ dissociation limit, and the holes in the d-subshells were in the subspace of delta orbitals (i.e., the so-dubbed δδ-states). In particular, the ground state was determined as X ¹Γ_g and had spectroscopic constants: bond length (R_e) = 2.26 Å, harmonic frequency (ω_e) = 276.0 cm⁻¹, and binding energy (D_e) = 1.75 eV; whereas the 1 ¹Σ_g⁺ excited state (with spectroscopic constants: R_e = 2.26 Å, ω_e = 276.8 cm⁻¹, and D_e = 1.75) of the ³D₃ + ³D₃ dissociation channel lay at only 16.4 cm⁻¹ (0.002 eV) above the ground state at the equilibrium geometry. Inclusion of scalar relativistic effects through the spin-free exact two component (sf-X2C) method reduced the bond lengths of both of these two states to 2.20 Å, and increased their binding energies to 1.95 eV and harmonic frequencies to 296.0 cm⁻¹ for X ¹Γ_g and 297.0 cm⁻¹ for 1 ¹Σ_g⁺. These values are in good agreement with experimental values of R_e = 2.1545 ± 0.0004 Å, ω_e = 280 ± 20 cm⁻¹, and D₀ = 2.042 ± 0.002 eV for the ground state. All states considered within the ³F₄ + ³F₄ dissociation channel proved to be energetically high-lying and van der Waals-like in nature. In contrast to most previous theoretical studies of Ni₂, full PECs of all considered electronic states of the molecule were produced.

Puzzle of magnetic moments of Ni clusters revisited using quantum Monte Carlo method

The puzzle of the magnetic moments of small nickel clusters arises from the discrepancy between values predicted using density functional theory (DFT) and experimental measurements. Traditional DFT approaches underestimate the magnetic moments of nickel clusters. Two fundamental problems are associated with this puzzle, namely, calculating the exchange-correlation interaction accurately and determining the global minimum structures of the clusters. Theoretically, the two problems can be solved using quantum Monte Carlo (QMC) calculations and the ab initio random structure searching (AIRSS) method correspondingly. Therefore, we combined the fixed-moment AIRSS and QMC methods to investigate the magnetic properties of Ni_n (n = 5-9) clusters. The spin moments of the diffusion Monte Carlo (DMC) ground states are higher than those of the Perdew-Burke-Ernzerhof ground states and, in the case of Ni_8-9, two new ground-state structures have been discovered using the DMC calculations. The predicted results are closer to the experimental findings, unlike the results predicted in previous standard DFT studies.

Electronic ground state of Ni2. J Chem Phys 2016;145:194302. [PMID: 27875883 DOI: 10.1063/1.4967821]

The Φ9/24 ground state of the Ni₂⁺ diatomic molecular cation is determined experimentally from temperature and magnetic-field-dependent x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap, where an electronic and rotational temperature of 7.4±0.2 K was reached by buffer gas cooling of the molecular ion. The contribution of the spin dipole operator to the x-ray magnetic circular dichroism spin sum rule amounts to 7T_z=0.17±0.06μ_B per atom, approximately 11% of the spin magnetic moment. We find that, in general, homonuclear diatomic molecular cations of 3d transition metals seem to adopt maximum spin magnetic moments in their electronic ground states.

Chemical bonding in groups 10, 11, and 12 transition metal homodimers — An electron density study

The properties of metal–metal bonding for transition metal homonuclear diatomics from groups 10, 11 and 12 are studied within the framework of the quantum theory of atoms in molecules (QTAIM) at the coupled cluster CCSD and CCSD(T) levels of theory. A novel approximate method developed by Keith and Frisch is used to augment electron densities calculated with pseudopotentials with the missing relativistic core densities to obtain approximations to the total densities of the dimers. The calculated delocalization indices for group 10 dimers are: Ni2 (1.6), Pd2 (0.44, an outlier in the group), and Pt2 (1.8); for group 11 dimers: Cu2 and Ag2 (1.01), and Au2 (1.13), all covalent bonds; for group 12: Zn2 (0.06), Cd2 (0.08), and Hg2 (0.09), all consistent with weak van der Waals complexes. The picture of bonding obtained by examining the values of the electron density at the bond critical points is consistent with the one obtained on the basis of these delocalization indices. A curious linear (instead of exponential) dependence of the delocalization index on the electron density at the bond critical point is presented here as an observation and will be investigated in more depth in later work. Several correlations between bond properties and bond dissociation energies are also explored. It is found that, with the exception of the Ni2 dimer that exhibits considerable multi-reference character, there are correlations between the calculated bond dissociation energies of the studied diatomics and several bond critical point properties. These correlations are novel as they span a set of bonds between different pairs of elements, while traditionally these correlations were reported for bonds between the same pair or elements but with different substituents.