Function-based Ab Initio Calculations Research Articles

Magneto-Structural Correlation of Five-Coordinate Trigonal Bipyramidal High Spin Cobalt(II) Complexes.

Here, we combined magnetometry, multi-frequency electronic paramagnetic resonance, and wave function based ab initio calculations to investigate magnetic properties of two high spin Co(II) complexes Co(BDPRP) (BDPRP=2,6-bis((2-(S)-di(4-R)phenylhydroxylmethyl-1-pyrrolidi-nyl)methyl)pyridine, R=H for 8; R=tBu for 9). Complexes 8 and 9 featuring effective D3h symmetry were found to possess D=24.0 and 32.0 cm-1, respectively, in their S=3/2 ground states of . Ligand field analyses revealed that the low-lying d-d excited states make either positive or vanishing contributions to D. Hence, total positive D values were measured for 8 and 9, as well as related D3h high spin Co(II) complexes. In contrast, negative D values are usually observed for C3v congeners. In-depth analyses suggested that lowering symmetry from D3h to C3v induces orbital mixing between and and admixes excited state into the ground state. Both factors turn the total D value progressively negative with the increasing distance (δ) of the Co(II) center out of the equatorial plane. Therefore, δ determines the sign and magnitude of final D values of five-coordinate trigonal bipyramidal S=3/2 Co(II) complexes as measured for a series of such species with varying δ.

Unusual Electronic Structures of an Electron Transfer Series of [Cr(μ-η1 : η1 -N2 )Cr]0/1+/2.

In dinitrogen (N2 ) fixation chemistry, bimetallic end-on bridging N2 complexes M(μ-η1 : η1 -N2 )M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2 (μ-N2 )}0/1+/2+ (1-3) are incapable to cleave N2 , in contrast to their Mo congeners. Remarkably, cross this series the N-N bond length of the N2 ligand and the N-N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand-mediated two-center spin transitions, distinct from discrete dinuclear spin crossovers. In-depth analyses using wave function based ab initio calculations reveal that the Cr-N2 -Cr bonding in complexes 1-3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1-3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ-η1 : η1 -N2 )M complexes compared to their 4d and 5d analogs.

Unusual Electronic Structures of an Electron Transfer Series of [Cr(μ‐η1 : η1‐N2)Cr]0/1+/2+

AbstractIn dinitrogen (N2) fixation chemistry, bimetallic end‐on bridging N2 complexes M(μ‐η1 : η1‐N2)M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2(μ‐N2)}0/1+/2+ (1–3) are incapable to cleave N2, in contrast to their Mo congeners. Remarkably, cross this series the N−N bond length of the N2 ligand and the N−N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand‐mediated two‐center spin transitions, distinct from discrete dinuclear spin crossovers. In‐depth analyses using wave function based ab initio calculations reveal that the Cr‐N2‐Cr bonding in complexes 1–3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1–3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ‐η1 : η1‐N2)M complexes compared to their 4d and 5d analogs.

Is Bi2+ Responsible for the Red-Orange Emission of Bismuth-Doped SrB4O7?

Red-orange luminescence from bismuth-doped SrB4O7 has previously been reported and assigned to 6p-6p transitions of divalent Bi. To provide support for this assignment and for the stability of this unusual valence state of Bi, we report here results from low-temperature luminescence spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and wave function based ab initio calculations. Low-temperature luminescence spectra reveal zero-phonon lines (ZPLs) in excitation and emission spectra, allowing an accurate determination of the energies for the electronic transitions. The influence of the Bi concentration on the emission intensity is shown to be small, and only a small increase of the red-orange emission is observed upon raising the nominal Bi concentration from 0.02% (200 ppm) to 2%. This result indicates that only a very low concentration of Bi2+ can be incorporated in SrB4O7. This observation is supported by EPR experiments, which do not show a signal that can be assigned to Bi2+, and by XANES experiments showing that most Bi is in the trivalent state. An upper limit of the Bi2+ concentration is estimated to be 20 ppm. Ab initio calculations on the (BiO9)16– cluster embedded in SrB4O7 give energies for excited states that are close to the experimentally observed energies. Also, the luminescence lifetime for the red-orange emission (∼12 μs) is consistent with the lifetime for 6p-6p emission calculated for the Bi2+ emission (3.5 μs). Equivalent ab initio calculations for Bi2+ luminescence are very far from the experimental results, providing independent evidence and additional support for the interpretation of stable Bi2+ species being responsible for the red-orange luminescence. The calculations provide a new interpretation of the third excitation band, which is not due to a 2S1/2 state of the 6s27s configuration of Bi2+, as previously assumed, but is due to a state with important characters of 6s6p2–4P (63%) and doublets of the 6s6p2, 6s26d, and 6s26p configurations; its higher intensity is due to its character of parity-allowed 6s → 6p and 6p → 6d excitations.

Host effects on the optically active 4f and 5d levels of Ce3+ in garnets

When Ce(3+) is doped in a garnet, it substitutes for some cations. The effect of the substitution on the optically active 4f and 5d levels of Ce(3+) can be analyzed in terms of an undistorted substitution followed by a structural relaxation, but, whereas the contribution of the undistorted substitution can be predicted/calculated using the crystallographic structure of the pure garnet, which is at hand, that of the structural relaxation demands the detailed local structure around the Ce(3+) impurity, which is hard to know. Hence the importance of knowing the role of the undistorted substitution. What we study in this paper is the role of the unrelaxed host effects on the 4f and 5d levels of Ce(3+)-doped garnets (i.e. the interactions between Ce and the second and more distant neighbors). When they are added to the (previously studied) unrelaxed first-neighbor effects, they give the contributions of the undistorted substitutions. We performed spin-orbit coupling, relativistic, embedded cluster, wave function based ab initio calculations on the (CeO8)(13-) cluster under the effects of the embedding potentials of 21 selected garnets of Si, Al, and Ga, which have been obtained in this work, using experimental unrelaxed structures of the pure garnets. The calculations reveal that the unrelaxed host effect: (1) produces a red shift of the first 4f → 5d transition and (2) is an important contribution to this transition which plays a very important role in differentiating the values it has in different garnet families. The unrelaxed host effect is found to be responsible for the low value of the first 4f → 5d transition in Ce(3+)-doped Lu2CaMg2Si3O12. It is analyzed in terms of 5d and 4f centroid energy contributions and crystal field splitting contributions in the 21 doped garnets. The effect on the splittings of the 4f and 5d levels is also discussed. The undistorted host approximation is found to give reasonable comparison with experiments, so that it represents a relatively fast way to provide reliable ab initio information on the 4f and 5d levels of Ce(3+) in garnets.

Universal Theoretical Approach to Extract Anisotropic Spin Hamiltonians.

Monometallic Ni(II) and Co(II) complexes with large magnetic anisotropy are studied using correlated wave function based ab initio calculations. Based on the effective Hamiltonian theory, we propose a scheme to extract both the parameters of the zero-field splitting (ZFS) tensor and the magnetic anisotropy axes. Contrarily to the usual theoretical procedure of extraction, the method presented here determines the sign and the magnitude of the ZFS parameters in any circumstances. While the energy levels provide enough information to extract the ZFS parameters in Ni(II) complexes, additional information contained in the wave functions must be used to extract the ZFS parameters of Co(II) complexes. The effective Hamiltonian procedure also enables us to confirm the validity of the standard model Hamiltonian to produce the magnetic anisotropy of monometallic complexes. The calculated ZFS parameters are in good agreement with high-field, high-frequency electron paramagnetic resonance spectroscopy and frequency domain magnetic resonance spectroscopy data. A methodological analysis of the results shows that the ligand-to-metal charge transfer configurations must be introduced in the reference space to obtain quantitative agreement with the experimental estimates of the ZFS parameters.

A theoretical study of the electronic structure of the Co 2O 2 molecule

The electronic structure of the Co 2O 2 molecule in its planar rhombic form has been studied theoretically by means of DFT and wave function based ab initio calculations. In the DFT part, based on the broken symmetry approach and the TPSS functional, four low-lying spin states were investigated, one singlet, triplet, quintet, and septet. The singlet state was found to have the lowest energy, its calculated equilibrium geometry and vibration spectrum are in fair agreement with the respective experimental data. Wave function based ab initio calculations at increasing level of accuracy (CASSCF, CAS-CI, MC-CEPA, and SOC-CI) were used to determine the whole manifold of low-lying electronic states. Without inclusion of spin–orbit coupling, several low-lying spin ladders exist, each consisting of a singlet, a triplet, a quintet, and a septet state. The spin coupling is antiferromagnetic in the four lowest ladders, with similar exchange coupling constants J of about −60 cm −1 at the CASSCF and −120 cm −1 at the correlated level. The overall ground state has the symmetry 1A g. Spin-orbit coupling has a strong effect on the order and energetics of the lowest levels: after its inclusion a nearly degenerate pair of states is found to be lowest in energy, followed by a second pair 300 cm −1 higher in energy and another bunch of six states after another 200 cm −1.