Ab Initio Calculations of the Magnetic Coupling between a Ni(II) Ion and Two Nitroxide Radicals in cis and trans Positions.

The magnetic couplings between a Ni(II) center in an octahedral environment and two nitroxides in its coordination sphere are calculated by ab initio methods. The system is well described by a Heisenberg model. Fully variational calculations (DDCI2) give results similar to those of combined variational/perturbative calculations (CASPT2), and some DFT results are obtained. The effect of some structural parameters is studied: the cis and trans positions and the angles and the distances of the nitroxides with respect to the nickel center. We show that the magnetic couplings are almost the same in the cis and in the trans complexes; thus, the only relevant geometric parameters for ferromagnetic or antiferromagnetic couplings are the orientations and the distances of the nitroxide ligands with respect to the Ni(II) center. Direct exchange between the magnetic centers is calculated: it depends neither on the angles nor on the cis or trans conformations, showing that the superexchange contribution is the only angle-dependent one.

Ab initio dynamic study of the reaction of Cl2LaR (R=H, CH3) with H2.

In this paper, a comparison between "static" and "dynamic" determination of the thermodynamic (DeltarF degrees) and kinetic data (DeltarF#) for the reaction of Cl2LaR (R=H, CH3) and H2 is given. A difference is obtained in the case of the reaction between Cl2LaH and H2 and can be attributed to a failure of the "static" approach based on the harmonic approximation. The influence of the zero point energy correction is also analyzed but does not explain the 30% difference between the two calculated activation energies. The influence of the flatness of the potential energy surface around the transition state is proved as no such an effect is observed for the reaction of Cl2LaCH3 and H2.

Berry pseudorotation mechanism for the interpretation of the 19F NMR spectrum in PF5 by ab initio molecular dynamics simulations.

For the first time, theoretical evidence that confirms the importance of the Berry pseudorotation process in the interpretation of the 19F NMR spectrum of phosphorus pentafluoride (PF5) is presented. Ab initio molecular dynamics simulations have been performed to generate a large number of configurations used for NMR parameter computations at the density functional theory level. Two different temperatures were set to highlight the effect of pseudorotation process on the NMR spectrum. Average 19F chemical shifts and spin-spin coupling constants calculated for the five fluorine atoms converge towards the NMR equivalence of the five atoms when the Berry pseudorotation mechanism is accounted for.

1,4- vs 1,3-prototropic mechanism for intramolecular double proton transfer reaction in monothiooxalic acid. Theoretical investigation of potential energy surface.

Proton transfer is a very common and important chemical step in many systems. Despite its apparent simplicity, a correct description of this chemical process is difficult from a theoretical point of view. It requires a correct and simultaneous description of a bond breaking and a bond formation. The situation is even much more complicated when two protons are implied. This is the case for monothiooxalic acid, for which two different types (1,3- and 1,4-prototropy) of proton transfers can be invoked. A further problem is the type of the reaction (concerted or not). This paper reports a complete investigation of the potential energy surfaces: characterization of equilibrium points and transitions states. The main conclusion is: the 1,4-prototropic mechanism, mainly considered as a one step concerted exchange of protons, is the most favored from an energetic point of view.

Towards accurate all-electron quantum Monte Carlo calculations of transition-metal systems: spectroscopy of the copper atom.

In this work we present all-electron fixed-node diffusion Monte Carlo (FN-DMC) calculations of the low-lying electronic states of the copper atom and its cation. The states considered are those which are the most relevant for the organometallic chemistry of copper-containing systems, namely, the (2)S, (2)D, and (2)P electronic states of Cu and the (1)S ground state of Cu(+). We systematically compare our FN-DMC results to CCSD(T) calculations using very large atomic-natural-orbital-type all-electron basis sets. The FN-DMC results presented in this work provide, to the best of our knowledge, the most accurate nonrelativistic all-electron correlation energies for the lowest-lying states of copper and its cation. To compare our results to experimental data we include the relativistic contributions for all states through numerical Dirac-Fock calculations, which for copper (Z=29) provide almost the entire relativistic effects. It is found that the fixed-node errors using Hartree-Fock nodes for the lowest transition energies of copper and the first ionization potential of the atom cancel out within statistical fluctuations. The overall accuracy achieved with quantum Monte Carlo for the nonrelativistic correlation energy (statistical fluctuations of about 1600 cm(-1) and near cancelation of fixed-node errors) is good enough to reproduce the experimental spectrum when relativistic effects are included. These results illustrate that, despite the presence of the large statistical fluctuations associated with core electrons, accurate all-electron FN-DMC calculations for transition metals are nowadays feasible using extensive but accessible computer resources.

Calculation of the ground and excited states of a mixed valence compound [Fe2(OH)3(NH3)6]2+: a class II or class III compound?

The effective group potentials (EGP) approach has been successfully used for the computation of the ground and excited states energies of the mixed valence compound [Fe2(OH)3(NH3)6]2+. It is the first time that for a system as big as the complex presented above the ground and excited states are computed with their own orbitals and studied in such a detailed way. First of all, the NH3 EGP was validated by comparing calculations where NH3 was treated explicitly at different levels of calculations. Once the validation was obtained, the complete spectrum of the compound under interest was calculated and compared with results obtained in a previous work by Barone et al. and the spin Hamiltonian of widespread use. Some deviations from these predictive approaches were observed. This allowed us to emphasize the importance of the dynamic correlation which is not included explicitly in the spin Hamiltonian. Then, the influence of vibration has been studied by computing the potential energy curves obtained when moving the (OH)3 plane. This study shows that our calculations lead to a delocalized compound (class III) as expected according to former experimental data.

Using effective group potential methodology for predicting organometallic complex properties.

Using the Effective Group Potentials (EGP) method, optimal geometries, harmonic vibrational frequencies, and relative energies of different sets of metal complexes are calculated. All of the systems under consideration contain the cyclopentadienyl (Cp) ligand. They are as follows: (i). Group V metal Atom complexes showing one Cp ligand, (ii). a tetrameric Al-Cp compound with four Cp ligands, (iii). homometallic lutetium hydrides containing six cyclopentadienyl rings. Various electron correlation treatments have been carried out. All of the results compare very satisfactorily with available experimental data and with all-electron ab initio calculations performed for this work or published in the literature. Furthermore, the performance of the EGP method was tested on a rather large complex for which experimental evidence exists, but no all-electron calculation has been reported so far.

Quantum chemistry-based interpretations on the lowest triplet state of luminescent lanthanides complexes. Part 1. Relation between the triplet state energy of hydroxamate complexes and their luminescence properties.

In this paper, we evaluate the potential use of theoretical calculations to obtain an energy scale of the lowest ligand-centred triplet excited state in luminescent terbium(III) complexes. In these complexes, non-radiative deactivation of the terbium emitting state via a back-energy transfer process (T1<--Tb(5D4)) is a common quenching process. Consequently the prediction of the energy gap between these two excited states should be useful for programming highly luminescent Tb(III) systems. We report on a strategy based upon experimental and theoretical investigations of the excited state properties of a series of four simple aromatic hydroxamate ligands coordinated to Tb(III) and Gd(III) ions. By using previously reported crystallographic data, the structural and energies properties of these systems were investigated in the ground and first excited triplet states at the density functional theory (DFT) level of calculations. Our theoretical results are consistent with a triplet excited state T1 which is localised on one ligand only and whose the energy level is independent of the lanthanide ion nature (Tb(III), Gd(III)). A good agreement between the calculated adiabatic transition energies and experimental data derived from emission spectra is obtained when a corrective term is considered. These satisfactory results are an indication that this type of modelling can lead to discriminate in terms of the position of the lowest ligand triplet energy level the best antenna among a family of chromophoric compounds. In addition this theoretical approach has provided indications that the difference between the adiabatic transition energies of all the investigated complexes can be mainly explained by metal-ligand electrostatic interactions. The influence of the number of antennae on the quantum yield and the luminescence lifetime is discussed.