-
[show abstract]
[hide abstract]
ABSTRACT: The low-energy spectroscopies of Mn(II) and Mn(III) porphyrin (P) complexes were investigated using complete active space and subsequent perturbative treatment (CASPT2) as well as DFT-based calculations. Starting from DFT optimizations of MnIIP and MnIIIPCl using crystallographic data, the CASPT2 results show that whatever the relative position of the Mn(II) ion with respect to the porphyrin cavity, the high-spin state S = 5/2 of the [MnP] unit lies much lower in energy than the intermediate S = 3/2 state. Not only are these results in agreement with experimental observations but they also differ from previous theoretical conclusions. In the Mn(III) complexes, σ and π charge redistributions compete to result in a S = 2 ground state. The performances of different functionals have been tested in the reproduction of the CASPT2 spin gaps. Our results confirm that the Mn(II) system is very challenging, as GGA functionals fail in the spin states ordering and in the reproduction of the gaps, unless a high percentage of exact HF exchange (55%), as in KMLYP, is incorporated. This inspection demonstrates the need for specific active space functional to investigate the low-energy spectroscopy of [MnP] units.
09/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The redox and spin versatilities of manganese-porphyrin complexes [Mn(II)P] are examined to construct a redox-switchable device. The electronic structure of [Mn(III)P](+) was analyzed by using wavefunction-based calculations (complete active spaces plus single excitations on top of the active spaces, that is, CAS+singles). A non-negligible σ-type electron-transfer configuration is present in the [Mn(III)P](+) S=2 ground state. By contrast, the [Mn(II)P(.)](+) valence tautomer is a purely π-type intramolecular charge transfer, thus reflecting an S=3 spin state as a result of the strong ferromagnetic interaction (J=30 meV) between the S=5/2 Mn(II) ion and the S=1/2 porphyrin radical cation P(.+). The change of the redox-sensitive site in the valence tautomer leads to a 'triangular scheme' that involves a critical step in which a simultaneous electron transfer and spin change are expected to induce bistability. From the wavefunction inspection, a meso-substituted porphyrin candidate was designed to support this scenario. The complete active-space second-order perturbation theory (CASPT2) adiabatic energy difference between the S=2 and the S=3 spin states was reduced down to 0.15 eV, thereby giving rise to a metastable S=3 state characterized by a 0.10 Å extension of the porphyrin cavity radius. These results not only confirm the rather versatile nature of these inorganic systems but also demonstrate that redox and spin changes are intermingled in this class of compounds and can be used for applied devices.
Chemistry 09/2011; 17(43):12045-50. · 5.93 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we present a DFT study of the proton reduction mechanism catalyzed by the complex [Ni(P₂(H)N₂(H))₂](2+), bioinspired from the hydrogenases. A detailed analysis of the reactive isomers is discussed together with the localizations of the transitions states and energy minima. The reactive catalytic species is a biprotonated Ni(0) complex that can show different conformations and that can be protonated on different sites. The energies of the different conformations and biprotonated species have been calculated and discussed. Energy barriers for two different reaction mechanisms have been identified in solvent and in gas phase. Frequency calculations have been performed to check the nature of the energy minima and for the calculations of entropic energetic terms and zero point energies. We show that only one conformation is mostly reactive. All the others species are nonreactive in their original form, and they have to pass through conformational barriers in order to transform in the reactive species.
The Journal of Physical Chemistry A 10/2010; 114(43):11861-7. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Stable complexes of pentavalent uranyl [UO(2)(salan-(t)Bu(2))(py)K](n) (3), [UO(2)(salan-(t)Bu(2))(py)K(18C6)] (4), and [UO(2)(salophen-(t)Bu(2))(thf)]K(thf)(2)}(n) (8) have been synthesized from the reaction of the complex {[UO(2)py(5)][KI(2)py(2)]}(n) (1) with the bulky amine-phenolate ligand potassium salt K(2)(salan-(t)Bu(2)) or the Schiff base ligand potassium salt K(2)(salophen-(t)Bu(2)) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [U(V)O(2)(salan-(t)Bu(2))(py)][Cp*(2)Co], 6, were also isolated from the reduction of [U(VI)O(2)(salan-(t)Bu(2))(py)], 5, with Cp*(2)Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO(2)py(5)]I.py (2). The measured redox potentials E(1/2) (U(VI)/U(V)) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give mu(eff) of 2.20 and 2.23 mu(B) at 300 K respectively, which is compared with a mu(eff) of 2.6(1) mu(B) (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g(x) = 1.98; g(y) = 1.25; g(z) = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the pi(u)*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO(2)(salan-(t)Bu(2))(py)K] and [UO(2)(salophen-(t)Bu(2))(py)K] complexes with respect to disproportionation and hydrolysis.
Journal of the American Chemical Society 12/2009; 132(2):495-508. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A description of the electronic structure of F(3)UCO, F(3)NdCO, F(4)UCO, and F(5)UCO has been obtained by Complete Active Space second-order perturbation theory CASPT2 calculations using a relativistic effective core potential. These multiconfigurational calculations have been compared to the DFT description combined with a quasi-relativistic ZORA scalar approach. Geometries have been optimized for both levels of calculations and frequencies computed in the DFT formalism. The bonding properties of U(III) have been compared to those of Nd(III) and of higher oxidation states of U(IV,V). Both methodologies are consistent and show a decrease of the covalent character of the U-CO bonding with a higher oxidation state, U(IV) or U(V), as well as its absence for for the isoelectronic Nd(III) species.
The Journal of Physical Chemistry A 11/2009; 113(52):14760-5. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present a full configuration interaction study of Be(N) (N=2,3,4,5) linear chains. A comparative study of the basis-set effect on the reproduction of the energy profile has been reported. In particular, the 3s1p, 4s2p, 4s2p1d, 5s3p2d, and 5s3p2d1f bases were selected. For the smallest chains (i.e., Be(2) and Be(3)), smaller basis sets give dissociative energy profiles, so large basis set is demanded for the reproduction of equilibrium minima in the structures. For Be(4) and Be(5) linear chains, the energy profiles show a minimum also by using the smallest basis sets, but the largest ones give a much stronger stabilization energy. For all the structures, two spin states have been studied: the singlet and the triplet. It is shown that the energy separation of the two states, in the equilibrium region, is small and decays exponentially with respect to the number of atoms in the chain. Finally an interpolative technique allowing for the estimation of the long-chain parameters from shorter ones is presented.
The Journal of chemical physics 02/2009; 130(2):024301. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The precursor of the metal-insulator transition is studied at ab initio level in linear chains of equally spaced lithium atoms. In particular, full configuration interaction calculations (up to 1 x 10(9) determinants) are performed, in order to take into account the different nature of the wave function at different internuclear distances. Several indicators of the Metal-Insulator transition (minimum of the energy gap, maximum of the localization tensor or of the polarizability) are considered and discussed. It is shown that the different indicators give concordant results, showing a rapid change in the nature of the wave function at an internuclear distance of about 7 bohrs.
The Journal of Chemical Physics 02/2008; 128(2):024701. · 3.33 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present a comparative Density Functional Theory (DFT) study based on two different implementations of relativistic effects within the Kohn-Sham (KS) approach, to describe the metal-ligand interaction in I(3)M-L complexes (L = NH(3), NCCH(3), CO and M = La, Nd, U). In the first model, the scalar corrections were included by a quasi-relativistic approach (QR) via the so-called ZORA or Pauli Hamiltonians, while in the second, these effects are taken into account in a quasi-Relativistic Effective Core Potential (RECP). These relativistic approaches were used in conjunction with various gradient corrected (GGA) or hybrid (SCH) functionals. The structural parameters obtained from geometry optimizations have been compared to experimental structural trends, and rationalized by a KS orbital analysis. Both approaches provide similar results for mainly ionic metal-ligand bonds (e.g., for the sigma-donor ligand L = NH(3)). For the pi-acceptor ligands (NCCH(3), CO), the QR approach is in agreement with experimental trends and consistent with the presence of a backbonding interaction between U(III) and the neutral ligand, which does not exist in the lanthanide homologues. The GGA/RECP methods also reproduce this phenomenon, while the SCH/RECP scheme fails to describe this interaction. The role of the RECP, of its size, and of additional polarization functions has also been examined. Finally, the failure of the SCH/RECP approach was interpreted as a consequence of a bad estimation of frontier orbital energy levels in the uranium and ligand species.
Journal of Computational Chemistry 06/2003; 24(7):850-8. · 4.58 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A comparative density functional study on metal–ligand (M–L) interaction has been performed on X3–M–L (X = F, I; M = U, La, Nd: L = NH3, NCCH3, CO) and X3–M–(CO)2 species including scalar relativistic effects by means of the zero-order regular approximation (ZORA) hamiltonian. The role of the halogen atoms in modeling the M–L interactions is discussed for π-acceptor ligands CO and NCCH3 and for the σ-donor NH3. The fluoride counterions, compared with previous calculations on the iodide complexes, stabilize more tedraedral structure with longer M–L distances. Finally, the addition of a second ligand to the previous species has permitted a more complete discussion on the backbonding effects on the uranium complexes. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003
International Journal of Quantum Chemistry 11/2002; 91(3):321 - 327. · 1.36 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A comparative density functional study of a new functional (PBE) and of the corresponding hybrid version (PBE0) has been performed on LnX3 species (X=F, Cl, Br, I and Ln=La, Gd, Lu), including a relativistic effective core potential. Geometry optimizations and harmonic frequency calculations were carried out. Both PBE and PBE0 give better results than previous B3LYP computations, and the hybrid PBE0 was found to perform better than the PBE functional for the reproduction of geometrical features, while both give similar good agreement for frequencies. Finally, the addition of polarization functions in the halogen basis set greatly improves both geometric parameters and frequencies.
Chemical Physics Letters.
