-
[show abstract]
[hide abstract]
ABSTRACT: We have studied the electronic structure (ground and excited states) and g matrix of a model compound for oxomolybdenum enzymes featuring the MoO–dithiolate moiety in C
s
symmetry, by means of multiconfigurational second-order perturbation theory (CASPT2) for a range of fold angles (5–29°),
i.e. the angle between the S–Mo–S and S–C–C–S planes of the dithiolate ligand. We found no evidence of a suggested 3-center
pseudo-σ bonding interaction between the singly occupied orbital of the ground state and the symmetric in-plane dithiolate
orbital (Inscore et al. in Inorg Chem 38:1401–1410, 1999). This is complemented with our alternative assignment of band 4 in the electronic spectrum as the transition out of the a″ instead of the a′ dithiolate in-plane orbital into the singly
occupied ground-state orbital, believed to probe the dominant hole superexchange pathway (Inscore et al. in Inorg Chem 38:1401–1410,
1999; Burgmayer et al. in J Inorg Biochem 101:1601–1616, 2007). Principal g values of 1.9652, 1.9090, 2.0003 were obtained at a fold angle of 21°. The latter value is so close to the free electron
g
e
factor is due to an important positive contribution from the LMCT transition corresponding to band 4, counteracting the negative contributions from the ligand field transitions.
Theoretical Chemistry Accounts 04/2012; 124(3):251-259. · 2.16 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A series of model transition-metal complexes, CrF(6), ferrocene, Cr(CO)(6), ferrous porphin, cobalt corrole, and FeO/FeO(-), have been studied using second-order perturbation theory based on a restricted active space self-consistent field reference wave function (RASPT2). Several important properties (structures, relative energies of different structural minima, binding energies, spin state energetics, and electronic excitation energies) were investigated. A systematic investigation was performed on the effect of: (a) the size and composition of the global RAS space, (b) different (RAS1/RAS2/RAS3) subpartitions of the global RAS space, and (c) different excitation levels (out of RAS1/into RAS3) within the RAS space. Calculations with active spaces, including up to 35 orbitals, are presented. The results obtained with smaller active spaces (up to 16 orbitals) were compared to previous and current results obtained with a complete active space self-consistent field reference wave function (CASPT2). Higly accurate RASPT2 results were obtained for the heterolytic binding energy of ferrocene and for the electronic spectrum of Cr(CO)(6), with errors within chemical accuracy. For ferrous porphyrin the intermediate spin (3)A(2g) ground state is (for the first time with a wave function-based method) correctly predicted, while its high magnetic moment (4.4 mu(B)) is attributed to spin-orbit coupling with very close-lying (5)A(1g) and (3)E(g) states. The toughest case met in this work is cobalt corrole, for which we studied the relative energy of several low-lying Co (II)-corrole pi radical states with respect to the Co(III) ground state. Very large RAS spaces (25-33 orbitals) are required for this system, making compromises on the size of RAS2 and/or the excitation level unavoidable, thus increasing the uncertainty of the RASPT2 results by 0.1-0.2 eV. Still, also for this system, the RASPT2 method is shown to provide distinct improvements over CASPT2, by overcoming the strict limitations in the size of the active space inherent to the latter method.
Journal of Chemical Theory and Computation 12/2011; 7:3961-3977. · 5.22 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, the results are presented from a comparative study of the electronic and geometric structure of copper corroles by means of either density functional theory (DFT, using both pure and hybrid functionals) and multiconfigurational ab initio methods, starting from either a complete active space (CASSCF) or restricted active space (RASSCF) reference wave function and including dynamic correlation by means of second-order perturbation theory (CASPT2/RASPT2). DFT geometry optimizations were performed for the lowest singlet and triplet states of copper corrole, both unsubstituted and meso-substituted with three phenyl groups. The effect of saddling on the electronic structure was investigated by comparing the results obtained for planar (C(2v)) and saddled (C(2)) structures. With DFT, the origin of the saddling distortion is found to be dependent on the applied functional: covalent Cu 3d-corrole π interactions with pure functionals (BP86, OLYP), antiferromagnetic exchange coupling between an electron in the corrolate (C(2)) b type π orbital, and an unpaired Cu(II) 3d electron with hybrid functionals (B3LYP, PBE0). The CASPT2 results essentially confirm the suggestion from the hybrid functionals that copper corroles are noninnocent, although the contribution of diradical character to the copper-corrole bond is found to be limited to 50% or less. The lowest triplet state is calculated at 0-10 kcal/mol and conform with the experimental observation (variable temperature NMR) that this state should be thermally accessible.
Inorganic Chemistry 10/2010; 49(22):10316-29. · 4.60 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: EPR spectroscopy has proven to be an indispensable tool in elucidating the structure of metal sites in proteins. In recent years, experimental EPR data have been complemented by theoretical calculations, which have become a standard tool of many quantum chemical packages. However, there have only been a few attempts to calculate EPR g tensors for exchange-coupled systems with more than two spins. In this work, we present a quantum chemical study of structural, electronic, and magnetic properties of intermediates in the reaction cycle of multicopper oxidases and of their inorganic models. All these systems contain three copper(II) ions bridged by hydroxide or O(2-) anions and their ground states are antiferromagnetically coupled doublets. We demonstrate that only multireference methods, such as CASSCF/CASPT2 or MRCI can yield qualitatively correct results (compared to the experimental values) and consider the accuracy of the calculated EPR g tensors as the current benchmark of quantum chemical methods. By decomposing the calculated g tensors into terms arising from interactions of the ground state with the various excited states, the origin of the zero-field splitting is explained. The results of the study demonstrate that a truly quantitative prediction of the g tensors of exchange-coupled systems is a great challenge to contemporary theory. The predictions strongly depend on small energy differences that are difficult to predict with sufficient accuracy by any quantum chemical method that is applicable to systems of the size of our target systems.
The Journal of Physical Chemistry B 06/2010; 114(22):7692-702. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The accuracy of the relative spin-state energetics of three small FeII or FeIII heme models from multiconfigurational perturbation theory (CASPT2) and density functional theory with selected functionals (including the recently developed M06 and M06-L functionals) was assessed by comparing with recently available coupled cluster results. While the CASPT2 calculations of spin-state energetics were found to be very accurate for the studied FeIII complexes (including FeP(SH), a model of the active site of cytochrome P450 in its resting state), there is a strong indication of a systematic error (around 5 kcal/mol) in favor of the high-spin state for the studied FeII complexes (including FeP(Im), a model of the active site of myoglobin). A larger overstabilization of the high-spin states was observed for the M06 and M06-L functionals, up to 22 and 11 kcal/mol, respectively. None of the tested density functionals consistently provides a better accuracy than CASPT2 for all model complexes.
01/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: The structural, electronic and magnetic properties of two different models of the heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides Cu(hfac)2L(R) have been studied by means of multiconfigurational perturbation theory based on a CASSCF (complete active space self-consistent field) wave function, i.e. the CASPT2 method. Our calculations reveal the presence of two minima in the electronic energy curve along the Cu-O(L) bond, separated by only 6 kcal/mol, and corresponding to the X-ray structures of the CuO6 centers in Cu(hfac)2L(Pr) at 115 and 293 K, respectively. The two energetic minima are characterized by a different electronic structure, thus giving rise to a different three-spin exchange coupling and explaining the thermally induced spin transitions in this family of compounds. The concomitant variations in the magnetic properties, i.e. g factors and magnetic moments mu(eff)(T) were calculated and compared with the experimental data of Cu(hfac)2L(Pr). Even if the correspondence is only qualitative, our calculations provide a convincing explanation of the observed magnetic peculiarities. In particular, at low temperatures, the predicted ground-state is 2A(u), well separated from the 2A(g), 4A(u) states and therefore exclusively populated. Its calculated g factors, g(parallel) = 1.848, g(perpendicular) = 1.965, 1.974, qualitatively correspond to the observed g < 2 signals in the low-temperature EPR spectra. The previously assumed formal spin assignment >N-O*-Cu-*O-N < for these linear spin triads is challenged by our calculations, pointing instead to a more important role of the end-standing NO in the exchange interactions with Cu(II).
The Journal of Physical Chemistry A 06/2009; 113(21):6149-57. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Calculations of the g tensor of three copper(II) complexes [Cu(NH3)4]2+, [CuCl4]2-, and plastocyanin are presented. Two different sum-over-states-based approaches are considered, making use of the multistate CASPT2 method for excitation energies and PMCAS (perturbation modified CAS) wave functions for the computation of the angular momentum and spin-orbit coupling matrix elements. Test calculations on [Cu(NH3)4]2+ and [CuCl4]2- point to the need of including in the MS-CASPT2 treatment the specific charge-transfer state with an electron excited out of the bonding counterpart of the ground-state SOMO. The computed g shifts for these two molecules present a considerable improvement with respect to the results obtained from our previous g tensor calculations based instead on CASSCF/CASPT2. This is shown to be related to an improved description of the covalency of the Cu-L bonds. For the calculations on plastocyanin, different models are used, taken from a recent (QM/MM) DFT study by Sinnecker and Neese. The effect of the surrounding protein is taken into account by surrounding the central cluster either with a dielectric continuum (epsilon = 4) or with a set of point charges. The second approach is found to be indispensable for an accurate description of environmental effects. With this approach, the calculated g values compare to within 30 ppt with the experimental data of plastocyanin.
The Journal of Physical Chemistry A 06/2008; 112(17):4011-9. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: High-level ab initio calculations using multiconfigurational perturbation theory [complete active space with second-order perturbation theory (CASPT2)] were performed on the transition energy between the lowest high-spin (corresponding to (5T2g) in Oh) and low-spin (corresponding to 1A1g in Oh) states in the series of six-coordinated Fe(II) molecules [Fe(L)(NHS4)], where NHS4 is 2,2'-bis(2-mercaptophenylthio)diethylamine dianion and L=NH3, N2H4, PMe3, CO, and NO+. The results are compared to (previous and presently obtained) results from density functional theory (DFT) calculations with four functionals, which were already shown previously by Casida and co-workers [Fouqueau et al., J. Chem. Phys. 120, 9473 (2004); Ganzenmuller et al., ibid. 122, 234321 (2005); Fouqueau et al., ibid. 122, 044110 (2005); Lawson Daku et al., ChemPhysChem 6, 1393 (2005)] to perform well for the spin-pairing problem in these and other Fe(II) complexes, i.e., OLYP, PBE0, B3LYP, and B3LYP*. Very extended basis sets were used both for the DFT and CASPT2 calculations and were shown to be necessary to obtain quantitative results with both types of method. This work presents a sequel to a previous DFT/CASPT2 study of the same property in the complexes [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+ [Pierloot et al., J. Chem. Phys. 125, 124303 (2006)]. The latter work was extended with new results obtained with larger basis sets and including the OLYP functional. For all considered complexes, the CASPT2 method predicts the correct ground state spin multiplicity. Since experimental data for the actual quintet-singlet (free) energy differences are not available, the performance of the different DFT functionals was judged based on the comparison between the DFT and CASPT2 results. From this, it was concluded that the generalized gradient OLYP functional performs remarkably well for the present series of ferrous compounds, whereas the success of the three hybrid functionals varies from case to case.
The Journal of Chemical Physics 02/2008; 128(3):034104. · 3.33 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The computation of the electronic g tensor by two multireference methods is presented and applied to a selection of molecules including CN, BO, AlO, GaO, InO, ZnH, ZnF, O(2), H(2)O(+), O(3) (-), and H(2)CO(+) (group A) as well as TiF(3), CuCl(4) (2-), Cu(NH(3))(4) (2+), and a series of d(1)-MOX(4) (n-) compounds, with M=V, Cr, Mo, Tc, W, Re and X=F, Cl, Br (group B). Two approaches are considered, namely, one in which spin-orbit coupling and the Zeeman effect are included using second-order perturbation theory and another one in which the Zeeman effect is added through first-order degenerate perturbation theory within the ground-state Kramers doublet. The two methods have been implemented into the MOLCAS quantum chemistry software package. The results obtained for the molecules in group A are in good agreement with experiment and with previously reported calculated g values. The results for the molecules in group B vary. While the g values for the d(1) systems are superior to previous theoretical results, those obtained for the d(9) systems are too large compared to the experimental values.
ChemPhysChem 09/2007; 8(12):1803-15. · 3.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: High-level ab initio calculations using the CASPT2 method and extensive basis sets were performed on the energy differences of the high-[(5)T(2g):t(2g) (4)e(g) (2)] and low-[(1)A(1g):t(2g) (6)] spin states of the pseudo-octahedral Fe(II) complexes [Fe(H(2)O)(6)](2+), [Fe(NH(3))(6)](2+), and [Fe(bpy)(3)](2+). The results are compared to the results obtained from density functional theory calculations with the generalized gradient approximation functional BP86 and two hybrid functionals B3LYP and PBE0, and serve as a calibration for the latter methods. We find that large basis set CASPT2 calculations may provide results for the high-spin/low-spin splitting DeltaE(HL) that are accurate to within 1000 cm(-1), provided they are based on an adequately large CAS[10,12] reference wave function. The latter condition was found to be much more stringent for [Fe(bpy)(3)](2+) than for the other two complexes. Our "best" results for DeltaE(HL) (including a zero-point energy correction) are -17 690 cm(-1) for [Fe(H(2)O)(6)](2+), -8389 cm(-1) for [Fe(NH(3))(6)](2+), and 3820 cm(-1) for [Fe(bpy)(3)](2+).
The Journal of Chemical Physics 10/2006; 125(12):124303. · 3.33 Impact Factor
-
Steven Vancoillie
[show abstract]
[hide abstract]
ABSTRACT: Doctor in de Wetenschappen: Scheikunde Afdeling Kwantumchemie en Fysicochemie Departement Chemie Faculteit Wetenschappen Doctoral thesis Doctoraatsthesis
