| 1. |
- Cimpoesu, Fanica, et al.
(författare)
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Disorder, exchange and magnetic anisotropy in the room-temperature molecular magnet V[TCNE](x) - A theoretical study
- 2014
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Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 91, s. 320-328
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Tidskriftsartikel (refereegranskat)abstract
- We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE](x) (TCNE = tetracyanoethylene, x similar to 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra- or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V[TCNE](x). The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D-c = -0.03 K) of the bulk as well as a sizeable random anisotropy (D-r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.
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| 2. |
- Frecus, Bogdan, et al.
(författare)
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Ab initio study of exchange coupling for the consistent understanding of the magnetic ordering at room temperature in V[TCNE] (x)
- 2014
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Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 133:5, s. 1470-
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Tidskriftsartikel (refereegranskat)abstract
- We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE](2) model system, describing the amorphous room temperature molecular magnet V[TCNE] (x) (TCNE = tetracyanoethylene, x similar to 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE](-) anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.
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| 3. |
- Frecus, Bogdan, et al.
(författare)
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EPR spin Hamiltonian parameters of encapsulated spin-labels : impact of the hydrogen bonding topology
- 2013
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 15:7, s. 2427-2434
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Tidskriftsartikel (refereegranskat)abstract
- Encapsulation of spin-labels into "host'' compounds, like cucurbit[n]urils or cyclodextrins, in solutions has profound effects on the EPR spin Hamiltonian parameters of the spin-labels. In this work we study the microscopic origin of the EPR spin Hamiltonian parameters of spin-labels enclosed in hydrophobic cavities. We focus on the dependence of the EPR properties of encapsulated spin-labels on the hydrogen bonding topologies that occur upon encapsulation, and quantize various contributions to these parameters according to specific hydrogen bonding patterns. The obtained results provide refined insight into the role of the hydrogen bonding induced encapsulation shifts of EPR spin Hamiltonian parameters in solvated "spin-label@host compound'' complexes.
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| 4. |
- Frecus, Bogdan
(författare)
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Theoretical studies of EPR parameters of spin-labels incomplex environments
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis encloses quantum chemical calculations performed in the framework of density functional response theory for evaluating electron paramagnetic resonance (EPR) spin Hamiltonian parameters of various spin-labels in different environments. These parameters are the well known electronic g-tensor and the nitrogen hyperfine coupling constants, which are extensively explored in this work for various systems. A special attention was devoted to the relationships that form between the structural and spectroscopic properties that can be accounted for as an environmental inuence. Such environmental effects were addressed either within a fully quantum mechanical formalism, involving simplified model structures that still capture the physical properties of the extended system, or by employing a quantum mechanics/molecular mechanics (QM/MM) approach. The latter implies that the nitroxide spin label is treated quantum mechanically, while the environment is treated in a classical discrete manner, with appropriate force fields employed for its description. The state-of- the art techniques employed in this work allow for an optimum accounting of the environmental effects that play an important role for the behaviour of EPR properties of nitroxides spin labels. One achievement presented in this thesis includes the first theoretical con_rmation of an empirical assumption that is usually made for inter-molecular distance measurement experiments in deoxyribonucleic acid (DNA), involving pulsed electron-electron double resonance (PELDOR) and site-directed spin labeling (SDSL) techniques. This refers to the fact that the EPR parameters of the spin-labels are not affected by their interaction with the nucleobases from which DNA is constituted. Another important result presented deals with the inuence of a supramolecular complex on the EPR properties of an encapsulated nitroxide spin-label. The enclusion complex affects the hydrogen bonding topology that forms around the R2NO moiety of the nitroxide. This, on the other hand has a major impact on its structure which further on governs the magnitude of the spectroscopic properties. The projects and results presented in this thesis offer an example of successful usage of modern quantum chemistry techniques for the investigation of EPR parameters of spin-labels in complex systems.
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| 5. |
- Frecus, Bogdan, et al.
(författare)
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π –stacking effects on the EPR parameters of a prototypical DNA spin label
- 2013
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : RSC Publishing. - 1463-9076 .- 1463-9084. ; 15:25, s. 10466-10471
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Tidskriftsartikel (refereegranskat)abstract
- The character and value of spin labels for probing environments like double-stranded DNA depends on the degree of changeof the spin Hamiltonian parameters of the spin label induced by the environment. Herein we provide a systematic theoreticalinvestigation of this issue, based on a density functional theory method applied to a spin labeled DNA model system, focusingon the dependence of the EPR properties of the spin label on the π stacking and hydrogen bonding that occur upon incorporatingthe spin label into selected base pair inside DNA. It is found that the EPR spin Hamiltonian parameters of the spin label is onlynegligibly affected by its incorporation into DNA, when compared to the its free form. This result gives theoretical ground forthe common empirical assumption regarding the behaviour of spin Hamiltonian parameters made in EPR based measurementsof distance between spin labels incorporated into DNA.
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| 6. |
- Oprea, Corneliu I., et al.
(författare)
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DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells
- 2011
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Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 109:21, s. 2511-2523
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Tidskriftsartikel (refereegranskat)abstract
- The paper reports Time Dependent Density Functional Theory (TD DFT) calculations providing the structure, electronic properties and spectra of [Ru(II)(bpy)(3-n)(dcbpy)(n)](2+) and [Rh(III)(bpy)(3-n)(dcbpy)(n)](3+) complexes, where bpy = 2,2'-bipyridyl, dcbpy = 4,4'-dicarboxy-2,2'-bipyridyl, and n = 0, 1, 2, 3, studied as possible pigments for dye-sensitized solar cells. The role of the metallic ion and of the COOH groups on the optical properties of these complexes are compared and contrasted and their relevance as dyes for hybrid organic-inorganic photovoltaic cells is discussed. It was found that the optical spectra are strongly influenced by the metallic ion, with visible absorption bands for the Ru(II) complexes and only ultraviolet bands for the Rh(III) complexes. Upon excitation, the extra positive charge of the Rh(3+) centre tends to draw electrons towards the metal ion, facilitating some charge transfer from the ligand to the metal, whereas in the case of the Ru(2+) ion the electron transfer is clearly from the metal to the ligand. The carboxyl groups play an important role in strengthening the absorption bands in solution in the visible region. Of the complexes studied, the most suited as pigments for dye-sensitized solar cells are the [Ru(II)(bpy)(3-n)(dcbpy)(n)](2+) complexes with n = 1 and 2. This is based on the following arguments: (i) their intense absorption band in the visible region, (ii) the presence of the anchoring groups allowing the bonding to the TiO(2) substrate and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte.
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| 7. |
- Oprea, Corneliu I., et al.
(författare)
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DFT study of structure-properties correlations in [MnTPP][TCNE] quasi-one-dimensional molecular magnets
- 2011
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Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 129:6, s. 847-857
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Tidskriftsartikel (refereegranskat)abstract
- We report the first band structure calculations of the quasi-one-dimensional [MnTPP][TCNE] compounds (TPP = meso-tetraphenylporphyrinato, TCNE = tetracyanoethylene), based on Density Functional Theory (DFT) methods, in order to interpret the magnetic ordering in these prototypic systems. We compare and contrast the results of broken-symmetry DFT calculations for extended systems, with periodic boundary conditions, and for finite systems, magnetic dimers modeling the actual molecular magnets. By varying systematically the main angles, we are able to determine the geometry dependence of the exchange interaction. Structure-properties correlations in these charge-transfer salts reveal the determinant role of the Mn-(N C) TCNE bond angle on the strength of the ferrimagnetic coupling between the S(1) = 2 spin located on the Mn(III)-porphyrin donor and the S(2) = 1/2 spin positioned on the cyanocarbon acceptor. When the Mn-(N C) TCNE angle is decreased, the intrachain magnetic coupling strengthens, correlated with the increase in the d(z)(2) - pi* orbital overlap. The exchange coupling constants resulting from DFT calculations of extended systems, with periodic boundary conditions, were found to be consistent with those obtained for the dimers, but systematically smaller. The exchange constants vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the experimental mean-field critical temperatures. The coupling constant varies significantly with the type of broken-symmetry approach, depending on the overlap between magnetic orbitals, but weakly on the basis set once polarization effects are included. The electronic structure calculations for the extended systems provide a density of states consistent with the energy spectrum of the corresponding dimer, allowing for an intuitive explanation of the intrachain ferrimagnetic ordering.
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| 8. |
- Rinkevicius, Zilvinas, et al.
(författare)
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Density Functional Restricted-Unrestricted/Molecular Mechanics Theory for Hyperfine Coupling Constants of Molecules in Solution
- 2011
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 7:10, s. 3261-3271
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Tidskriftsartikel (refereegranskat)abstract
- A density functional restricted unrestricted approach, capable of evaluating hyperfine coupling constants with the inclusion of spin polarization effects in a spin-restricted Kohn-Sham method, has been extended to incorporate environmental effects. This is accomplished by means of a hybrid quantum mechanics/molecular mechanics formalism which allows for a granular representation of the polarization and electrostatic interactions with the classically described medium. By this technique, it is possible to trace the physical origin of hyperfine coupling constants in terms of spin polarization and spin density contributions and disentangle the dependence of these contributions on molecular geometry and solvent environment, something that increases the prospects for optimal design of spin labels for particular applications. A demonstration is given for the nitrogen isotropic hyperfine coupling constant in di-tert-butyl nitroxide solvated in water. The results indicate that the direct spin density contribution is about 5 times smaller than the spin polarization contribution to the nitrogen isotropic hyperfine coupling constant and that the latter contribution is solely responsible for the solvent shift of the constant. The developed approach is found capable of achieving satisfactory accuracy in prediction of the hyperfine coupling constants of solvated di-tert-butyl nitroxide and other similar nitroxides without the inclusion of solvent molecules in the quantum region provided polarizable force fields are used for the description of these molecules.
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| 9. |
- Rinkevicius, Zilvinas, et al.
(författare)
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Encapsulation Influence on EPR Parameters of Spin-Labels : 2,2,6,6-Tetramethyl-4-methoxypiperidine-1-oxyl in Cucurbit[8]uril
- 2012
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 8:1, s. 257-263
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Tidskriftsartikel (refereegranskat)abstract
- Encapsulation of a nitroxide spin label into a host cavity can prolong the lifetime of the spin label in biological tissues and other environments. Although such paramagnetic supramolecular complexes have been extensively studied experimentally, there is yet little understanding of the role of the encapsulation on the magnetic properties of the spin labels and their performance at the atomistic level. In this work, we approach this problem by modeling encapsulation induced changes of the magnetic properties of spin labels for a prototypical paramagnetic guest host complex, 2,2,6,6-tetramethyl-4-methoxypiperidine-1-oxyl, enclosed in the hydrophobic cavity of cucurbit[8]uril, using state-of-the-art hybrid quantum mechanics/molecular mechanics methodology. The results allow a decomposition of the encapsulation shift of the electronic g-tensor and the nitrogen isotropic hyperfine coupling constant of nitroxide radical into a set of distinct contributions associated with the host cavity confinement and with changes of the local solvent environment of the spin label upon encapsulation. It is found that the hydrophobic cavity of cucurbit[8]uril only weakly influences the electronic g-tensor of the 2,2,6,6-tetramethyl-4-methoxypiperidine-1-oxyl but induces a significant encapsulation shift of the nitrogen hyperfine coupling constant. The latter is caused by the change of topology of the hydrogen bonding network and the nature of the hydrogen bonds around the spin label induced by the hydrophobic cavity of the inclusion host. This indirect effect is found to be more important than the direct influence of the cavity exerted on the radical. The ramification of this finding for the use of approximate methods for computing electron paramagnetic resonance spectra of spin labels and for designing optimal spin labels based on guest-host templates is discussed.
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