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1.
J Mol Model ; 24(8): 216, 2018 Jul 26.
Artigo em Inglês | MEDLINE | ID: mdl-30051296

RESUMO

In this work, we demonstrate the viability of using distributed Gaussian orbitals as a basis set for the calculation of the properties of electrons subjected to an external potential. We validate our method by studying one-electron systems for which we can compare to exact analytical results. We highlight numerical aspects that require particular care when using a distributedGaussian basis set. In particular, we discuss the optimal choice for the distance between two neighboring Gaussian orbitals. Finally, we show how our approach can be applied to many-electron problems.

2.
J Am Chem Soc ; 140(24): 7698-7704, 2018 06 20.
Artigo em Inglês | MEDLINE | ID: mdl-29888914

RESUMO

A cyano-bridged Fe(II)-Cr(III) single-chain magnet designed to ensure a parallel orientation of the axial anisotropy of the building units is reported. This ferromagnetic chain compound consists of a pentagonal bipyramid Fe(II) complex with Ising-type anisotropy and a dicyanide Cr(III) complex interlinked through their apical positions. It is characterized by an energy gap for the magnetization reversal of Δeff/ kB = 113 K and exhibits magnetic hysteresis with a coercive field of 1400 Oe at 2 K which positions this compound among the very few examples of SCMs with spin reversal barriers above 100 K. The quite remarkable performances of this single-strand SCM are attributed to the alignment of the local anisotropy axes, which is supported by ab initio modeling. A discrete Cr2Fe complex based on the same building units and behaving as a SMM in zero field is also reported.

3.
Dalton Trans ; 47(31): 10636-10645, 2018 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-29796506

RESUMO

Three monomeric polyoxometalates [M(C10H8N2)3][α-PMoMoO40Zn2(C10H8N2)2]·2H2O (M-PMo12Zn2, M = Fe, Co, Ru) with {Zn(bpy)2}2+ units capped on reduced α-Keggin polyanions and [M(bpy)3]2+ counter-ions were synthesized under hydrothermal conditions. The 1D polymer [N(C4H9)4][Ru(C10H8N2)3][α-PMoMoO43] (Ru-PMo14) was prepared by a similar strategy, in the absence of 2,2'-bpy ligands. In this chain capped reduced Keggin anions are linked via Mo-O-Mo bridges and are surrounded by both tetrabutylammonium cations and [Ru(bpy)3]2+ counter-ions. The compounds were characterized in the solid state by single crystal and powder X-ray diffraction and IR spectroscopy and in solution by 31P NMR spectroscopy. 31P diffusion ordered NMR spectroscopy (DOSY) indicates that the diffusion coefficient of the dissolved species of Ru-PMo14 corresponds to a dimeric structure. Magnetic susceptibility measurements performed on Ru-PMo14 show the existence of antiferromagnetic interactions between the d1 electrons of the six MoV centers, with a singlet spin ground state. However, attempts to fit the data in the 2-300 K temperature range with Heisenberg Hamiltonians adapted for 0 or 1D systems suggest that these electrons are delocalized. Density Functional Theory (DFT) and Wave Function Theory (WFT) calculations indicate a migration of the electrons of the capping MoV centers into the PMo12 units at high temperature, allowing the rationalization of the experimental observations.

4.
Inorg Chem ; 56(24): 14809-14822, 2017 Dec 18.
Artigo em Inglês | MEDLINE | ID: mdl-29181984

RESUMO

A series of mononuclear [M(hfa)2(pic)2] (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; pic = 4-methylpyridine; M = FeII, CoII, NiII, ZnII) compounds were obtained and characterized. The structures of the complexes have been resolved by single-crystal X-ray diffraction, indicating that, apart from the zinc derivative, the complexes are in a trans configuration. Moreover, a dramatic lenghthening of the Fe-N distances was observed, whereas the nickel(II) complex is almost perfectly octahedral. The magnetic anisotropy of these complexes was thoroughly studied by direct-current (dc) magnetic measurements, high-field electron paramagnetic resonance, and infrared (IR) magnetospectroscopy: the iron(II) derivative exhibits an out-of-plane anisotropy (DFe = -7.28 cm-1) with a high rhombicity, whereas the cobalt(II) and nickel(II) complexes show in-plane anisotropy (DCo ∼ 92-95 cm-1; DNi = 4.920 cm-1). Ab initio calculations were performed to rationalize the evolution of the structure and identify the excited states governing the magnetic anisotropy along the series. For the iron(II) complex, an out-of-phase alternating-current (ac) magnetic susceptibility signal was observed using a 0.1 T dc field. For the cobalt(II) derivative, the ac magnetic susceptibility shows the presence of two field-dependent relaxation phenomena: at low field (500 Oe), the relaxation process is beyond single-ion behavior, whereas at high field (2000 Oe), the relaxation of magnetization implies several mechanisms including an Orbach process with Ueff = 25 K and quantum tunneling of magnetization. The observation by µ-SQUID magnetization measurements of hysteresis loops of up to 1 K confirmed the single-ion-magnet behavior of the cobalt(II) derivative.

5.
Chemistry ; 23(18): 4380-4396, 2017 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-28118518

RESUMO

Pentagonal bipyramid FeII complexes have been investigated to evaluate their potential as Ising-spin building units for the preparation of heteropolynuclear complexes that are likely to behave as single-molecule magnets (SMMs). The considered monometallic complexes were prepared from the association of a divalent metal ion with pentadentate ligands that have a 2,6-diacetylpyridine bis(hydrazone) core (H2 LN3O2R ). Their magnetic anisotropy was established by magnetometry to reveal their zero-field splitting (ZFS) parameter D, which ranged between -4 and -13 cm-1 and was found to be modulated by the apical ligands (ROH versus Cl). The alteration of the D value by N-bound axial CN ligands, upon association with cyanometallates, was also assessed for heptacoordinated FeII as well as for related NiII and CoII derivatives. In all cases, N-coordinated cyanide ligands led to large magnetic anisotropy (i.e., -8 to -18 cm-1 for Fe and Ni, +33 cm-1 for Co). Ab initio calculations were performed on three FeII complexes, which enabled one to rationalize the role of the ligand on the nature and magnitude of the magnetic anisotropy. Starting from the pre-existing heptacoordinated complexes, a series of pentanuclear compounds were obtained by reactions with paramagnetic [W(CN)8 ]3- . Magnetic studies revealed the occurrence of ferromagnetic interactions between the spin carriers in all the heterometallic systems. Field-induced slow magnetic relaxation was observed for mononuclear FeII complexes (Ueff /kB up to 53 K (37 cm-1 ), τ0 =5×10-9  s), and SMM behavior was evidenced for a heteronuclear [Fe3 W2 ] derivative (Ueff /kB =35 K and τ0 =4.6 10-10  s), which confirmed that the parent complexes were robust Ising-type building units. High-field EPR spectroscopic investigation of the ZFS parameters for a Ni derivative is also reported.

6.
Nat Commun ; 7: 13646, 2016 12 08.
Artigo em Inglês | MEDLINE | ID: mdl-27929089

RESUMO

A challenge in molecular spintronics is to control the magnetic coupling between magnetic molecules and magnetic electrodes to build efficient devices. Here we show that the nature of the magnetic ion of anchored metal complexes highly impacts the exchange coupling of the molecules with magnetic substrates. Surface anchoring alters the magnetic anisotropy of the cobalt(II)-containing complex (Co(Pyipa)2), and results in blocking of its magnetization due to the presence of a magnetic hysteresis loop. In contrast, no hysteresis loop is observed in the isostructural nickel(II)-containing complex (Ni(Pyipa)2). Through XMCD experiments and theoretical calculations we find that Co(Pyipa)2 is strongly ferromagnetically coupled to the surface, while Ni(Pyipa)2 is either not coupled or weakly antiferromagnetically coupled to the substrate. These results highlight the importance of the synergistic effect that the electronic structure of a metal ion and the organic ligands has on the exchange interaction and anisotropy occurring at the molecule-electrode interface.

7.
Inorg Chem ; 55(21): 10968-10977, 2016 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-27783500

RESUMO

A family of four-coordinate FeII complexes formed with N,N'-chelating amido-pyridine ligands was synthesized, and their magnetic properties were investigated. These distorted tetrahedral complexes exhibit significant magnetic anisotropy with zero-field splitting parameter D ranging between -17 and -12 cm-1. Ab initio calculations enabled identification of the structural factors that control the nature of the magnetic anisotropy and the rationalization of the variation of D in these complexes. It is shown that a reduced N-Fe-N angle involving the chelating nitrogen atoms of the ligands is at the origin of the negative D value and that the torsion between the two N-Fe-N planes imposed by steric hindrances further increases the |D| value. Field-induced slow relaxation of magnetization was observed for the three compounds, and a single-molecule magnet behavior with an energy barrier for magnetization flipping (Ueff) of 27 cm-1 could be evidenced for one of them.

8.
Phys Chem Chem Phys ; 18(5): 3545-57, 2016 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-26750534

RESUMO

We present an extension of the constrained density functional tight binding scheme combined with configuration interaction (DFTB-CI) to efficiently compute excited states of molecular cluster cations and their oscillator strengths from the ground state. The present extension consists of generalizing the initial model, relying on configurations with holes in the monomer HOMOs only, to configurations involving sub-HOMO holes, allowing for the description of higher excited states. The extended scheme is benchmarked on selected energy pathways with respect to available ab initio and new CASPT2 reference calculations on the benzene, naphthalene and pyrene dimer cations. The ability of the model to describe the potential energy surfaces and the transition dipole moments is discussed. The vertical electronic absorption spectra of the three dimer cations are calculated and compared with the theoretical litterature and available experimental data. Finally, the electronic absorption spectra of low energy isomers of the trimer and tetramer pyrene cluster cations are also predicted.

9.
J Chem Theory Comput ; 11(2): 550-9, 2015 Feb 10.
Artigo em Inglês | MEDLINE | ID: mdl-26580912

RESUMO

Accurate quantum chemical calculations on real-world magnetic systems are challenging, the inclusion of electron correlation being the bottleneck of such task. One method proposed to overcome this difficulty is the embedded fragment approach. It tackles a chemical problem by dividing it into small fragments, which are treated in a highly accurate way, surrounded by an embedding included at an approximate level. For the vast family of medium-to-large sized polyoxometalates, two-electron-reduced systems are habitual and their magnetic properties are interesting. In this paper, we aim at assessing the quality of embedded fragment calculations by checking their ability to reproduce the electronic spectra of a complete system, here the mixed-metal series [MoxW6-xO19](4-) (x = 0-6). The microscopic parameters extracted from fragment calculations (electron hopping, intersite electrostatic repulsion, local orbital energy, etc.) have been used to reproduce the spectra through model Hamiltonian calculations. These energies are compared to the results of the highly accurate ab initio difference dedicated configuration interaction (DDCI) method on the complete system. In general, the model Hamiltonian calculations using parameters extracted from embedded fragments nearly exactly reproduce the DDCI spectra. This is quite an important result since it can be generalized to any inorganic magnetic system. Finally, the occurrence of singlet or triplet ground states in the series of molecules studied is rationalized upon the interplay of the parameters extracted.

10.
J Phys Chem A ; 119(21): 5207-17, 2015 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-25474414

RESUMO

The intermediate effective Hamiltonians are designed to provide M exact energies and the components of the corresponding eigenvectors in the N-dimensional model space, with N > M. The effective Hamiltonian is not entirely defined by these N × M conditions, and several dressings of the Hamiltonian matrix in the model space are possible. Some of them lead to unreliable N - M roots associated with the intermediate model space. This defect appears dramatically when one refers to the weak separability property, namely, the fact that in a noninteracting A···B problem where the model space only involves excitations on A, the consideration of the excitations on B should not affect the spectrum of A. We suggest variants that should maintain the physical meaning of the intermediate roots. Numerical comparisons illustrate the relevance of this proposal.

11.
Chemistry ; 21(2): 763-9, 2015 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-25430555

RESUMO

Herein we evaluate the influence of an electric field on the coupling of two delocalized electrons in the mixed-valence polyoxometalate (POM) [GeV14 O40 ](8-) (in short V14 ) by using both a t-J model Hamiltonian and DFT calculations. In absence of an electric field the compound is paramagnetic, because the two electrons are localized on different parts of the POM. When an electric field is applied, an abrupt change of the magnetic coupling between the two delocalized electrons can be induced. Indeed, the field forces the two electrons to localize on nearest-neighbors metal centers, leading to a very strong antiferromagnetic coupling. Both theoretical approaches have led to similar results, emphasizing that the sharp spin transition induced by the electric field in the V14 system is a robust phenomenon, intramolecular in nature, and barely influenced by small changes on the external structure.

12.
J Phys Chem A ; 118(31): 5876-84, 2014 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-24580130

RESUMO

This work compares three descriptions of the unpaired electrons distribution in conjugated monoradical and diradical hydrocarbons involving one or two methylene groups attached to an aromatic skeleton. The first one is the simple Hückel topological Hamiltonian, the singly occupied molecular orbitals (SOMO) of which may be analytically obtained. The second one is the restricted open-shell self-consistent field (ROHF-SCF) method. The so-obtained distribution of the unpaired electrons on the skeleton appears deeply different from that predicted by the Hückel Hamiltonian, being more strongly localized on the external methylene groups. More elaborate methods treat all π electrons in the π valence molecular orbitals (MOs) through a full valence π complete active space self-consistent field (CASSCF) treatment. The distributions of the unpaired electrons (given by the natural MOs of occupation number close to 1) are surprisingly similar to those predicted by the Hückel model. The spin density distributions, including spin polarization effects, can be improved by further configuration interactions involving one hole-one particle excitations and compared with the experimental hyperfine coupling constant ratios. This comparison confirms the lack of delocalization of the magnetic orbitals defined from the self-consistent single-reference treatment. We show that, provided correct SOMO are used, a single excitation CI performed on top of a single reference gives accurate spin densities. Finally, a rationalization of the role of the dynamic correlation in correcting the excessive localization of the unpaired electron(s) at the ROHF level on the exocyclic methylene group(s) is given, attributing it to the dynamic charge polarization of the charge transfer configurations between methylene and the aromatic frame.

13.
Chem Commun (Camb) ; 49(83): 9621-3, 2013 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-24022124

RESUMO

The two-electron reduced mixed-valence polyoxometalate [GeV14O40](8-) presents an unusual paramagnetic behaviour as a consequence of the partial trapping of these electrons. The effect of applying an electric field is that of inducing antiferromagnetic coupling between the two delocalized electronic spins.

14.
J Chem Phys ; 137(22): 224304, 2012 Dec 14.
Artigo em Inglês | MEDLINE | ID: mdl-23249000

RESUMO

In spin-crossover (SCO) compounds exhibiting a light induced excited spin state trapping (LIESST) effect, the thermodynamic T(1∕2) and kinetic T(LIESST) temperature values depend on the features of the potential energy surfaces (PES) of the two lowest singlet and quintet states but also on vibrational contributions, collective effects, such as electrostatics, for instance, spin-orbit couplings to a lesser extent, etc. In this work, the question of the link between the shape of the PES of SCO compounds exhibiting a LIESST effect and their first coordination sphere structure is addressed from wave function theory based ab initio calculations. Fe(II) complexes based on model ligands suited to reproduce the main characteristics of the PES of such compounds are distorted to emphasize selectively the role played by the metal-ligand distances and the ligand-metal-ligand angles. The studied angular deformations are those usually observed in many Fe(L)(2)(NCS)(2) complexes. It is shown that the larger the deformation between the low spin and high spin equilibrium geometries, the higher the energy barrier from the high spin state and the weaker the energy difference between the bottom of the wells. These results corroborate observations made by experimentalists on a large number of complexes. While the PES features only constitutes one of the contributions to these temperatures, it is worth noticing that, relating T(1∕2) to the energy difference between the bottoms of the singlet and quintet wells and the T(LIESST) to the energy barrier from the quintet bottom well, the same slope of the empirical law T(LIESST) = -0.3T(1∕2)+T(0) is observed.

15.
J Comput Chem ; 33(21): 1748-61, 2012 Aug 05.
Artigo em Inglês | MEDLINE | ID: mdl-22610880

RESUMO

The electrical conductivities and plausible charge-ordering states in the room temperature (r.t.) phase for MMX chains [Ni(2)(dta)(4)I](∞) and [Pt(2)(dta)(4)I](∞) (dta = CH(3)CS(2)(-)) have been analyzed with periodic density functional theory (DFT) and correlated ab initio calculations combined with the effective Hamiltonian theory. Periodic DFT calculations show a more delocalized nature of the ground state in [Pt(2)(dta)(4)I](∞) compared to [Ni(2)(dta)(4)I](∞), which features a rather large energy gap between the occupied and empty bands, and charge polarized dimer units. A larger electrical conductivity for the Pt chain can be expected, especially because the Fermi level lies within a band with contributions from Pt and I orbitals. Electronic structure parameters extracted from ab initio cluster calculations show that the large difference between the observed conductivities at 300 K for Ni and Pt compounds, of 3 orders of magnitude, cannot be explained from the parameters extracted from an embedded M(2)(dta)(4)I(2) dimer fragment alone. When tetramer fragments are considered, we observe that the interdimer transfer integral (t) between neighboring M(2) units connected by an iodine atom at correlated level is comparable in both chains. On the other hand, the energy to transfer an electron from a dimer to the neighboring one (Coulomb repulsion U) is three times larger in the Ni compound with respect to the Pt chain, in line with the poor conductivity of the former. The electronic structure of the M(4)(dta)(8)I(3) fragment points to an alternate charge-polarization state for Ni and an average valence state for Pt when the r.t. X-ray structure is considered.


Assuntos
Níquel/química , Compostos Organometálicos/química , Platina/química , Teoria Quântica , Temperatura
16.
J Chem Theory Comput ; 8(11): 4127-37, 2012 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-26605579

RESUMO

This work addresses the following paradox observed in diradicalar conjugated hydrocarbons: while the natural orbitals occupation numbers clearly indicate only two open-shell orbitals, i.e. two unpaired electrons, the minimal CAS zero-order description fails to reproduce accurately the electronic structures of the lowest states (spin density distribution and singlet-triplet energy gap, i.e., magnetic coupling). We will focus on the question of the optimization of both magnetic and nonmagnetic orbitals for the determination of accurate magnetic interactions in organic compounds. It is analytically demonstrated (in the Appendix) and numerically shown from multireference configuration interaction calculations performed on a series of original organic ferro- and antiferromagnetic compounds that, (i) some double excitations must be considered to obtain reliable magnetic orbitals for the calculation of magnetic couplings, (ii) the account of these excitations results in a larger spatial extent of the magnetic orbitals on the surrounding ligands and hence better drives the interaction between several magnetic centers, and (iii) the reliability of the orbitals is a crucial ingredient for the determination of accurate magnetic couplings. A strategy which optimizes the orbitals at a reasonable computational cost is proposed. It relies on a CAS(2,2) zero-order description and provides orbitals of the same quality as the CAS(full valence π)SCF orbitals. The values of the magnetic couplings computed using the difference dedicated configuration interaction on top of the CAS(2,2) references with the new orbital set are very close to those obtained at the much more computationally demanding CAS(full valence π)PT2 level of treatment.

17.
Chemphyschem ; 12(16): 3020-36, 2011 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-22021220

RESUMO

This work addresses the conception of purely organic magnetic materials by properly bridging high-spin polycyclic hydrocarbons A and B, through covalent ligands L. The strategy varies two degrees of freedom that govern the magnetic character of the A-L--B sequence, namely, the bridge response to spin polarization and the relative signs of spin density on carbon atoms to which the bridge is attached. Topological prescriptions based on Ovchinnikov's rule are proposed to predict ground-state spin multiplicities of various A-L-B sets. The relevance of these guiding principles is essentially confirmed through DFT calculations on dimers connected by conjugated bridges. The transferability of interunit magnetic couplings to larger assemblies is further checked, the building blocks tending to maintain their high-spin character whatever the environment. Such local designs open the way to periodic lattices of ferromagnetic, antiferromagnetic, ferrimagnetic, or paramagnetic materials.

18.
Chemistry ; 16(29): 8762-72, 2010 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-20572170

RESUMO

High-spin organic structures can be obtained from fused polycyclic hydrocarbons, by converting selected peripheral HC(sp(2)) sites into H(2)C(sp(3)) ones, guided by Ovchinnikov's rule. Theoretical investigation is performed on a few examples of such systems, involving three to twelve fused rings, and maintaining threefold symmetry. Unrestricted DFT (UDFT) calculations, including geometry optimizations, confirm the high-spin multiplicity of the ground state. Spin-density distributions and low-energy spectra are further studied through geometry-dependent Heisenberg-Hamiltonian diagonalizations and explicit correlated ab initio treatments, which all agree on the high-spin character of the suggested structures, and locate the low-lying states at significantly higher energies. In particular, the lowest-lying state of lower multiplicity is always found to be higher than kT at room temperature (at least ten times higher). Simplification of the ferromagnetic organization based on sets of semilocalized nonbonding orbitals is proposed. Molecular architectures are thus conceived in which the ferromagnetically-coupled unpaired electrons tally up to one third of the involved conjugated carbons. Connecting such building blocks should provide bidimensional materials endowed with robust magnetic properties.

19.
J Am Chem Soc ; 131(2): 715-22, 2009 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-19072230

RESUMO

This paper provides a qualitative analysis of the physical content of the low-energy states of a spin-transition compound presenting a light-induced excited spin state trapping (LIESST) phenomenon, namely, [Fe(dipyrazolpyridine)2](BF4)2, which has been studied using the wave function-based CASPT2 method. Both the nature of the low-energy states and the relative position of their potential energy wells as a function of the geometry are rationalized from the analysis of the different wave functions. It is shown that the light-induced spin transition occurring in such systems could follow several pathways involving different excited spin states. In an ideal octahedral geometry, the interconversion from the excited singlet state to the triplet of lower energy, which is usually seen as an intermediate state in the LIESST mechanism, is quite unlikely since there is no crossing between the potential energy curves of these two states. On the contrary, in lower-symmetry complexes, the geometrical distortion of the coordination sphere due to ligand constraints is responsible for the occurrence of a crossing between these two states in the Franck-Condon region, leading to a possible participation of this triplet state in the LIESST mechanism. In the reverse LIESST process, a crossing between the potential energy curves of another triplet state and the excited quintet state occurs in the Franck-Condon region as well.

20.
J Chem Theory Comput ; 5(11): 2977-84, 2009 Nov 10.
Artigo em Inglês | MEDLINE | ID: mdl-26609979

RESUMO

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.

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