Calculation Of Exchange Coupling Constants Research Articles

Tetrabromidocuprate(II) complexes with diprotonated 1-hydroxy-4methyl-2-(pyridin-2-yl)-5-phenylimidazole and 1-hydroxy-2-(pyridin-2-yl)-4,5,6,7-tetrahydrobenzimidazole ligands: Relationship between the structure and magnetic properties

Two new tetrabromidocuprate(II) complexes with diprotonated 1-hydroxy-2-(pyridin-2-yl)-4,5,6,7-tetrahydrobenzimidazole (1) and 1-hydroxy-4methyl-2-(pyridin-2-yl)-5-phenylimidazole (2) were synthesized and their structure established from single-crystal X-ray data. Compounds crystallized in the monoclinic space groups P21/n and P21/c, respectively, where tetrabromidocuprate anions formed magnetic chains as inferred from the distances between the halide ions of different [CuBr4]2− units. Magnetization measurements showed that both compounds behave as well-isolated uniform S = 1/2 antiferromagnetic chains described by the one-dimensional Heisenberg antiferromagnetic model with intra-chain exchange interaction 2 J = 23 K (1) and 2 J = 33 K (2) (the Hamiltonian Hˆ=2J∑SˆiSˆi+1). Electron paramagnetic resonance, optical absorption, and density functional theory investigations provided complementary results for 1 and 2. A calculation of exchange coupling constants gave values of 2 J = 24 K (1) and 2 J = 39 K (2), in good agreement with the experiment. The quantum theory of atoms in molecules and non-covalent interaction analyses showed that halogen bonding determined the exchange interaction between [CuBr4]2− units in magnetic chains of these cation–anion compounds.

Assessment of density functional approximations for calculation of exchange coupling constants in thiocyanato and cyanato double bridged binuclear Ni(II) complexes

In the present work, we examine the magnetic properties of 8 ?endto- end? thiocyanato, and 3 ?end-to-end? cyanato double bridged Ni(II) binuclear complexes. Thiocyanato complexes are weakly ferromagnetic. Cyanato bridged complexes exhibit weak antiferromagnetic coupling. Therefore, it is a challenge for computational chemistry to calculate the exchange coupling constant in these systems accurately. 17 different density functional approximations with different flavours are used to find the method of choice to study magnetic properties in binuclear Ni(II) complexes within the broken-symmetry approach. It is found that M06-2X and PWPB95 performed the best compared to the experimental values for the entire set of examined complexes. Furthermore, the magneto-structural correlation rationalizes the results.

Assessing the Calculation of Exchange Coupling Constants and Spin Crossover Gaps Using the Approximate Projection Model To Improve Density Functional Calculations.

This work evaluates the quality of exchange coupling constant and spin crossover gap calculations using density functional theory corrected by the approximate projection model. Results show that improvements using the approximate projection model range from modest to significant. This study demonstrates that, at least for the class of systems examined here, spin projection generally improves the quality of density functional theory calculations of J-coupling constants and spin crossover gaps. Furthermore, it is shown that spin projection can be important for both geometry optimization and energy evaluations. The approximate projection model provides an affordable and practical approach for effectively correcting spin-contamination errors in such calculations.

Calculation of Exchange Coupling Constants in Triply-Bridged Dinuclear Cu(II) Compounds Based on Spin-Flip Constricted Variational Density Functional Theory

The performance of the second-order spin-flip constricted variational density functional theory (SF-CV(2)-DFT) for the calculation of the exchange coupling constant (J) is assessed by application to a series of triply bridged Cu(II) dinuclear complexes. A comparison of the J values based on SF-CV(2)-DFT with those obtained by the broken symmetry (BS) DFT method and experiment is provided. It is demonstrated that our methodology constitutes a viable alternative to the BS-DFT method. The strong dependence of the calculated exchange coupling constants on the applied functionals is demonstrated. Both SF-CV(2)-DFT and BS-DFT affords the best agreement with experiment for hybrid functionals.

Calculation of the exchange coupling constants of copper binuclear systems based on spin-flip constricted variational density functional theory

We have recently developed a methodology for the calculation of exchange coupling constants J in weakly interacting polynuclear metal clusters. The method is based on unrestricted and restricted second order spin-flip constricted variational density functional theory (SF-CV(2)-DFT) and is here applied to eight binuclear copper systems. Comparison of the SF-CV(2)-DFT results with experiment and with results obtained from other DFT and wave function based methods has been made. Restricted SF-CV(2)-DFT with the BH&HLYP functional yields consistently J values in excellent agreement with experiment. The results acquired from this scheme are comparable in quality to those obtained by accurate multi-reference wave function methodologies such as difference dedicated configuration interaction and the complete active space with second-order perturbation theory.

Calculation of Exchange Coupling Constants of Transition Metal Complexes with DFT

A broken-symmetry method for the calculation of exchange coupling constants from DFT calculations, using the Heisenberg-Dirac-van Vleck spin Hamiltonian, has been validated for a dinuclear copper(II) complex. Hybrid functionals in combination with a large basis set on the metal centers and their first coordination sphere, and a smaller basis set on the ligand backbone are shown to be efficient and acceptable with respect to the computational cost and precision in comparison with experimental data. This method was thoroughly tested with a series of oligonuclear transition metal complexes with Cr(III), Cu(II), Fe(III), Mn(II), Mn(III), Mn(IV), Ni(II), and V(IV) as magnetic centers. The computed values of J are within approximately 50 cm(-1) of the experimental values for most of the examples; with combined basis sets, there generally is a similar accuracy to that obtained with a large basis set for the entire spin cluster but with significantly reduced computational expense. When the experimentally observed structural data are refined prior to the calculation of the exchange coupling constants, the computed values of J are in most cases in slightly better agreement with the experimental data than those obtained from single point calculations based on the X-ray data.

A New Quantum Chemical Approach to the Magnetic Properties of Oligonuclear Transition‐Metal Complexes: Application to a Model for the Tetranuclear Manganese Cluster of Photosystem II

The reliable correlation of structural features and magnetic or spectroscopic properties of oligonuclear transition-metal complexes is a critical requirement both for research into innovative magnetic materials and for elucidating the structure and function of many metalloenzymes. We have developed a novel method that for the first time enables the extraction of hyperfine coupling constants (HFCs) from broken-symmetry density functional theory (BS-DFT) calculations on clusters. Using the geometry-optimized tetranuclear manganese complex [Mn(4)O(6)(bpy)(6)](4+/3+) as a model, we first examine in detail the calculation of exchange coupling constants J through the BS-DFT approach. Complications arising from the indeterminacy of experimentally fitted J constants are identified and analyzed. It is found that only the energy levels derived from Hamiltonian diagonalization are a physically meaningful basis for comparing theory and experiment. Subsequently, the proposed theoretical scheme is applied to the calculation of (55)Mn HFCs of the Mn(III,IV,IV,IV) state of the complex, which is similar to the S(2) state of the oxygen-evolving complex (OEC) in photosystem II of oxygenic photosynthesis. The new approach performs reliably and accurately, and yields calculated HFCs that can be directly compared with experimental data. Finally, we carefully examine the dependence of HFC on the J value and draw attention to the sensitivity of the calculated values to the exchange coupling parameters. The proposed strategy extends naturally to hetero-oligonuclear clusters of arbitrary shape and nuclearity, and hence is of general validity and usefulness in the study of magnetic metal clusters. The successful application of the new approach presented here is a first step in the effort to establish correlations between the available spectroscopic information and the structural features of complex metalloenzymes like OEC.

Broken symmetry approach and density functional theory calculations for heterospin system consisting of copper(II) and aminoxyl radicals

The magnetic-structural correlation in metal-radical heterospin system such as bis(hexafluoroacetylacetonato)copper(II) [Cu(hfac) 2] ligated with 3- and 4-(N-oxyl-tert-butylamino)-pyridines (3NOPy and 4NOPy) complexes has been investigated on the basis of unrestricted density functional theory (UDFT) and unrestricted Hartree–Fock (UHF) calculations combined with the broken symmetry (BS) approach, in order to study the magnetic properties of the complexes [Cu(hfac) 2(4NOPy) 2] and [Cu(hfac) 2(3NOPy) 2]. The effective exchange coupling constants J 1 have been obtained by different methods (UPBE, UB3PW91, UB3LYP, UB3P86, UPBE0 and UHF). The best calculated exchange coupling constant for complex [Cu(hfac) 2(4NOPy) 2] was found to be J = 55.8 K, while the smallest magnitude calculated exchange coupling constant for complex [Cu(hfac) 2(3NOPy) 2] was found to be J = −33.1 K. For both two complexes UPBE0, UB3PW91, UB3LYP, and UB3P86 methods with LANL2DZ basis set are suitable for the calculation of exchange coupling constant.

Reply to “Comment on ‘About the calculation of exchange coupling constants using density-functional theory: The role of the self-interaction error’” [J. Chem. Phys.123, 164110 (2005)

This reply focuses on two crucial aspects, the role of the self-interaction error on the calculation of the exchange coupling constants, which has been proposed by other authors and ourselves, and the excellent agreement between the results obtained with our theoretical approach and the experimental data for all kinds of magnetic transition metal compounds.

Comment on “About the calculation of exchange coupling constants using density-functional theory: The role of the self-interaction error” [J. Chem. Phys. 123, 164110 (2005)

The use of density functional theory to obtain energy differences related to magnetic coupling constants is debated with special emphasis to the claims by Ruiz et al. [J. Chem. Phys.123, 164110 (2005)] that good agreement with experiment using the B3LYP potential is obtained by ignoring spin symmetry in case self-interaction error cannot be removed.

About the calculation of exchange coupling constants using density-functional theory: The role of the self-interaction error

The effect of the correction of the self-interaction error on the calculation of exchange coupling constants with methods based on density-functional theory has been tested in simple model systems. The inclusion of the self-interaction correction cancels the nondynamical correlation energy contributions simulated by the commonly used functionals. Hence, such correction should be important in the accurate determination of exchange coupling constants. We have also tested several recent functionals to calculate exchange coupling constants in transition-metal complexes, such as meta-GGA functionals or new formulations of hybrid functionals. The influence of the basis set and of the use of pseudopotentials on the calculated J values has also been evaluated for a Fe(III) dinuclear complex in which the paramagnetic centers bear several unpaired electrons.

Calculation of exchange coupling constants in solid state transition metal compounds using localized atomic orbital basis sets

The application of theoretical methods based on density functional theory using hybrid functionals and localized, atomic orbital type basis sets is shown to provide good estimates for exchange coupling constants in non-metallic, solid state transition metal compounds with relatively complex crystal structures. The accuracy of the calculated exchange coupling constants is similar to that previously obtained for dinuclear and polynuclear molecular compounds. As an application of this procedure, the magnetic properties of the high-temperature phase of CuGeO 3, the recently synthesized silver copper oxide Ag 2Cu 2O 3, and the family of M[N(CN) 2] 2 ( M=Cr(II), Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)) compounds are analyzed via the computation of their most relevant exchange coupling constants.

About the calculation of exchange coupling constants in polynuclear transition metal complexes.

The application of theoretical methods based on the density functional theory with hybrid functionals provides good estimates of the exchange coupling constants for polynuclear transition metal complexes. The accuracy is similar to that previously obtained for dinuclear compounds. We present test calculations on simple model systems based on H. He and CH(2). He units to compare with Hartree-Fock and multiconfigurational results. Calculations for complete, nonmodeled polynuclear transition metal complexes yield coupling constants in very good agreement with available experimental data.

Broken symmetry approach to calculation of exchange coupling constants for homobinuclear and heterobinuclear transition metal complexes

The application of broken symmetry density functional calculations to homobinuclear and heterobinuclear transition metal complexes produces good estimates of the exchange coupling constants as compared to experimental data. The accuracy of different hybrid density functional theory methods was tested. A discussion is presented of the different methodological approaches that apply when a broken symmetry wave function is used with either Hartree–Fock or density functional calculations. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1391–1400, 1999

On the calculation of exchange coupling constants in transition metal dimers by the Xα-SW method

The main theoretical models for the calculation of the singlet triplet separation in dinuclear systems containing transition metal ions are briefly reviewed and some examples of applications are reported. The Xα-SW model is applied to calculate the magnetic structure of [(OH)4 Cu (μ -0)Cu (OH)4]6- and of the heterodinuclear ferromagnetic dimer [(NH3 )2 Cu (μ-OH)2 VO (OH)2 ]. A nice agreement with the experimental results has been obtained.