Calculation Of Hyperfine Coupling Constants Research Articles

Integrated Multiscale Multilevel Approach to Open Shell Molecular Systems.

We present a novel multiscale approach to study the electronic structure of open shell molecular systems embedded in an external environment. The method is based on the coupling of multilevel Hartree-Fock (MLHF) and Density Functional Theory (MLDFT), suitably extended to the unrestricted formalism, to Molecular Mechanics (MM) force fields (FF). Within the ML region, the system is divided into active and inactive parts, thus describing the most relevant interactions (electrostatic, polarization, and Pauli repulsion) at the quantum level. The surrounding MM part, which is formulated in terms of nonpolarizable or polarizable FFs, permits a physically consistent treatment of long-range electrostatics and polarization effects. The approach is extended to the calculation of hyperfine coupling constants and applied to selected nitroxyl radicals in an aqueous solution.

Broken-Symmetry Density Functional Theory Analysis of the Ω Intermediate in Radical S-Adenosyl-l-methionine Enzymes: Evidence for a Near-Attack Conformer over an Organometallic Species.

Radical S-adenosyl-l-methionine (SAM) enzymes are found in all domains of life and catalyze a wide range of biochemical reactions. Recently, an organometallic intermediate, Ω, has been experimentally implicated in the 5'-deoxyadenosyl radical generation mechanism of the radical SAM superfamily. In this work, we employ broken-symmetry density functional theory to evaluate several structural models of Ω. The results show that the calculated hyperfine coupling constants (HFCCs) for the proposed organometallic structure of Ω are inconsistent with the experiment. In contrast, a near-attack conformer of SAM bound to the catalytic [4Fe-4S] cluster, in which the distance between the unique iron and SAM sulfur is ∼3 Å, yields HFCCs that are all within 1 MHz of the experimental values. These results clarify the structure of the ubiquitous Ω intermediate and suggest a paradigm shift reversal regarding the mechanism of SAM cleavage by members of the radical SAM superfamily.

Calculated hyperfine coupling constants of some sigma-type radicals

The hyperfine coupling constants of some type radicals were calculated by using the DFT(B3LYP) method with LanL2DZ basic set. These are phenyl, naphthyl, pyridyl, and quinolyl radicals. Also, the study was enriched by the calculations of the spin densities for all the radicals. From the results it was concluded that the spin density of the unpaired electron is mainly in one sp-hybrid orbital of the carbon atom from which hydrogen is removed. The electron paramagnetic resonance (EPR) spectra of all the radicals are generally based on the hyperfine splittings belonging to only the close hydrogen or azote atoms of the benzene ring to the unpaired electron since the effect of the other atoms is disappeared in the linewith of the EPR spectrum lines. The EPR spectra were also simulated by using their calculated isotropic hyperfine coupling constants and, compared with the experimental data. It was found a good agreement beetwen them.

Important components for accurate hyperfine coupling constants: electron correlation, dynamic contributions, and solvation effects

ABSTRACT The calculation of hyperfine coupling constants is a challenging task in balancing accuracy and computational effort. While previous work has shown the importance of electron correlation and molecular dynamic contributions, we present a systematic study simultaneously analyzing the influence of both effects on hyperfine coupling constants. To this end, we thoroughly study two organic radicals, namely the dimethylamino radical and ethanal radical cation, proving the need to account for conformational flexibility as well as the large influence of electron correlation. Based on these results, we analyse the effect of electron correlation and dynamic simulations on a set of 12 organic radicals, illustrating that both effects are vital for an accurate in silico description on the same scale. Furthermore, we study the influence of solvation using the efficient nuclei-selected algorithm to obtain hyperfine coupling constants with electron correlation for large systems, indicating the necessity to include explicit solvent molecules. Finally, we introduce a composite approach to incorporate all contributions for hyperfine coupling of radicals in solution at comparatively low computational cost. This is successfully tested on the hydroxylated TEMPO radical in aqueous solution, where we are able to compute a 14 N-HFCC of 44.4 MHz compared to the experimentally measured 47.6 MHz.

Hybrid density functional calculations of hyperfine coupling tensor for hole-type defects in MgAl2O4

We have performed the density functional calculations (DFT) on the hole-type defects (V-centres) in magnesium aluminate spinel (MgAl2O4) following the results of recent paramagnetic resonance measurements (EPR) in Nucl. Inst. Methods Phys. Res. B 435 (2018) 31–37. The hybrid B3LYP functional calculations using large supercells of 448 atoms have demonstrated excellent results not only for bulk properties but also properties of the V-centres in MgAl2O4. Three types of V-centres have been considered and confirmed, namely V1, V2 and V22. The DFT calculations have revealed the atomic relaxation pattern and spin density distribution around the hole-type defects that is suggested as an important complement to the experiments. Moreover, the calculated hyperfine coupling constants (HCCs) have been analyzed and compared with those from the measured EPR spectra. A good agreement between the calculated and measured HCC values is observed and discussed.

First-Principles Calculation of Transition Metal Hyperfine Coupling Constants with the Strongly Constrained and Appropriately Normed (SCAN) Density Functional and its Hybrid Variants

Density functional theory (DFT) is used extensively for the first-principles calculation of hyperfine coupling constants in both main-group and transition metal systems. As with many other properties, the performance of DFT for hyperfine coupling constants is of variable quality, particularly for transition metal complexes, because it strongly depends on the nature of the chemical system and the type of approximation to the exchange-correlation functional. Recently, a meta-generalized-gradient approximation (mGGA) functional was proposed that obeys all known exact constraints for such a method, known as the Strongly Constrained and Appropriately Normed (SCAN) functional. In view of its theoretically superior formulation a benchmark set of complexes is used to assess the performance of SCAN for the challenging case of transition metal hyperfine coupling constants. In addition, two global hybrid versions of the functional, SCANh and SCAN0, are described and tested. The values computed with the new functionals are compared with experiment and with those of other DFT approximations. Although the original SCAN and the SCAN-based hybrids may offer improved hyperfine coupling constants for specific systems, no uniform improvement is observed. On the contrary, there are specific cases where the new functionals fail badly due to a flawed description of the underlying electronic structure. Therefore, despite these methodological advances, systematically accurate and system-independent prediction of transition metal hyperfine coupling constants with DFT remains an unmet challenge.

Dimer Radical Anions of Polyfluoroarenes. Two More to a Small Family.

While there is a body of experimental data concerning dimers formed by an aromatic molecule and its radical cation, information on the corresponding dimer radical anions (DRAs) is scarce. In this work, evidence for the formation of the DRAs of decafluorobiphenyl and 4-aminononafluorobiphenyl has been obtained by the optically detected electron paramagnetic resonance and the time-resolved magnetic field effect techniques. Theoretical investigation (DFT B3LYP-D3/6-31+G*) of these DRAs and the DRAs of octafluoronaphtalene and 1,2,4,5-tetrafluorobenzene previously detected by Werst has been undertaken to gain greater insight into the structure of the polyfluoroarene DRAs. Without substituents different from a fluorine atom, an extra electron is evenly delocalized over two fragments; the bonding interaction is π stacking. On the potential energy surfaces (PES), there are two minima of nearly equal energy corresponding to the structures of perfect and parallel displaced sandwiches. Such a PES structure is due to a conical intersection between two electronic states of different symmetry. The DRA of 4-aminononafluorobiphenyl is an ion-molecular associate stabilized by electrostatic interactions involving NH2 groups. The complex cyclic structure of the PES of this DRA suits the successive electron transfers between the dimer fragments. The calculated hyperfine coupling constants averaged over the PES minima agree well with the experimental ones.

Effective yet reliable computation of hyperfine coupling constants in solution by a QM/MM approach: Interplay between electrostatics and non-electrostatic effects.

In this paper, we have extended to the calculation of hyperfine coupling constants, the model recently proposed by some of the present authors [Giovannini et al., J. Chem. Theory Comput. 13, 4854-4870 (2017)] to include Pauli repulsion and dispersion effects in Quantum Mechanical/Molecular Mechanics (QM/MM) approaches. The peculiarity of the proposed approach stands in the fact that repulsion/dispersion contributions are explicitly introduced in the QM Hamiltonian. Therefore, such terms not only enter the evaluation of energetic properties but also propagate to molecular properties and spectra. A novel parametrization of the electrostatic fluctuating charge force field has been developed, thus allowing a quantitative reproduction of reference QM interaction energies. Such a parametrization has been then tested against the prediction of EPR parameters of prototypical nitroxide radicals in aqueous solutions.

Calculation of hyperfine coupling constant and the g tensor of alanine radicals at different crystal temperatures based on Density Functional Theory (DFT)

Calculation of hyperfine coupling constant and the g tensor of alanine radicals at different crystal temperatures based on Density Functional Theory (DFT)

Hyperfine structure of 2Σ molecules containing alkaline-earth-metal atoms

Ultracold molecules with both electron spin and an electric dipole moment offer new possibilities in quantum science. We use density-functional theory to calculate hyperfine coupling constants for a selection of molecules important in this area, including RbSr, LiYb, RbYb, CaF and SrF. We find substantial hyperfine coupling constants for the fermionic isotopes of the alkaline-earth and Yb atoms. We discuss the hyperfine level patterns and Zeeman splittings expected for these molecules. The results will be important both to experiments aimed at forming ultracold open-shell molecules and to their applications.

Frontispiece: Relativistic DFT Calculations of Hyperfine Coupling Constants in 5d Hexafluorido Complexes: [ReF6]2− and [IrF6]2−

Frontispiece: Relativistic DFT Calculations of Hyperfine Coupling Constants in 5d Hexafluorido Complexes: [ReF₆]²⁻ and [IrF₆]²⁻

Relativistic DFT Calculations of Hyperfine Coupling Constants in 5d Hexafluorido Complexes: [ReF6 ]2- and [IrF6 ]2.

The performance of relativistic density functional theory (DFT) methods has been investigated for the calculation of the recently measured hyperfine coupling constants of hexafluorido complexes [ReF6]2− and [IrF6]2−. Three relativistic methods were employed at the DFT level of theory: the 2‐component zeroth‐order regular approximation (ZORA) method, in which the spin–orbit coupling was treated either variationally (EV ZORA) or as a perturbation (LR ZORA), and the 4‐component Dirac–Kohn–Sham (DKS) method. The dependence of the results on the basis set and the choice of exchange‐correlation functional was studied. Furthermore, the effect of varying the amount of Hartree–Fock exchange in the hybrid functionals was investigated. The LR ZORA and DKS methods combined with DFT led to very similar deviations (about 20 %) from the experimental values for the coupling constant of complex [ReF6]2− by using hybrid functionals. However, none of the methods were able to reproduce the large anisotropy of the hyperfine coupling tensor of complex [ReF6]2−. For [IrF6]2−, the EV ZORA and DKS methods reproduced the experimental tensor components with deviations of ≈10 and ≈5 % for the hybrid functionals, whereas the LR ZORA method predicted the coupling constant to be around one order of magnitude too large owing to the combination of large spin–orbit coupling and very low excitation energies.

Calculated hyperfine coupling constants for 5,5-dimethyl-1-pyrroline N-oxide radical products in water and benzene

The optimized structures of some radical adducts of 5,5-dimethyl-1-pyrroline N-oxide were computed by different methods on ESR spectra. As trapped radicals, H, N3, NH2, CH3, CCl3, OOH in water and F, OH, CF3, CH2OH, OC2H5 in benzene solutions were used. The calculated isotropic hyperfine coupling constants of all the trapped radicals were compared with the corresponding experimental data. The hyperfine coupling constant due to the β proton of the nitroxide radical was seen to be consist with the McConnel’s relation αβ = B 0 + B 1cos2θ and, to be effected with the opposite spin density of oxygen nucleus bonded to the nitrogen. It was concluded that in hyperfine calculations the DFT(B3PW91)/LanL2DZ level is superior computational quantum model relative to the used other level. Also, the study has been enriched by the computational of the optimized geometrical parameters, the hyper conjugative interaction energies, the atomic charges and spin densities for all the radical adducts.

Energies and hyperfine structures of the 1s22s2p 3Po state of Be-like ions with Z = 11–18

Nonrelativistic energies and wave functions of the 1s22s2p 3Po states of Be isoelectronic sequence (Z = 11–18) are calculated using the full core plus correlation method (FCPC). To obtain the accurate energy level, the relativistic corrections and mass polarization effect are included by using the first-order perturbation theory. The calculated excitation energies (relative to the 1s22s2 ground state) are compared with the experiment. Most of the calculated [Formula: see text] energies agree with the experiment to within a few inverse centimetres. The calculated hyperfine coupling constants are in good agreement with the latest theoretical data in the literature. Our results may provide valuable reference data for spectral analysis and identification in the future.

DFT calculation of isotropic hyperfine coupling constants of spin adducts of 5,5-dimethyl-1-pyrroline-N-oxide in benzene and water

Isotropic hyperfine coupling constants (HFCCs) of spin adducts of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) are calculated by DFT method with solvation effect. Geometries of ring conformers and rotational conformers of trapped radicals are optimized and their HFCCs are calculated. Boltzmann weighted average of HFCCs of low energy conformations shows very good agreement with the experimental value in benzene. Two or four water molecules hydrogen bonded to the spin adduct have to be explicitly incorporated in the case of water. The results show time-independent averaging of relevant conformers is a very practical method for calculation of HFCCs of DMPO spin adducts.

DFT calculations of hyperfine coupling constants of organic π radicals and comparison with empirical equations and experiment.

Density functional theory calculations (UB3LYP/EPR-III) for a series of radicals and radical ions were performed to check the internal consistency of the method and its implications to the theoretical concepts of electron paramagnetic resonance such as π-σ spin polarization, hyperconjugation and phenyl hyperconjugation. In the second part, experimental data for seven radicals (43 hyperfine coupling constants) are compared with calculations, yielding a correlation of r2 = 0.97. Copyright © 2016 John Wiley & Sons, Ltd.

EPR study of gamma-irradiated 2-Bromo-4′-methoxyacetophenone single crystals

ABSTRACTThe gamma-irradiated single crystals of 2-Bromo-4′-methoxyaceto-phenone (2B4MA) were investigated using electron paramagnetic resonance (EPR) technique. Density-functional theory calculations were employed to investigate and identify the radicals that have been assumed to be formed upon irradiation of 2B4MA single crystals. The EPR spectra of 2B4MA were recorded at different orientations in the magnetic field at room temperature. Taking into account the chemical structure and experimental spectra of irradiated single crystal of 2B4MA, it was assumed that at least two different radicals were produced in the sample. Following this assumption, six possible radicals were modeled and EPR parameters were calculated by using the DFT, B3LYP/6-311+G(d), for the modeled radicals individually. The calculated hyperfine coupling constants and g-tensors were used as initial values for simulation studies. The three crystallographic axes on the simulated spectra were well matched with experimental spectra for the two modeled radicals. Thus, we identified the R1 type radical and R4 type radical as paramagnetic species produced in gamma-irradiated 2B4MA.

Calculations of hyperfine coupling constant of copper(II) in aqueous environment. Finite temperature molecular dynamics and relativistic effects.

The presented paper is focused on the calculation of hyperfine coupling constants (HFCC) of Cu (2+) ion in water environment. To simulate the conditions of the electron paramagnetic resonance (EPR) experiment in aqueous phase, molecular dynamics using the density functional theory (DFT) was employed. In total three different functionals (BLYP, B3LYP, M06) were employed for studying their suitability in describing coordination of Cu (2+) by water molecules. The system of our interest was composed of one Cu (2+) cation surrounded by a selected number (between thirty and fifty) of water molecules. Besides the non-relativistic HFCCs (Fermi contact terms) of Cu (2+) also the four-component relativistic HFCC calculations are presented. The importance of the proper evaluation of HFCCs, the inclusion of spin-orbit term, for Cu (2+) containing systems (Neese, J. Chem. Phys. 118, 3939 2003; Almeida et al., Chem. Phys. 332, 176 2007) is confirmed at the relativistic four-component level of theory.

17O and 1H relaxometric and DFT study of hyperfine coupling constants in [Mn(H2O)6]2+

Nuclear Magnetic Relaxation Dispersion (NMRD) profiles and 17O NMR chemical shifts and transverse relaxation rates of aqueous solutions of [Mn(H2O)6]2+ were recorded to determine the parameters governing the relaxivity in this complex and the 17O and 1H hyperfine coupling constants (HFCCs). The analysis of the NMRD and 17O NMR data provided a water exchange rate of kex298 = 28.2 × 106 s−1, and AO/ħ and AH/ħ hyperfine coupling constants of −34.6 and 5.4 rad s−1, respectively. DFT calculations (TPSSh model) performed on the [Mn(H2O)6]2+ and [Mn(H2O)6]2+·12H2O systems were used to evaluate theoretically the 17O and 1H HFCCs responsible for the 17O NMR chemical shifts and the scalar contributions to 17O and 1H NMR relaxation rates. The use of a mixed cluster–continuum approach with the explicit inclusion of second-sphere water molecules is critical for an accurate calculation of HFCCs of coordinated water molecules. The impact of complex dynamics on the calculated HFCCs was evaluated with the use of molecular dynamics simulations within the atom-centered density matrix propagation (ADMP) approach. These molecular dynamics simulations show that the Aiso values are critically affected by the distance between the oxygen atom of the coordinated water molecule and the MnII ion, as well as by the orientation of the water molecule plane with respect to the Mn–O vector. The substantial scalar contribution to relaxivity observed for [Mn(H2O)6]2+ is related to a combination of a slow water exchange rate and a slow electron spin relaxation.

Hyperfine Coupling Constants on Inner-Sphere Water Molecules of a Triazacyclononane-based Mn(II) Complex and Related Systems Relevant as MRI Contrast Agents

We report the synthesis of the ligand H2MeNO2A (1,4-bis(carboxymethyl)-7-methyl-1,4,7-triazacyclononane) and a detailed experimental and computational study of the hyperfine coupling constants (HFCCs) on the inner-sphere water molecules of [Mn(MeNO2A)] and related Mn(2+) complexes relevant as potential contrast agents in magnetic resonance imaging (MRI). Nuclear magnetic relaxation dispersion (NMRD) profiles, (17)O NMR chemical shifts, and transverse relaxation rates of aqueous solutions of [Mn(MeNO2A)] were recorded to determine the parameters governing the relaxivity in this complex and the (17)O and (1)H HFCCs. DFT calculations (TPSSh model) performed in aqueous solution (PCM model) on the [Mn(MeNO2A)(H2O)]·xH2O and [Mn(EDTA)(H2O)](2-)·xH2O (x = 0-4) systems were used to determine theoretically the (17)O and (1)H HFCCs responsible for the (17)O NMR chemical shifts and the scalar contributions to (17)O and (1)H NMR relaxation rates. The use of a mixed cluster/continuum approach with the explicit inclusion of a few second-sphere water molecules is critical for an accurate calculation of HFCCs of coordinated water molecules. The impact of complex dynamics on the calculated HFCCs was evaluated with the use of molecular dynamics simulations within the atom-centered density matrix propagation (ADMP) approach. The (17)O and (1)H HFCCs calculated for these complexes and related systems show an excellent agreement with the experimental data. Both the (1)H and (17)O HFCCs (A(iso) values) are dominated by the spin delocalization mechanism. The A(iso) values are significantly affected by the distance between the oxygen atom of the coordinated water molecule and the Mn(2+) ion, as well as by the orientation of the water molecule plane with respect to the Mn-O vector.