Theoretical Spin Density Research Articles

Graphene oxide @ nickel phosphate nanocomposites for photocatalytic hydrogen production

The graphene oxide @nickel phosphate (GO:NPO) nanocomposites (NCs) are prepared by using a one-pot in-situ solar energy assisted method by varying GO:NPO ratio i.e. 0.00, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, and 2.00 without adding any surfactant or a structure-directing reagent. As produced GO:NPO nanosheets exhibited an improved photocatalytic activity due to the spatial seperation of charge carriers through interface, where photoinduced electrons transferred from NiPO4 to the GO sheets without charge-recombination. Out of the series, the system 1.00 GO:NPO NC show the optimum hydrogen production activity (15.37 μmol H2 h−1) towards water splitting under the visible light irradiation. The electronic environment of the nanocomposite GO-NiO6/NiO4-PO4 elucidated in the light of advance experimental analyses and theoretical DFT spin density calculations. Structural advanmcement of composites are well correlated with their hydrogen production activity.

Comparative Analysis of DFT+U, ACBN0, and Hybrid Functionals on the Spin Density of YTiO3 and SrRuO3

We present a quantitative analysis of the theoretical spin density map of two ferromagnetic perovskites, YTiO3 and SrRuO3. We calculated the spin density using the standard density functional theory (DFT)+U method, where the Hubbard U correction is applied to the Ti and Ru ions, and with the pseudo-hybrid ACBN0 method, where the Hubbard U parameters are determined self-consistently. The ACBN0 calculations yielded a large value of the Hubbard U of the oxygen 2p orbitals. We also used the screened hybrid HSE06 functional, which is widely used to describe the electronic structure of oxides. We used the Quantum Theory of Atoms in Molecules (QTAIM) theory and integrated the spin density in the atomic basins instead of projecting on atomic orbitals. This way, our results can be compared to experimental reports as well as to other DFT calculations.

Breaking the ring: 53Cr-NMR on the Cr8Cd molecular nanomagnet

An accurate experimental characterization of finite antiferromagnetic (AF) spin chains is crucial for controlling and manipulating their magnetic properties and quantum states for potential applications in spintronics or quantum computation. In particular, finite AF chains are expected to show a different magnetic behaviour depending on their length and topology. Molecular AF rings are able to combine the quantum-magnetic behaviour of AF chains with a very remarkable tunability of their topological and geometrical properties. In this work we measure the 53Cr-NMR spectra of the Cr8Cd ring to study the local spin densities on the Cr sites. Cr8Cd can in fact be considered a model system of a finite AF open chain with an even number of spins. The NMR resonant frequencies are in good agreement with the theoretical local spin densities, by assuming a core polarization field AC = −12.7 T μB−1. Moreover, these NMR results confirm the theoretically predicted non-collinear spin arrangement along the Cr8Cd ring, which is typical of an even-open AF spin chain.

Chrysin/β-cyclodextrin supramolecular system: a quantum mechanical investigation

Chrysin is a bioflavonoid which possesses a wide range of important biological activities. In present study, we used a quantum mechanical approach to shed light on the antioxidant ability and antioxidant mechanism of chrysin to scavenge hydroxyl radical (˙OH) in solution phase. The analysis of the theoretical bond dissociation enthalpy (BDE) values and spin density of the radicals to determine the delocalization possibilities at B3LYP/6-311++G** level clearly shows the importance of the A-ring and the 7-OH group in antioxidant reactivity. In the next step, the inclusion of chrysin with β-CD has been investigated extensively using theoretical methods. Density functional theory (M05-2X) employing the 6–31+G* basis set has been used to research the lowest energy geometry for all studied complexes in gas and water mediums. The results of calculations show that the A ring of chrysin with large cavity side of β-CD significantly formed the favorable β-CD/chrysin complex in both phases. Our observations also inferred that hydrogen bonds were the key interactions which stabilized the chrysin/β-CD complex. We used Bader’s atoms in molecules (AIM) theory to perform a topological study on chrysin/β-CD complex to emphasize the hydrogen bonds. According to our findings, the process of inclusion increased not only the solubility of chrysin but also its antioxidant potential.

H‐Mediated Magnetic Interactions between Layers in a 2D MnII–Dicyanamide Polymer: Neutron Diffraction, DFT, and Quantum Monte Carlo Calculations

We report neutron‐diffraction investigations of the quasi‐2D MnII(dca)2(pym)(H2O) (pym = N2C4H4) compound, where high‐spin MnII ions are bridged by dicyanamide anions, [N(CN)2]– (herein abbreviated dca). Inside the layers, Mn2+ ions are connected by single or double dca bridges. The magnetic phase diagram was established by neutron diffraction on a single crystal. In the low‐field phase, the MnII ions are antiferromagnetically ordered in the layers, with moments nearly parallel to the c axis, and the layers are antiferromagnetically coupled. The spin‐flop phase corresponds to ferromagnetic coupling between the antiferromagnetic layers, in which the MnII moments are nearly perpendicular to the c axis. The induced spin‐density distribution in the paramagnetic phase, determined by polarized neutron diffraction, visualizes the superexchange pathways through the dca ligands within the layers and through H bonding between neighboring layers. The theoretical spin density obtained by bidimensional periodic DFT calculations is compared with the experimental results. Furthermore, quantum Monte Carlo simulations have been performed to compare the DFT results with experimental susceptibility measurements.

Spin density in YTiO3 : I. Joint refinement of polarized neutron diffraction and magnetic x-ray diffraction data leading to insights into orbital ordering

Orbital ordering below 30 K was previously observed in the ferromagnetic ${\text{YTiO}}_{3}$ compound both by polarized neutron diffraction (PND) and x-ray magnetic diffraction (XMD). In this paper we report a procedure for the joint refinement of a unique spin-density model based on both PND and XMD data. The distribution of the unpaired $3d$ electron of titanium is clearly seen on the magnetization density reconstructed by the maximum entropy method from the PND data collection at 5 K. The ${\text{Ti}}^{3+}$ $3d$ orbital populations obtained by joint model refinement are discussed in terms of the orbital ordering scheme. Small but significant magnetic moments on apical oxygen ${\text{O}}_{1}$ and yttrium atoms are found. The agreement between experimental and theoretical spin densities obtained using density functional theory is discussed.

Structure and properties of the anions MF4−, MCl4− and MBr4− (M = C, Si, Ge)

Density functional theory (DFT), Møller–Plesset (MP2) and coupled cluster with single and double substitutions including non-iterative triple excitations (CCSD(T)) calculations on the anions MX4−, with M = C, Si, Ge and X = F, Cl, Br, show that GeF4−, SiCl4−, GeCl4− and SiBr4− prefer a C2v conformation, but CCl4− is an elongated C3v structure. CBr4− has Td symmetry in MP2, but is slightly more stable in elongated C3v form with DFT and CCSD(T). GeBr4− has Td symmetry. CF4− and SiF4− are unstable with respect to loss of an electron. Vertical electron affinities (EAs) are negative also for CCl4 and SiCl4, and close to zero for GeF4 and SiBr4. Adiabatic EAs range from 0.47 eV for SiCl4 to 1.78 eV for GeBr4. The lowest excited states at Td symmetry are 2T2 resonances with energies of 2.1–3.5 eV, resulting from excitation of the a1 singly occupied molecular orbital to vacant t2 orbitals. Vertical excitation energies (VEEs) and vibrational frequencies are given for the most stable anionic geometries. Comparison with experimental VEEs for CCl4− is made. From dissociation energies of MX4, MX4−, MX3 and MX3−, appearance energies of X−, MX3−, X2− and MX2− were calculated. Most were found to be in reasonable agreement with experimental values. Theoretical spin densities and g-factors have been compared with experimental results available for CCl4−, SiCl4− and GeCl4−.

Characterizing Radiation-Induced Oxidation of DNA by Way of the Monohydrated Guanine−Cytosine Radical Cation

The interaction of one water molecule with the guanine-cytosine radical cation has been studied with ab initio and density functional methods in order to help elucidate the nature of oxidized aqueous DNA. The theoretical spin density of [GC]*(+) reveals that the radical center is localized on guanine. The adiabatic ionization potential lowers from 7.63 to 6.71 eV in concurrence with the formation of the Watson-Crick base pair and hydration by one water molecule. A natural bond orbital analysis of partial charges shows that approximately 80% of the positive charge persists on guanine upon hydration and formation of the Watson-Crick base pair with cytosine. Hydration energies were computed with second-order Z-averaged perturbation theory (ZAPT2) using the aug-cc-pVDZ basis set at 11 stationary points on the B3LYP/DZP++ potential energy surface. The hydration energy at the global minimum is 14.2 kcal mol(-1). The lowest energy structures correspond to hydration near the glycosidic bond sites. Structural changes in the Watson-Crick base pair are predominantly seen for monohydration in the groove regions of double-helix DNA.

Radical Dynamics of Puerarin as Revealed by Laser Flash Photolysis and Spin Density Analysis

Puerarin, a C-glycoside of daidzein, forms upon direct photoexcitation in acetonitrile an excited-state with a lifetime of 4.2 micros assigned by oxygen quenching and sensitized formation of triplet zeaxanthin as a triplet and phenoxyl radicals of ms lifetime insensitive to oxygen and with spin density delocalized over the ACB isoflavonoid ring system, [ACB]*, as shown by laser flash photolysis and theoretical spin density calculations. Photoexcitation of A-ring 7-phenolate puerarin yields a [AC]* radical, which converts into the [ACB]* radical with a rate constant of 3.6 x 10(5) s(-1) in 5% methanolic acetonitrile in a process triggered by B-ring deprotonation (4'-phenol). For the 7-phenolate with the 4'-phenol derivatized to yield a propyl anisole, no rearrangement of the initially formed [AC]* radical was observed. With the A-ring phenol derivatized, the 7-propyl-4'-phenolate forms a radical with spin density delocalized over the CB ring system, [CB]*, together with a minor fraction of [ACB]* due to propyl radical dissociations confirmed by BDE-calculations. Dianionic puerarin forms initially the [ACB]* radical, which is converted into the [CB]* radical in a slower process (1.6 x 10(4) s(-1)) assigned to 7-methylation. The radical dynamics is discussed in relation to puerarin/carotenoid antioxidant synergism at water/lipid interphases.

Electron density distribution of an oxamato bridged Mn(II)-Cu(II) bimetallic chain and correlation to magnetic properties.

The electron density distribution of the ferrimagnetic MnCu(pba)(H2O)3.2H2O chain compound, where pba stands for 1,3-propylenebis(oxamato), has been derived from high resolution X-ray diffraction measurements at 114 K using a multipolar model. The analysis of the chemical bonding has been carried out through the "Atoms in Molecules" formalism and thoroughly interpreted with regards to the strong intrachain and weak interchain magnetic couplings. The topological properties of the electron density on the oxamato bridge indicate large electron delocalization and conjugation effects, in addition to high charge transfer from both metals to the bridge. The resulting positive charges on Mn (+1.45 e) and Cu (+1.56 e) induce charge polarization of the bridge, leading to a shift of electron density from the central C atoms to the metal coordinating O and N atoms. The Mn-bridge interactions are mainly closed-shell interactions with low electron density at the corresponding bond critical points, whereas the Cu-bridge interactions exhibit significant covalent character. The Cu-N bonds are moreover stronger than the Cu-O bonds. The 3d Cu and Mn orbital populations are consistent with pyramidal and regular octahedral environments, respectively, in agreement with the loss of degeneracy due to ligand field effects. Interchain interaction pathways are evidenced by the existence of four bond critical points in hydrogen bond regions. Finally, these intrachain and interchain bonding features are correlated to the results of experimental and theoretical spin density distributions, as well as magnetic measurements.

Investigation of ferromagnetic spinel semiconductors by hyperfine interactions of implanted nuclear probes

The semiconducting ferromagnetic spinel compounds CdCr 2Se 4, CdCr 2S 4, HgCr 2Se 4 and CuCr 2Se 4 (metallic) were investigated by the perturbed angular correlations (PAC) method with the radioactive probes 111In, 111mCd, 111Ag, 117Cd, 199mHg and 77Br. The probes were implanted at the ISOLDE on-line separator (CERN-Geneva) into single crystals. From the time dependence of the PAC spectra and the measured hyperfine interaction parameters: electric field gradient and magnetic hyperfine field, the probe positions and the thermal behavior of the probes could be determined. Cd, Ag and Hg are substituted at the A-site, In at the A- and B-site in the semiconducting compounds and Br at the anion position. Electric and magnetic hyperfine fields were used as test quantities for theoretical charge and spin density distributions of LAPW calculations (WIEN97).

Exploiting the theoretical electron and spin densities

Exploiting the theoretical electron and spin densities

Experimental and Theoretical Spin Density in a Ferromagnetic Molecular Complex Presenting Interheteromolecular Hydrogen Bonds [J. Am. Chem. Soc. 1999, 121, 10126-10133

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionORIGINAL ARTICLEThis notice is a correctionExperimental and Theoretical Spin Density in a Ferromagnetic Molecular Complex Presenting Interheteromolecular Hydrogen Bonds [J. Am. Chem. Soc.1999, 121, 10126-10133].Yves Pontillon, Takeyuki Akita, Andre Grand, Keiji Kobayashi, Eddy Lelievre-Berna, Jacques Pécaut, Eric Ressouche, and Jacques SchweizerCite this: J. Am. Chem. Soc. 1999, 121, 47, 11026Publication Date (Web):November 4, 1999Publication History Published online4 November 1999Published inissue 1 December 1999https://doi.org/10.1021/ja995537vCopyright © 1999 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views202Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (13 KB) Get e-AlertsSupporting Info ()»Supporting Information Supporting Information Get e-Alerts

PE 2PF 6·2/3THF analysed by high resolution 13C-CP-MAS-NMR

We report high resolution 13C nuclear magnetic resonance measurements of the quasi-one dimensional organic conductor (PE) 2PF 6·2/3THF (PE = perylene, THF = tetrahydrofurane) including magic angle spinning and cross polarisation between 13C and 1H. In the spectrum there are found two types of different line groups in the metallic phase. The molecules of the conducting stack exhibit pairs of Knight shifted lines, having the same temperature dependence as the susceptibility of the conduction electrons, whereas the molecules which separate the conducting stacks from each other show only chemically shifted lines. Measurements with the gated decoupling technique allow to identify carbons bonded to protons. The complete line assignment is further controlled by calculated Knight shift positions from theoretical spin densities. A conclusive picture is thus obtained.

Additivity of the Electronic Meta-Substituent Effect in 3,5-Disubstituted Cumyl Radicals Assessed by the EPR D Parameter of 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals.

The D parameter (EPR zero-field splitting) of the 3,3',5,5'-tetrasubstituted triplet diradicals 6 (X = X' = H, NO(2), CH(3), OAc, OCH(3), NH(2), and OH) were determined in a MTHF matrix at 77 K and serves as a spectroscopic tool for the determination of electronic substituent effects in multiply-substituted benzyl-type monoradicals through its spin density dependence. The linear correlation (m = 2.00 +/- 0.01, r(2) = 0.974) of the experimental D values of these meta-disubstituted triplet diradicals 6 versus the 3,3'-disubstituted triplet diradicals 5 demonstrates the additivity of the electronic meta-substituent effect in the corresponding 3,5-disubstituted cumyl monoradicals 4 and is corroborated by theoretical (PM3-AUHF) spin density calculations for the latter. Thus, the combined use of the experimental D parameter and semiempirically calculated alpha spin density has provided for the first time the unambiguous demonstration of the additivity of electronic effects exerted by meta-substituents in the cumyl monoradicals 4.

Electronic Structures and Endor Spectra of Polarons in Poly(p-Phenylene Vinylene)

Abstract Polaron states in electroluminescent conjugated polymer, poly(p-phenylene vinylene) (PPV) are studied theoretically. The Pariser-Parr-Pople model which includes a long range electron-electron interaction is applied to a π electron system on a PPV chain. Using the unrestricted Hartree-Fock method, we calculate the properties of polarons, such as the structures of spin and charge densities. Electron-nuclear double-resonance spectra calculated from the theoretical spin density of a polaron well explain corresponding experiments in undoped, oriented PPV.

Electronic Structure of Neutral Tryptophan Radicals in Ribonucleotide Reductase Studied by EPR and ENDOR Spectroscopy

Two different tryptophan radicals (Wa• and Wb•) with lifetimes of several minutes at room temperature are formed during the reconstitution of the diiron center in the Escherichia coli ribonucleotide reductase mutant protein R2 Y122F. Detailed hyperfine parameters are for the first time determined for protein-linked oxidized neutral tryptophan radicals. Wa• is freeze-trapped and investigated by EPR and ENDOR in protonated and selectively deuterated proteins at 20 K. Two hyperfine couplings from the β-methylene protons, hyperfine tensors of two α-protons, and the complete nitrogen hyperfine tensor are determined. Based on the absence of a large hyperfine coupling from the N−H proton, which would be expected for a cation radical, and on comparison of the experimental data with theoretical spin densities from density functional calculations, Wa• is assigned to an oxidized neutral tryptophan radical. A small anisotropic hyperfine coupling detected in selectively deuterated Wa• is tentatively assigned to a prot...

An imino nitroxide free radical: Experimental and theoretical spin density and electronic structure

An imino nitroxide, the 2-(3-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl (m-NPIM), a potential building block for molecular-based magnets, was investigated by conventional and polarized neutron single crystal diffraction. Several methods, including the recently developed Default Model Maximum of Entropy (DMME) technique, were used to reconstruct the spin density distribution in the radical. Quantitative (atomic spin populations) and qualitative (shape of the spin density around the nuclei) results were obtained. Our data provide the basis for the discussion of the electronic configuration of imino nitroxides. The nodes of the singly occupied molecular orbital (SOMO) were directly observed. The spin polarization effect, which gives rise to negative spin densities of several atomic sites in the radical and is liable for the spin density transfer to the nitrophenyl group, is discussed. The spin distribution is compared to that previously observed experimentally in nitronyl nitroxides and with theoretical predictions obtained by density functional first principle calculations.

Ligand Substitution Effect on Optical Properties in Conducting Tetraazaporphyrines

ABSTRACTSelf-consistent Density Functional calculations have been performed on a variety of planar conjugated Ni-centered macrocycles with a basic tetraazaporphyrinic core and dithiolene groups (PZ) or fused-benzo groups (PC). Theoretical energy diagrams, charge and spin distributions and densities of states have been obtained in order to understand the electronic structure modifications due to peripheral ligand substitution. The substituents role in altering electronic properties and charge distribution of the porphyrazine macrocycles has been used to interpret the observed variations in optical absorption profiles. In the Q-band (∼ 680 nm) region, a single peak is seen for high symmetry (D4h) macrocycles and a double peak for lower symmetry (D2h and C2v) systems. Calculated intensities and band splittings are compared in detail with qualitative molecular orbital models and experiment in the visible and UV regions. Predictions are made for the infrared absorption and semiconducting band gap.

Experimental spin density in the first purely organic ferromagnet: the β para-nitrophenyl nitronyl nitroxide

A purely organic ferromagnet, the β form of the para-nitrophenyl nitronyl nitroxide (p-NPNN), was investigated by conventional and polarized neutron diffraction. Several approaches were used to reconstruct the spin density distribution in the radical. Quantitative results in the form of atomic spin populations were obtained, and are discussed in comparison with those previously obtained experimentally for a related phenyl-substituted nitronyl nitroxide. The spin delocalization and/or spin polarization effects are responsible for the transfer of the spin density to the nitrophenyl group. In the framework of the McConnell mechanism these effects play a key role in the intermolecular ferromagnetic interaction, which passes through both the nitro and the phenyl fragments. Hartree-Fock-based ab initio theoretical spin densities found in the literature are inconsistent with the experimental results.