Spin Density Changes Research Articles

Regarding certain stages of the development of quantum chemistry in Russia: Experience from the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies

Objectives. To analyze the history of the development of quantum chemistry and software for quantum chemical calculations in Russia at the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies of RTU MIREA.Results. This work presents a historical overview of the development of quantum chemistry at the Ya.K. Syrkin Department of Physical Chemistry from Academician Ya.K. Syrkin to Professor V.R. Flid. It provides a summary of the work with the participation of the author in 1980s–1990s. Quantum-chemical models used to describe some of the intercalation reactions in a bond are considered in comparison with the well-known Woodward–Hoffman and Fukui approaches. The work outlines fundamentals of studies on the design of bifunctional compounds.Conclusions. The physical significance of the exchange interaction constant is given a visual meaning: it establishes the change in spin density on the metals forming complexes of the type in question when passing from isolated cations in the composition of the complexes. The work provides recommendations to synthetic chemists regarding the selection of components in the synthesis of magnetic sublattices of bifunctional materials. It also examines the high level of scientific research carried out at the Ya.K. Syrkin Department of Physical Chemistry and its relevance to the world science level.

Regarding certain stages of the development of quantum chemistry in Russia: Experience from the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies

Objectives. To analyze the history of the development of quantum chemistry and software for quantum chemical calculations in Russia at the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies of RTU MIREA.Results. This work presents a historical overview of the development of quantum chemistry at the Ya.K. Syrkin Department of Physical Chemistry from Academician Ya.K. Syrkin to Professor V.R. Flid. It provides a summary of the work with the participation of the author in 1980s–1990s. Quantum-chemical models used to describe some of the intercalation reactions in a bond are considered in comparison with the well-known Woodward–Hoffman and Fukui approaches. The work outlines fundamentals of studies on the design of bifunctional compounds.Conclusions. The physical significance of the exchange interaction constant is given a visual meaning: it establishes the change in spin density on the metals forming complexes of the type in question when passing from isolated cations in the composition of the complexes. The work provides recommendations to synthetic chemists regarding the selection of components in the synthesis of magnetic sublattices of bifunctional materials. It also examines the high level of scientific research carried out at the Ya.K. Syrkin Department of Physical Chemistry and its relevance to the world science level.

Simulations of the Aqueous "Brown-Ring" Complex Reveal Fluctuations in Electronic Character.

Ab initio molecular dynamics (AIMD) simulations of the aqueous [Fe(H2O)5(NO)]2+ "brown-ring" complex in different spin states, in combination with multiconfigurational quantum chemical calculations, show a structural dependence on the electronic character of the complex. Sampling in the quartet and sextet ground states show that the multiplicity is correlated with the Fe-N distance. This provides a motivation for a rigid Fe-N scan in the isolated "brown-ring" complex to investigate how the multiconfigurational wave function and the electron density change around the FeNO moiety. Our results show that subtle changes in the Fe-N distance produce a large response in the electronic configurations underlying the quartet wave function. However, while changes in spin density and potential energy are pronounced, variations in charge are negligible. These trends within the FeNO moiety are preserved in structural sampling of the AIMD simulations, despite distortions present in other degrees of freedom in the bulk solution.

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Study of Charge Transfer in Radical Reactions of Hydrogen Atom Detachment Involving Triplet Nitro Compounds

The hypothesis of the formation of a radical anion during the dark stage of the reaction of hydrogen atom transfer from the hydrogen sulfide molecule to a triplet molecule of a nitro compound has been verified by quantum chemistry methods. Analysis of changes in spin density during the reaction, as well as analysis of changes in electron density using real-time time-dependent density functional theory (RT TD-DFT), has not revealed charge transfer between the reactants. At the same time, the detected weak dependence of the energy parameters of the reaction on the dielectric permittivity of the solvent supports the hypothesis of charge transfer.

The reaction of O(3P) with alkynes: a dynamic and computational study focusing on formyl radical production.

Production of formyl radical, HCO, from reactions of O(3P) with alkynes (acetylene, propyne, 1-butyne, and 1-pentyne) has been investigated using cavity ringdown laser absorption spectroscopy (CRDLAS) and computational methods. No HCO was detected from reaction with acetylene, while the amount of HCO increased for propyne and 1-butyne, dropping off somewhat for 1-pentyne. These results differ from trends previously observed for reactions of O(3P) with alkenes, which exhibit the largest HCO production for the smallest alkene and drop off as the alkene size increases. Computational studies employing density functional and coupled cluster methods have been employed to investigate the triplet and singlet state pathways for HCO production. Because intersystem crossing (ISC) has been shown to be important in these processes, the minimum energy crossing point (MECP) between the triplet and singlet surfaces has been studied. We find the MECP for propyne to possess C1 symmetry and to lie lower in energy than previous studies have found. Natural Bond Orbital and Natural Resonance Theory analyses have been performed to investigate the changes in spin density and bond order along the reaction pathways for formation of HCO. Explanations are suggested for the trend in HCO formation observed for the alkynes. The trend in alkyne HCO yield also is compared and contrasted with the trend previously observed for the alkenes.

Time-resolved electron spectroscopy for chemical analysis of photodissociation: Photoelectron spectra of Fe(CO)5, Fe(CO)4, and Fe(CO)3.

The prototypical photoinduced dissociation of Fe(CO)5 in the gas phase is used to test time-resolved x-ray photoelectron spectroscopy for studying photochemical reactions. Upon one-photon excitation at 266 nm, Fe(CO)5 successively dissociates to Fe(CO)4 and Fe(CO)3 along a pathway where both fragments retain the singlet multiplicity of Fe(CO)5. The x-ray free-electron laser FLASH is used to probe the reaction intermediates Fe(CO)4 and Fe(CO)3 with time-resolved valence and core-level photoelectron spectroscopy, and experimental results are interpreted with ab initio quantum chemical calculations. Changes in the valence photoelectron spectra are shown to reflect changes in the valence-orbital interactions upon Fe-CO dissociation, thereby validating fundamental theoretical concepts in Fe-CO bonding. Chemical shifts of CO 3σ inner-valence and Fe 3p core-level binding energies are shown to correlate with changes in the coordination number of the Fe center. We interpret this with coordination-dependent charge localization and core-hole screening based on calculated changes in electron densities upon core-hole creation in the final ionic states. This extends the established capabilities of steady-state electron spectroscopy for chemical analysis to time-resolved investigations. It could also serve as a benchmark for how charge and spin density changes in molecular dissociation and excited-state dynamics are expressed in valence and core-level photoelectron spectroscopy.

C–H Bond Activation on Bimetallic Two-Atom Co-M Oxide Clusters Deposited on Zr-Based MOF Nodes: Effects of Doping at the Molecular Level

Cluster-based density functional theory calculations show that energy barriers for the dissociative adsorption of propane on two-cation, Co-M oxide clusters supported on Zr-based nodes of NU-1000, a metal–organic framework material, vary from 57 to 9 kcal mol–1 based on the identity of the dopant. Systematic changes in spin density and positive partial charge on oxygen atoms bridging the two metal atoms (Co–O-M) are noted upon addition of dopants to cobalt, with increasing values of both giving lower enthalpic barriers to C–H scission. These observed correlations can be rationalized in terms of concepts applicable to bulk systems and provide target materials for synthesis.

Functional MRI during hyperbaric oxygen: Effects of oxygen on neurovascular coupling and BOLD fMRI signals

Hyperbaric oxygen (HBO) therapy is used to treat a number of ailments. Improved understanding of how HBO affects neuronal activity, cerebral blood flow (CBF) and blood-oxygenation-level dependent (BOLD) changes could shed light on the role of oxygen in neurovascular coupling and help guide HBO treatments. The goal of this study was to test two hypotheses: i) activation-induced CBF fMRI response is not dependent on hemoglobin deoxygenation, and ii) activation-induced BOLD fMRI is markedly attenuated under HBO. CBF and BOLD fMRI of forepaw stimulation in anesthetized rats under HBO at 3 atmospheres absolute (ATA) were compared with normobaric air. Robust BOLD and CBF fMRI were detected under HBO. Inflow effects and spin-density changes did not contribute significantly to the BOLD fMRI signal under HBO. Analysis of the T2⁎-weighted signal at normobaric air and 1, 2 and 3ATA oxygen in the tissue and the superior sagittal sinus showed a strong dependence on increasing inhaled [O2]. Spontaneous electrophysiological activity and evoked local-field potentials were reduced under HBO. The differences between normobaric air and HBO in basal and evoked electrical activity could not fully account for the strong BOLD responses under HBO. We concluded that activation-induced CBF regulation in the brain does not operate through an oxygen-sensing mechanism and that stimulus-evoked BOLD responses and the venous T2⁎-weighted signals still have room to increase under 3ATA HBO. To our knowledge, this is the first fMRI study under HBO, providing insights into the effects of HBO on neural activity, neurovascular coupling, tissue oxygenation, and the BOLD signal.

Probe‐assisted spin manipulation in one‐dimensional quantum dots

We study a spin structure that arises in a one‐dimensional quantum dot with zero total spin under the action of a charged tip of a scanning probe microscope in the presence of a weak magnetic field. The evolution of spin structure with changing the probe position is traced to show that the movable probe can be an effective tool to manipulate the spin. The spin structures are formed when the probe is located in certain regions along the dot due to Coulomb interaction of electrons as they are redistributed between the two sections in which the quantum dot is divided by the potential barrier created by the probe. There are two main states: spin‐polarized and non‐polarized ones. The transition between them is accompanied by a spin precession governed by the Rashba spin–orbit interaction induced by the electric field of the probe. In the transition region the spin density changes strongly while charge distribution remains nearly unchanged. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution.

Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site that need to be controlled to optimize complexes for photocatalytic hydrogen production and selective carbon-hydrogen bond activation. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst with an electron deficiency at the Fe centre, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16 - 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.

Nature of electrogenerated intermediates in nitro-substituted nor-β-lapachones: the structure of radical species during successive electron transfer in multiredox centers.

Electrochemical, spectroelectrochemical, and theoretical studies of the reduction reactions in nor-β-lapachone derivatives including a nitro redox center showed that reduction of the compounds involves the formation of several radical intermediates, including a biradical dianion resultant from the separate reduction of the quinone and nitro groups in the molecules. Theoretical descriptions of the corresponding Fukui functions f(αα)⁺ and f(ββ)⁺(r) and LUMO densities considering finite differences and frozen core approximations for describing the changes in electron and spin densities of the system allowed us to confirm these results. A description of the potential relationship with the obtained results and biological activity selectivity indexes suggests that both the formation of stable biradical dianion species and the stability of the semiquinone intermediates during further reduction are determining factors in the description of their biological activity.

Comparative electron paramagnetic resonance investigation of reduced graphene oxide and carbon nanotubes with different chemical functionalities for quantum dot attachment

Electron paramagnetic resonance (EPR) spectroscopy has been applied to different chemically treated reduced graphene oxide (rGO) and multiwalled carbon nanotubes (CNTs). A narrow EPR signal is visible at g = 2.0029 in both GO and CNT-Oxide from carbon-related dangling bonds. EPR signals became broader and of lower intensity after oxygen-containing functionalities were reduced and partially transformed into thiol groups to obtain thiol-functionalized reduced GO (TrGO) and thiol-functionalized CNT (CNT-SH), respectively. Additionally, EPR investigation of CdSe quantum dot-TrGO hybrid material reveals complete quenching of the TrGO EPR signal due to direct chemical attachment and electronic coupling. Our work confirms that EPR is a suitable tool to detect spin density changes in different functionalized nanocarbon materials and can contribute to improved understanding of electronic coupling effects in nanocarbon-nanoparticle hybrid nano-composites promising for various electronic and optoelectronic applications.

Study of Fe<sub>75</sub>Al<sub>25</sub> Alloys by the Mössbauer Spectroscopy Positron Annihilation Lifetime Spectroscopy (PALS), XRD and SEM

The structure, point defect and ordering parameter of Fe25Al samples is examined with the Mössbauer spectroscopy Positron Annihilation Lifetime Spectroscopy XRD and SEM. The studies are carried out for samples in as-cast state and after heat treatments: annealing for 24 hours at 900°C (or 1050°C) and either slow cooling with furnace or quenching to oil. Among the research method used, Mössbauer spectroscopy for determination of hyperfine structure parameters was adopted. These parameters, sensitive to changes in spin and charge electron densities in the nearest neighbourhood of a Mössbauer isotope nucleus, caused by specific configurations of atoms, are directly connected with the degree of ordering of a compound. Spectral analysis has been carried out using an authors’ software developed based on a theoretical model relating the shape of a Mössbauer spectrum to the sample microstructure. It has been shown that Mössbauer spectroscopy enables quantitative evaluation of the degree of ordering of phases occurring in samples characterised by large graining, in the case of which it is not possible to determine the long-range order parameter by X-ray diffraction. The PALS method only one type of defects is detected. The positron lifetime in these defects (V) suggests that they are quenched-in Fe-monovacancies (VFe). The vacancy concentration strongly depends on the rate of cooling.

First-principles study of O2 activation on ligand-protected Au32 clusters

Poly(N-vinyl-2-pyrrolidone) (PVP) is often used to protect active Au clusters from coalescence. The influences of PVP on the O2 adsorption on Au32 clusters were investigated using density functional theory calculations. Various low-lying structures of O2:Au32 and O2:Au32:PVP complexes, in which the Au32 is either neutral or anionic and the O2 is either molecular or dissociative, were identified. The PVP influences were evaluated in terms of the changes in geometry, adsorption energy, charge redistribution, spin density, and density of states upon PVP pre-adsorption. Our calculations reveal that PVP weakly adsorbs on the cluster surface, with rather small changes in the structural, geometrical and electronic properties that are relevant to the O2 activation. The activity of neutral or anionic Au32 towards O2 is kept or slightly enhanced by PVP because of the cooperative adsorption of PVP and O2. This is the structural basis of choosing PVP as the protective ligand for Au clusters.

Spin density in frustrated magnets under mechanical stress: Mn-based antiperovskites

In this paper we present results of first-principle calculations of the noncollinear spin density distribution in the systems with frustrated triangular magnetic structure [Mn-based antiperovskites, Mn3AN (A=Ga,Zn)] in the ground state and under external mechanical biaxial strain. We show that the spin density in the (111)-plane of the unit cell is distinctly nonuniform. In particular, both the direction and the magnitude of spin density depend strongly on the distance from Mn site within atomic sphere. We show that the change of spin density under stress exhibits more diverse features than assumed in the rigid spin model. There are regions within atomic sphere where the spin density rotates in opposite directions under stress.

Spin reorientation in Al/Metglas 2605S2/Al trilayers induced by magnetoelastic effect

Mössbauer spectroscopy, in a broad temperature interval of 12–425 K, has been applied to investigate the spin reorientation dynamics caused by the temperature induced magnetoelastic effect on Al(x μm)/Metglas 2605S2 (20 μm)/Al(x μm) trilayers (x=0; 2.5; 5 and 20). It was found that the angle between the average sample magnetization and gamma ray direction (perpendicular to the sample plane) depends on the Al layer thickness. For temperatures smaller than 260 K, saturation of spin reorientation, which can be controlled by adjusting the Al thickness, was reached for Al thicknesses larger than and equal to 5 μm. For a 20 μm Al thickness, changes in the F57e atom spin and charge densities have also been observed. A simple spin model has been proposed to describe qualitatively the spin reorientation effect as well as the influence of the Al thickness on the spin reorientation sensitivity.

Dual descriptors within the framework of spin-polarized density functional theory

Spin-polarized density functional theory (SP-DFT) allows both the analysis of charge-transfer (e.g., electrophilic and nucleophilic reactivity) and of spin-polarization processes (e.g., photophysical changes arising from electron transitions). In analogy with the dual descriptor introduced by Morell et al. [J. Phys. Chem. A 109, 205 (2005)], we introduce new dual descriptors intended to simultaneously give information of the molecular regions where the spin-polarization process linking states of different multiplicity will drive electron density and spin density changes. The electronic charge and spin rearrangement in the spin forbidden radiative transitions S(0)-->T(n,pi(*)) and S(0)-->T(pi,pi(*)) in formaldehyde and ethylene, respectively, have been used as benchmark examples illustrating the usefulness of the new spin-polarization dual descriptors. These quantities indicate those regions where spin-orbit coupling effects are at work in such processes. Additionally, the qualitative relationship between the topology of the spin-polarization dual descriptors and the vertical singlet triplet energy gap in simple substituted carbene series has been also discussed. It is shown that the electron density and spin density rearrangements arise in agreement with spectroscopic experimental evidence and other theoretical results on the selected target systems.

Ground- and Excited-State Properties of the Mixed-Valence Complex [(NH3)5RuIIINCRuII(CN)5]−

A computational study of the ground- and excited-state properties of the mixed-valence complex [(NH 3) 5Ru (III)NCRu (II)(CN) 5] (-) is presented. Employing DFT and TDDFT calculations for the complex in the gas phase and in aqueous solution, we investigate the vibrational and electronic structure of the complex in the electronic ground state as well as the character of the electronically excited states. The relevance of the various excited states for the intervalence metal-metal charge-transfer process in the complex is analyzed based on the change of charge density, spin density, and dipole moment upon photoexcitation as well as by a Mulliken-Hush analysis. Furthermore, those intramolecular modes, which are important for the charge-transfer process, are identified and characterized.

Bulk and surface analysis of Hägg Fe carbide(Fe5C2): a density functional theory study

A comprehensive density functional theory (DFT) study analysingthe bulk and various low Miller index surfaces of Hägg Fe carbide(Fe5C2), considered to be the active phase in Fe-catalysed Fischer–Tropsch synthesis (FTS),has been carried out. The DFT determined bulk structure of Hägg Fe carbide(Fe5C2) is found to be in good agreement with reported monoclinic(C 2/c) XRD data, independently of whether a monoclinic(C 2/c) or triclinic () bulk structure is used as input for calculations. Attention is focused on the constructionof a surface energy stability trend with subsequent correlation with particular surfaceproperties. It is found that a (010) Miller index plane results in the most stable surface(2.468 J m−2), while a (101) surface is the least stable(3.281 J m−2). The systematic comparison of calculated surface energies with surface properties such asthe number of dangling bonds and surface atom density (within a broken bond model), aswell as unrelaxed surface energies, relative ruggedness of surfaces, degree of surfacerelaxation upon optimization, total spin density changes of surfaces compared to the bulk,etc, result in only an approximate correlation with the surface stability trend in selectedcases. From the results it is concluded that the relative surface energies fall in a narrowrange and that a large number of additional surfaces may be defined, e.g. fromhigher Miller index planes, sharing similar surface energy values. The results serveto demonstrate the rich complexity and diverse nature of the Fe carbide phaseresponsible for FTS, effectively laying the foundation for further fundamental studies.