Spin Inversion Research Articles

Antitumor Platinium(IV) Prodrugs: A Systematic Computational Exploration of Their Reduction Mechanism by l-Ascorbic Acid.

The reduction mechanism of Pt(IV) anticancer prodrugs, still today a matter of debate, assisted by one of the dominant reductants in human plasma, that is l-ascorbic acid in its monodeprotonated form, has been computationally examined in this work. In order to check what should be the influence on the reduction rate of the identity of the ligands in axial and equatorial position, both cisplatin and oxaliplatin derivatives have been studied, varying the ligands in axial position in connection with the role they should play as bridges, trans leaving species, and proton acceptors. OH, OAc, Cl, and Br ligands have been tested as bridging/leaving ligands, whereas Cl and aspirin have been used as trans labile and less labile ligands, respectively. The most recent theoretical and experimental investigations have demonstrated that the generally adopted grouping of reduction mechanisms into inner- and outer-sphere does not properly take into account all the viable alternatives. Therefore, inner-sphere mechanisms, classified as ligand-bridged, ligand-bridged-H transfer and enolate β-carbon attack, have been explored for all the complexes under investigation. Concerning the outer-sphere mechanism, redox potentials have been calculated adopting a recently proposed procedure based on the separation between electrochemical and chemical events to evaluate their propensity to be reduced. Moreover, according to the hypothesis that the outer-sphere reduction mechanism involves the sequential addition of two electrons causing the formation of a Pt(III) intermediate, the possibility that singlet and triplet pathways can cross for the Pt(IV) cisplatin derivative having two chlorido ligands in axial position has been explored in detail. Results show that the mechanism indicated as base-assisted outer sphere can become competitive with respect to the inner one if two singlet-triplet spin inversions occur. Results presented here are helpful in addressing synthetic strategies as they show that Pt(IV) prodrugs propensity to be reduced can be properly tuned and give indications on how this aim can be accomplished.

Theoretical investigation on acetylene cyclotrimerization catalysed by TiO2 and Ti

AbstractThe reaction mechanisms for acetylene cyclotrimerization using TiO2 and Ti catalysts were studied by the density functional theory (DFT) method. Interestingly, the reaction catalysed by TiO2 occurs on the potential energy surface (PES) with a singlet state. For the same reaction catalysed by Ti, spin‐orbit coupling (SOC) calculations were performed to discuss spin inversion between the triplet and singlet PESs. The chance that an electron hops in the vicinity of the minimum‐energy crossing point (MECP) was verified regarding the Landau‐Zener model. The possibilities of single ( ) and double ( ) at MECP1 (SOC = 253.38 cm−1) are about 0.35 and 0.46, respectively. The energetic span model developed by Kozuch was applied in the above reactions. The turnover frequency (TOF)‐determining transition state (TDTS) and TOF‐determining intermediate (TDI) were verified. The TOF value indicates that Ti is a more active catalyst compared with TiO2 in C2H2 cyclotrimerization.

Thermal Redistribution of Exciton Population in Monolayer Transition Metal Dichalcogenides Probed with Plasmon–Exciton Coupling Spectroscopy

Inversion symmetry breaking and spin–orbit coupling result in spin-splitting of both valence and conduction bands in transition metal dichalcogenide (TMDC) monolayers. The optical transitions between band edges with opposite spins are termed dark excitons that are decoupled with in-plane polarized photons. Here, we find that the presence of dark excitons modifies the temperature-dependent plasmon–bright-exciton coupling strength of a TMDC monolayer interacting with a single plasmonic nanocavity. Quite interestingly, we observe that the modifications are in an opposite manner for WS2 and MoS2 monolayers. Coupled-oscillator analysis reveals that the WS2–nanocavity coupling strength increases with rising temperature, yet that for the MoS2–nanocavity diminishes, which both follow the temperature evolution of the respective exciton oscillator strength obtained by fitting the reflectance spectra of pristine TMDC monolayers with a multi-Lorentz oscillator model. Full-wave electromagnetic simulations with experim...

Probability of Inversion of a Large Spin in the Form of an Asymptotic Expansion in a Series of Bessel Functions

An exact expression for the probability of inversion of a large spin is established in the form of an asymptotic expansion in the series of Bessel functions with orders belonging to an arithmetic progression. Based on the new asymptotic expansion, a formula for the inversion time of the spin is derived.

The influence of magnetic fields on calcium carbonate scale formation within monoethylene glycol solutions at regeneration conditions

One of the most discussed topics related to the effects of external magnetic fields (MF) on aqueous solutions is the influence on the scale formation of calcium carbonate (CaCO3). However, the extent of the effect of these forces on the scale formation in the non-aqueous solutions has not been investigated so far. So MFs will be applied to non-aqueous mixtures to find out the behavior of scale formation. This study presents the results of inorganic scale formation within MEG solutions containing Ca2+ and HCO3− ions, which has been investigated using both static and dynamic scale loop (DSL) evaluation techniques. Furthermore, the influence of MFs on scale formation using the dynamic technique has also been studied. Results were generated using brine/MEG solutions exposed to an external MF produced by a 0.65 T Neodymium magnet for 2.5 s. The degree of scale formation was examined by measuring the pressure build-up across a capillary coil as scale was developed. Moreover, differences in CaCO3 morphologies were evaluated for the exposed and blank trials via the DSL technique and compared with the results obtained from the static scale evaluation method.The results of this research have demonstrated that the short exposure (2.5 s) to a powerful MF can significantly reduce scale-formation in the rich MEG solutions within the capillary coil. This is due to the alteration of the proton spin inversion in the field of diamagnetic salts. Furthermore, a significant difference in CaCO3 morphology was observed for the scale formed during dynamic and static conditions. The generating results help to reduce the use of chemical scale inhibitors with MEG solution during the gas hydrate treatments, especially when the concentration of MEG in formation water is low and scale formation is more likely to occur.

Mechanistic details of the MnO+ + H2/D2 reaction through temperature-dependent kinetics and statistical modeling

The temperature dependencies of the reactions MnO+ + H2/D2 from 150 to 600 K are measured in a selected- ion flow tube apparatus. The reactions are approximately 15% efficient at room temperature with rate constants (k = 2.7 ± 0.7 × 10−10 cm3 s-1 for H2 and 1.5 ± 0.4 × 10−10 cm3 s-1 for D2 at 300 K) that vary as T-1.1±0.2 and T-1.3±0.2, respectively. Both Mn+ and MnOH+/D+ products are observed, with Mn+ being the minority product in the H2 reaction, but the majority product in the D2 reaction. Reaction coordinates are explored using a variety of density functional methods, with quantitative but not qualitative differences in the results. Unlike the analogous FeO+ + H2 reaction, no evidence for spin inversion prior to the rate-limiting transition state is found. A crossing between septet and quintet potential energy surfaces in the product well exists, and the possibility of spin inversion on the way to products is discussed. The reaction coordinates inform a statistical treatment of the reaction, which well reproduces all kinetic results, including the product branching inversion observed due to isotope substitution.

Measurement of T1e, T1N, T1HE, T2e, and T2HE by Pulse EPR at X-Band for Nitroxides at Concentrations Relevant to Solution DNP

Relaxation times were measured at X-band (9.5 GHz) at concentrations up to 20.8 mM for two nitroxides that are widely used in Overhauser dynamic nuclear polarization (ODNP) experiments. Carboxy proxyl (CP, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy) in water and tempol (TP, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) in toluene have been studied with oxygen removed. For comparison, solutions of CP were also studied in air-saturated solution, which is the typical preparation for ODNP experiments. T2 and T2HE (HE = Heisenberg exchange) were measured by electron spin echo. The spin magnetization recovery time constant from inversion recovery experiments, which we denote as T1e*, includes contributions from T1e and T1HE. In the absence of oxygen, values of T1e* for both radicals decrease with increasing concentration up to ~ 1 mM, then increase again towards 10 mM. The concentration dependence results from changes in the relative contributions from T1e and T1HE. In air-saturated solutions of CP T1e* decreases with concentration to about 200 ns at 1 mM, and then remains independent of further concentration increases. T1e and T1N were also measured with a digital saturation recovery spectrometer. Using the combined results from spin echo, inversion recovery, and saturation recovery we could extract the values of T1e, T1HE, T1N, T2e, and T2HE for both radicals in this fast tumbling regime.

Electron spin inversion in gated silicene nanoribbons

We study locally gated silicene nanoribbons as spin active devices. We find that the gated segments of nanoribbons with zigzag edge can be used to perform a spin inversion for the electron spins injected with an in-plane orientation. The strong intrinsic spin-orbit coupling for low Fermi energy in presence of an external vertical electric field provides a fast spin precession around the axis perpendicular to the silicene plane. The spin inversion length can be as small as 10 nm. On the other hand in the armchair nanoribbons the spin inversion occurs via the Rashba effect which is weak and the spin inversion lengths are of the order of $\mu$m.

Spin inversion in graphene spin valves by gate-tunable magnetic proximity effect at one-dimensional contacts

Graphene has remarkable opportunities for spintronics due to its high mobility and long spin diffusion length, especially when encapsulated in hexagonal boron nitride (h-BN). Here, we demonstrate gate-tunable spin transport in such encapsulated graphene-based spin valves with one-dimensional (1D) ferromagnetic edge contacts. An electrostatic backgate tunes the Fermi level of graphene to probe different energy levels of the spin-polarized density of states (DOS) of the 1D ferromagnetic contact, which interact through a magnetic proximity effect (MPE) that induces ferromagnetism in graphene. In contrast to conventional spin valves, where switching between high- and low-resistance configuration requires magnetization reversal by an applied magnetic field or a high-density spin-polarized current, we provide an alternative path with the gate-controlled spin inversion in graphene.

Supercolloidal Spinners: Complex Active Particles for Electrically Powered and Switchable Rotation

AbstractA class of supercolloidal particles that controllably spin about their central axis in AC electric fields is reported. The rational design of these “microspinners” enables their rotation in a switchable manner, which gives rise to several interesting and programmable behaviors. It is shown that due to their complex shape and discrete metallic patches on their surfaces, these microspinners convert electrical energy into active motion via the interplay of four mechanisms at different electric field frequency ranges. These mechanisms of rotation include (in order of increasing frequency): electrohydrodynamic flows, reversed electrohydrodynamic flows, induced charge electrophoresis, and self‐dielectrophoresis. As the primary mechanism powering their motion transitions from one phenomenon to the next, these microspinners display three directional spin inversions (i.e., from clockwise to anticlockwise, or vice versa). To understand the mechanisms involved, this experimental study is coupled with scaling analyses. Due to their frequency‐switchable rotation, these microspinners have potential for applications such as interlocking gears in colloidal micromachines. Moreover, the principles used to power their switchable motion can be extended to design other types of supercolloidal particles that harvest electrical energy for motion via multiple electrokinetic mechanisms.

Large tunable valley splitting in edge-free graphene quantum dots on boron nitride.

Coherent manipulation of the binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid-state systems, whereas exploitation of the valley has only recently been started, albeit without control at the single-electron level. Here, we show that van der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunnelling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.

Density functional theory study on the geometric and electronic structures of Fe2O2 and the reaction of Fe2 + O2

Density functional theory calculations are performed on all possible structures of Fe2O2 using the hybrid B3LYP functional and the B3LYP functional combined with the broken-symmetry (BS) approach to obtain the most stable isomers. Based on the obtained stable isomers, the reaction mechanism of Fe2 + O2 toward rhombic Fe2(μ-O)2 is considered. The BS-singlet state of the rhombic Fe2(μ-O)22.1 is found to be the ground state of all Fe2O2 isomers. The 9A″ state of the open-cycle (η1-O)Fe2(µ-O) 2.11 and 3A state of the near-linear OFeOFe 1.4 are found to have the second and third lowest energy states, which are higher than the ground state 2.1 by 109.7 and 120.0 kJ mol−1, respectively. The lowest-lying energy states of the bare Fe2O2 clusters do not favor three-dimensional structures, but favor the linear and planar structures. Numerous electronic states are found for the symmetric optimized rhombic 2.1. The energy ordering of the BS-singlet ground state and the four lowest-lying states (9Ag, BS-septet, BS-triplet, and BS-quintet) at the B3LYP and BS-B3LYP levels agrees with the energy ordering obtained at the CCSD(T) level. The characteristic frequencies calculated for the stable isomers are similar to the previous theoretical and experimental studies. The singlet and nonet energy surfaces calculated for the reaction of Fe2 + O2 toward rhombic Fe2(μ-O)2 suggest that the reaction adiabatically proceeds with spin inversion from the nonet to singlet state and is expected to be fast because the reaction takes place with a very low barrier relative to that of the reactants (or without any activation energy). The NBO analyses show that the FeO bonds in the intermediates and final product exhibit both ionic and covalent characteristics and have highly polarities because of large charge transfer from Fe to O.

Recent Applications of [Cu(dap)2]Cl in Visible Light-Mediated Photoredox Catalysis

Since 2008, visible light-mediated photoredox catalysis has enjoyed a renaissance in organic synthesis. [1–5] The most commonly employed metal-based photoredox catalysts typically utilise second- and third-row transition metals featuring polypyridyl ligand systems such as [Ru(bpy) 3 ]Cl 2 (1). [1] In contrast, the synthetic applications of copper-based photoredox catalysts have received much less attention. [6] A class of photoactive homoleptic, phenanthroline-based copper(I) complexes, including [Cu (dap) 2 ]Cl (2), were first prepared some 35 years ago (Chart 1). [7,8] This class of complexes share several interesting properties that can be exploited to facilitate chemical transformations. The absorption of a photon in the visible range leads to a metal-to-ligand charge transfer (MLCT) excited state. Subsequent intersystem crossing (ISC) results in spin inversion and allows access to long-lived triplet photoexcited states. [9] In its photoexcited state, complex 2 is a strong reductant (-1.43 V versus saturated calomel electrode) capable of reducing organic acceptor molecules via single electron transfer (SET) processes. [8] The ensuing CuIIspecies can undergo a second SET process to oxidise an organic donor molecule, which reduces the catalyst back to its ground state and closes the catalytic cycle. [1] The coordinatively unsaturated nature of these copper(I) complexes typically provides lower coordinative stability relative to saturated octahedral complexes (e.g. 1). Also, in the ground state, these copper(I) complexes typically exhibit a distorted tetrahedral coordination geometry. Upon photoexcitation, oxidation of the metal centre occurs as a result of the population of the MLCT excited state. This causes a flattening distortion that results in a more square-planar coordination geometry. These two factors are primarily responsible for reducing excited state lifetimes. The present focus article aims to highlight recent applications of [Cu(dap) 2 ]Cl in organic synthesis.

Electron spin inversion in fluorinated graphene nanoribbons

We consider a dilute fluorinated graphene nanoribbon as a spin-active element. The fluorine adatoms introduce a local spin-orbit Rashba interaction that induces spin-precession for electron passing by. In the absence of the external magnetic field the transport is dominated by multiple scattering by adatoms which cancels the spin precession effects, since the direction of the spin precession depends on the electron momentum. Accumulation of the spin precession effects is possible provided that the Fermi level electron passes many times near the same adatom with the same momentum. In order to arrange for these conditions a circular n-p junction can be introduced to the ribbon by e.g. potential of the tip of an atomic force microscope. In the quantum Hall conditions the electron current gets confined along the junction. The electron spin interaction with the local Rashba field changes with the lifetime of the quasi-bound states that is controlled with the coupling of the junction to the edge of the ribbon. We demonstrate that the spin-flip probability can be increased in this manner by as much as three orders of magnitude.

Loss and Reformation of Ruthenium Alkylidene: Connecting Olefin Metathesis, Catalyst Deactivation, Regeneration, and Isomerization.

Ruthenium-based olefin metathesis catalysts are used in laboratory-scale organic synthesis across chemistry, largely thanks to their ease of handling and functional group tolerance. In spite of this robustness, these catalysts readily decompose, via little-understood pathways, to species that promote double-bond migration (isomerization) in both the 1-alkene reagents and the internal-alkene products. We have studied, using density functional theory (DFT), the reactivity of the Hoveyda-Grubbs second-generation catalyst 2 with allylbenzene, and discovered a facile new decomposition pathway. In this pathway, the alkylidene ligand is lost, via ring expansion of the metallacyclobutane intermediate, leading to the spin-triplet 12-electron complex (SIMes)RuCl2 (3R21, SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene). DFT calculations predict 3R21 to be a very active alkene isomerization initiator, either operating as a catalyst itself, via a η3-allyl mechanism, or, after spin inversion to give R21 and formation of a cyclometalated Ru-hydride complex, via a hydride mechanism. The calculations also suggest that the alkylidene-free ruthenium complexes may regenerate alkylidene via dinuclear ruthenium activation of alkene. The predicted capacity to initiate isomerization is confirmed in catalytic tests using p-cymene-stabilized R21 (5), which promotes isomerization in particular under conditions favoring dissociation of p-cymene and disfavoring formation of aggregates of 5. The same qualitative trends in the relative metathesis and isomerization selectivities are observed in identical tests of 2, indicating that 5 and 2 share the same catalytic cycles for both metathesis and isomerization, consistent with the calculated reaction network covering metathesis, alkylidene loss, isomerization, and alkylidene regeneration.

Externally controlled high degree of spin polarization and spin inversion in a conducting junction: Two new approaches

We propose two new approaches for regulating spin polarization and spin inversion in a conducting junction within a tight-binding framework based on wave-guide theory. The system comprises a magnetic quantum ring with finite modulation in site potential is coupled to two non-magnetic electrodes. Due to close proximity an additional tunneling is established between the electrodes which regulates electronic transmission significantly. At the same time the phase associated with site potential, which can be tuned externally yields controlled transmission probabilities. Our results are valid for a wide range of parameter values which demonstrates the robustness of our proposition. We strongly believe that the proposed model can be realized in the laboratory.

Activation of acetonitrile by gas-phase uranium: bond structure analysis and spin–flip reaction mechanism

The reaction mechanism of uranium atom with acetonitrile molecule has been systematically studied on the quintet and triplet spin-state potential energy surfaces (PESs) at B3LYP level of density functional theory. Reaction site prediction and bonding evolution were analyzed using different methods. Crossing seams and possible spin inversion processes between different PESs are discussed by means of spin–orbit coupling (SOC) calculations. The results show that there are three crossing points in the reaction, which appear in the process of capturing hydrogen atom. Larger SOC constant (1545.80 cm−1) and intersystem crossing (ISC) probability ( = 0.72) between quintet and triplet indicate that the effective ISC would occur in the vicinity of the minimum energy crossing points.

SHARPER Reaction Monitoring: Generation of a Narrow Linewidth NMR Singlet, without X-Pulses, in an Inhomogeneous Magnetic Field.

We report a new pure-shift method, termed SHARPER (Sensitive, Homogeneous, And Resolved PEaks in Real time) designed for the analysis of reactions and equilibria by NMR. By focusing on a single selected signal, SHARPER removes all heteronuclear couplings of a selected nucleus without the need to pulse on X channels, thus overcoming hardware limitations of conventional spectrometers. A more versatile decoupling scheme, termed sel-SHARPER, removes all heteronuclear and homonuclear couplings of the selected signal. Both methods are characterized by a periodic inversion of the active spin during the real-time acquisition. In addition to decoupling, they also compensate for pulse imperfections and magnetic field inhomogeneity, generating an extremely narrow singlet with a linewidth approaching limits dictated by the spin–spin relaxation. The decoupling and line narrowing effected by (sel)-SHARPER provide significant increases in the signal-to-noise (S/N) ratio. Increases of 20-fold were routinely achieved for 19F detection. sel-SHARPER is also applicable to first- and higher-order 1H spectra. The sensitivity gains are substantially greater for inhomogeneous magnetic fields, including dynamic inhomogeneity caused by gas sparging. The parameters of the pulse sequences have been analyzed in detail to provide guidelines for their most effective application. The considerable reduction in the detection threshold induced by (sel)-SHARPER make the technique particularly suited for in situ monitoring of reaction kinetics. The approach is illustrated by a 19F NMR study of the protodeboronation of an aryl boronic acid. Here, the high S/N allowed reliable determination of the net protodeoboronation kinetics, and the excess line broadening of 19F singlets was utilized to characterize the boronic acid/boronate equilibrium kinetics. Oxidation of diphenylphosphine, monitored by 31P NMR under optimized gas-flow conditions, demonstrated the high tolerance of SHARPER to dynamic inhomogeneity. The principles of the (sel)-SHARPER sequences are expected to find numerous applications in the design of new NMR experiments.

Unconventional band inversion and intrinsic quantum spin Hall effect in functionalized group-V binary films

Adequately understanding band inversion mechanism, one of the significant representations of topological phase, has substantial implications for design and regulation of topological insulators (TIs). Here, by identifying an unconventional band inversion, we propose an intrinsic quantum spin Hall (QSH) effect in iodinated group-V binary (ABI2) monolayers with a bulk gap as large as 0.409 eV, guaranteeing its viable application at room temperature. The nontrivial topological characters, which can be established by explicit demonstration of Z2 invariant and gapless helical edge states, are derived from the band inversion of antibonding states of px,y orbitals at the K point. Furthermore, the topological properties are tunable under strain engineering and external electric field, which supplies a route to manipulate the spin/charge conductance of edge states. These findings not only provide a new platform to better understand the underlying origin of QSH effect in functionalized group-V films, but also are highly desirable to design large-gap QSH insulators for practical applications in spintronics.

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

A theoretical analysis was carried out on the mechanism of methane oxidation to methanol occurring on single site palladium oxide species [PdO]2+ supported on a model of Al-MCM-41 silica. Both 6- and 8-membered ring structures were considered to represent the support. The energy profile for each elementary reaction was determined from density functional theory calculations with the OPBE functional. The calculated overall activation energies are close to the experimental values. Our calculations confirm that spin inversion can play a significant role in decreasing the barrier heights for the pathways. Indeed, in this type of reactions we could show a crossing between singlet and triplet reaction paths. We showed that the mechanism for the C-H bond cleavage and for the formation of methanol has a radical nature. According to our results, the [PdO]2+ species located on a 8-membered ring of silica is more active than that deposited on a 6-membered ring. The calculated activation energies to cleave the methane C-H bond are 35 and 84 kJ/mol for the radical and ionic pathways, respectively. The activation barrier and the transition state geometry of this H-abstraction step are directly correlated with the optimal angle at which the substrate should approach the [Pd═O]2+ moiety, with the elongation of the Pd-Ooxo bond and finally with the energy of the π* acceptor orbital.