Doublet Spin States Research Articles

Magnetic interactions as well as microscopic origins of the spin-Hamiltonian parameters for 6S(3d5) state ions in cubic symmetry crystal filed

The magnetic interactions as well as the microscopic origins of the spin-Hamiltonian(SH) parameters including a, g, and Δg for 6S(3d5) state ion in cubic symmetry crystal field, taking into account the spin-spin (SS), the spin-other-orbit (SOO), the orbit-orbit (OO) magnetic interactions besides the well-known spin-orbit (SO) magnetic interaction, have been investigated using the complete diagonalization method (CDM). It is shown that the SH parameters arise from the five microscopic mechanisms of SO coupling, SS coupling, SOO coupling, OO coupling, and SO-SS-SOO-OO combined coupling. The relative importance of the contributions of the mechanisms to the SH parameters are investigated. It is shown that the SO coupling mechanism and the SO-SS-SOO-OO combined coupling mechanism are the most important of these coupling mechanisms. The contributions from SS coupling mechanism, SOO coupling mechanism, OO coupling mechanism to the SH parameter are quite small but their combined effects (SO-SS-SOO-OO combined coupling mechanism) is appreciable. The zero-field splitting (ZFS) arises from the net spin quartet states as well as the combined effects between the spin doublet states and the spin quartets states whereas the Zeeman g (or Δg)factor arises from the net spin quartet states. The contributions to the SH parameters from the net spin doublet states are zero. It is found that the relations: a>0，a(-|Dq|)a(|Dq|)，g(-Dq)=g(Dq)，a(-Dq,-ξd, B, C)=a(Dq,ξd, B, C) and Δg(-Dq, -ξd, B, C)=Δg(Dq, ξd, B, C) always hold for the selected crystal field area.Illustrative evaluations are performed for the four typical materials. Mn2+: KZnF3, Mn2+: RbCdF3, Mn2+: MgO and Mn2+: CaO. Good agreement between the theoretical values and the experimental results are obtained.

The effect of stretching thiyl– and ethynyl–Au molecular junctions

We perform density functional theory (DFT) calculations of the stretching ofAu(111)–X–Au(111) molecular junctions where X is either a thiyl or ethynyl biradical. Theequilibrium geometries for the radicals adsorbing on the surface are first calculated and theradicals then placed in the junction geometry. The unit cell is stepwise increased in lengthand the geometry relaxed at each step. When stretching the ethynyl junction, a single goldatom is detached from the rest of the surface and the gold–carbon bond does not break. Incontrast, the gold–sulfur bond in the thiyl junction breaks without detaching anygold atoms. This behaviour can be attributed to the enhanced strength of theAu–C interaction over the Au–S interaction. In both junctions the conductancecalculated using the non-equilibrium Green’s function formalism (NEGF) decreasesas the junction is stretched. After breakage of the Au–S bond, the thiyl radicalcontains an unpaired electron on the sulfur atom and the system is in a spindoublet state. Transmission spectra were calculated for the spin-unpolarized caseonly; evaluation of the spin-polarized density of states suggests that an enhancedconductance for electrons of one spin type may be observed after the Au–S bond isbroken.

Optical Spectroscopy of Potassium-Doped Argon Clusters. Experiments and Quantum-Chemistry Calculations

Rare-gas clusters produced in a supersonic expansion into vacuum are doped with alkali-metal atoms using the pickup technique. We give here a detailed description of our experimental apparatus, which is suitable for electronic spectroscopy of any desired gas clusters doped with low-temperature melting metals. Potassium-doped Ar clusters (average size N approximately 2000) are investigated by beam-depletion (BD) spectroscopy as well as laser-induced fluorescence (LIF) spectroscopy. The observed BD spectra are strong and exhibit several broad peaks within the scan range of the Ti:sapphire excitation laser; only one weak band is observed instead in LIF spectra. This situation resembles the one previously observed for K-doped H2 clusters [Callegari et al., J. Phys. Chem. A 1998, 102, 4952] where it was argued that fluorescence is either quenched or strongly red-shifted by the cluster. We investigate BD spectra as a function of the potassium vapor pressure in the pickup cell, and we perform an extensive analysis of the pickup process, necessary to separate the overlapping spectral contributions of different species. We tentatively assign parts of the spectra to the monomer, singlet dimer, and doublet trimer, of potassium residing on the surface of the cluster. To support our trimer assignment, we perform complete active space self-consistent field (CASSCF) calculations to determine the lowest 12 electronic spin-doublet states of free K3. We find several candidate transitions in the region of interest. Although a definite assignment is not possible because the energy shifts due to the perturbation by the Ar cluster are not known, our theoretical and experimental data compare favorably with existing spectra, and with simulations, of the closely related Na3-molecule, upon suitable scaling of its energy-level structure.

How does the push/pull effect of the axial ligand influence the catalytic properties of Compound I of catalase and cytochrome P450?

Density functional theory (DFT) calculations on the chemoselective epoxidation versus hydroxylation reactions of propene by oxoiron porphyrin models mimicking the active sites of catalase, cytochrome P450 (P450) and horseradish peroxidase Compound I (CpdI) are presented. The catalase reactions are concerted and proceed via two-state reactivity patterns on competing doublet and quartet spin state surfaces, but the lowest barrier is the one leading to epoxide products on the doublet spin surface. The results are compared with earlier DFT studies of models of cytochrome P450, horseradish peroxide (HRP), taurine/α-ketoglutarate dioxygenase and some synthetic oxoiron catalysts. The catalase barriers are midway in between those obtained for HRP and P450 models, so that tyrosinate ligated heme systems should be able to catalyze C–H hydroxylation and C C epoxidation reactions. We show that for heme systems the barrier height of epoxidation linearly correlates with the electron affinity of Compound I as expected from the electron transfer mechanism of the rate determining step. Our studies show that the axial ligand does not influence the chemoselectivity of a reaction but that it does regulate the barrier heights and rate constants. Finally, we estimated the effect of the axial ligand on the oxoiron group and derived that it contributes from a field effect due to the charge of the ligand and a quantum mechanical effect as a result of orbital mixing. In catalase, the major component is the field effect, while the quantum mechanical effect is negligible. This is in contrast to P450 CpdI, where both effects are of similar order of magnitude.

Theoretical Study of Low-Spin, High-Spin, and Intermediate-Spin States of [FeIII(pap)2]+(pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato). Mechanism of Light-Induced Excited Spin State Trapping

The mechanism of light-induced excited spin state trapping (LIESST) of [FeIII(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) was discussed on the basis of potential energy surfaces (PESs) of several important spin states, where the PESs were evaluated with the DFT(B3LYP) method. The PES of the quartet spin state crosses those of the doublet and sextet spin states around its minimum. This means that the spin transition occurs from the quartet spin state to either the doublet spin state or the sextet spin state around the PES minimum of the quartet spin state. The PES minimum of the sextet spin state is slightly less stable than that of the doublet spin state by 0.18 eV (4.2 kcal/mol). This small energy difference is favorable for the LIESST. The doublet-sextet spin crossover point is 0.41 eV (9.6 kcal/mol) above the PES minimum of the sextet spin state. Because of this considerably large activation barrier, the thermal spin transition and the tunneling process do not occur easily. In the doublet spin state, the ligand to ligand charge transfer (LLCT) transition is calculated to be 2.16 eV with the TD-DFT(B3LYP) method, in which the pi orbital of the phenoxy moiety and the pi* orbital of the imine moiety in the pap ligand participate. This transition energy is moderately smaller than the visible light of 550 nm used experimentally. In the sextet spin state, the ligand to metal charge transfer (LMCT) transition is calculated to be at 2.36 eV, which is moderately higher than the visible light (550 nm). These results indicate that the irradiation of the visible light induces the LIESST to generate the sextet spin state but the reverse-LIESST is also somewhat induced by the visible light, indicating that the complete spin conversion from the doublet spin state to the sextet one does not occur, as reported experimentally.

A Density Functional Study of the Factors That Influence the Regioselectivity of Toluene Hydroxylation by Cytochrome P450 Enzymes

AbstractDensity functional theory calculations have been performed to elucidate the factors that influence the regioselectivity of toluene hydroxylation by a model of the active species of cytochrome P450 enzymes, so‐called Compound I (Cpd I). Cpd I can hydroxylate the benzylic C–H and generate benzyl alcohol, or it can activate the phenyl group and produce p‐cresol and p‐methylcyclohexanone products. The reactions take place via two‐state reactivity on competing doublet and quartet spin state surfaces. In the gas phase, the benzyl alcohol is preferred over p‐cresol by more than three orders of magnitude. Environmental perturbations, namely, NH···S hydrogen bonding to the thiolate ligand and bulk polarity of the protein, lower this preference to roughly 10:1 in favour of benzyl alcohol. Substitution of methyl hydrogen atoms by deuterium atoms raises the barriers that lead to benzyl alcohol without affecting those leading to p‐cresol. Therefore combining toluene deuteration with the effects of NH···S hydrogen bonding and bulk polarity lowers the free energy difference between the two processes to only 0.4 kcal mol–1, and the two processes become competitive. These results as well as the calculated kinetic isotope effects are in good general agreement with experimental data. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Formation of the Active Species of Cytochrome P450 by Using Iodosylbenzene: A Case for Spin‐Selective Reactivity

The generation of the active species for the enzyme cytochrome P450 by using the highly versatile oxygen surrogate iodosylbenzene (PhIO) often produces different results compared with the native route, in which the active species is generated through O(2) uptake and reduction by NADPH. One of these differences that is addressed here is the deuterium kinetic isotope effect (KIE) jump observed during N-dealkylation of N,N-dimethylaniline (DMA) by P450, when the reaction conditions change from the native to the PhIO route. The paper presents a theoretical analysis targeted to elucidate the mechanism of the reaction of PhIO with heme, to form the high-valent iron-oxo species Compound I (Cpd I), and define the origins of the KIE jump in the reaction of Cpd I with DMA. It is concluded that the likely origin of the KIE jump is the spin-selective chemistry of the enzyme cytochrome P450 under different preparation procedures. In the native route, the reaction proceeds via the doublet spin state of Cpd I and leads to a low KIE value. PhIO, however, diverts the reaction to the quartet spin state of Cpd I, which leads to the observed high KIE values. The KIE jump is reproduced here experimentally for the dealkylation of N,N-dimethyl-4-(methylthio)aniline, by using intra-molecular KIE measurements that avoid kinetic complexities. The effect of PhIO is compared with N,N-dimethylaniline-N-oxide (DMAO), which acts both as the oxygen donor and the substrate and leads to the same KIE values as the native route.

The spectral fine structure, zero-field splitting parameters and Jahn-Teller effect of LiNbO3∶Fe3+crystals

The 252 rank completely diagonalized Hamiltonian matrix of the 3d5 configuration with trigonal symmetry have been established by group theory, crystal field theory and irreducible tensor operator method. The spectral fine structure, zero-field splitting parameters, and Jahn-Teller effect, as well s the influence of spin doublet state and spin quartet state on the ground-state energy levels in LiNbO3∶Fe3+ crystals were studied with this matrix. The results show that the contribution of spin quartet state on the ground- state energy levels is the most important, the contribution of spin doublet state is weak but can not be neglected. The calculated results are in good agreement with the experiments. On their basis, the influence of spin-orbit interaction and spin-spin interaction on the spectra fine structure and zero-field splitting parameters were further studied, and we found that the influence of spin-orbit interaction is dominant, but the influence of spin-spin interaction can not be neglected. The research shows this substance has J-T effect in the spectral structure of spin quartet state. The reason is the combined action of spin-orbit interaction and trigonal distortion.

A linear trinuclear complex of copper(II) with N,N′-bis(2-hydroxy-5-methoxybenzelidene)-1,3-diiminopropane

The synthesis and X-ray crystal structure of a linear phenolate-bridged Cu(II) complex 1 with a Cu–Cu bond distance of 2.9260(5) Å is reported. The complex consists of three Cu(II) ions with two molecules of the N,N′-bis(2-hydroxy-5-methoxybenzelidene)-l,3-diiminopropane ligand and two nitrate ions in such a manner that one ligand is connected with two Cu(II) ions. The complex is monoclinic, space group P21/n, with a = 10.6305(5), b = 13.0719(7), c = 14.6336(8) Å and β = 102.549(1)° at 293 K, Z = 2. The structure shows deprotonation of the phenolate oxygen to form a μ2 bridge. The magnetic moment (1.627 BM per Cu3 unit) at 300 K reveals that the spin doublet state is the ground state.

Four-body structure ofΛ7Li andΛNspin-dependent interaction

Two spin-doublet states in ${}_{\ensuremath{\Lambda}}^{7}$Li are studied on the basis of the $\ensuremath{\alpha}+\ensuremath{\Lambda}+n+p$ four-body model. We employ the two-body interactions that reproduce the observed properties of any subsystems composed of $\ensuremath{\alpha}N,\ensuremath{\alpha}\ensuremath{\Lambda}$, $\ensuremath{\alpha}\mathit{NN}$, and $\ensuremath{\alpha}\ensuremath{\Lambda}N$. Furthermore, the $\ensuremath{\Lambda}N$ interaction is adjusted so as to reproduce the ${0}^{+}\text{\ensuremath{-}}{1}^{+}$ splitting of in ${}_{\ensuremath{\Lambda}}^{4}$H. The calculated energy splittings of $3/{2}^{+}\text{\ensuremath{-}}1/{2}^{+}$ and $7/{2}^{+}\text{\ensuremath{-}}5/{2}^{+}$ states in ${}_{\ensuremath{\Lambda}}^{7}$Li are 0.69 and 0.46 MeV, which are in good agreement with the recent observed data. The spin-dependent components of the $\ensuremath{\Lambda}N$ interaction are discussed.

What External Perturbations Influence the Electronic Properties of Catalase Compound I?

We have performed density functional theory calculations on an active-site model of catalase compound I and studied the responses of the catalytic center to external perturbations. Thus, in the gas phase, compound I has close-lying doublet and quartet spin states with three unpaired electrons: two residing in pi(FeO) orbitals and the third on the heme. The addition of a dielectric constant to the model changes the doublet-quartet energy ordering but keeps the same electronic configuration. By contrast, the addition of an external electric field along one of the principal axes of the system can change the doublet-quartet energy splitting by as much as 6 kcal mol(-1) in favor of either the quartet or the doublet spin state. This sensitivity is much stronger than the effect obtained for iron heme models with thiolate or imidazole axial ligands. Moreover, an external electric field is able to change the electronic system from a heme-based radical [Fe=O(Por*+)OTyr-] to a tyrosinate radical [Fe=O(Por)OTyr*]. This again shows that oxo-iron heme systems are chameleonic species that are influenced by external perturbations and change their character and catalytic properties depending on the local environment.

Propene Activation by the Oxo-Iron Active Species of Taurine/α-Ketoglutarate Dioxygenase (TauD) Enzyme. How Does the Catalysis Compare to Heme-Enzymes?

Density functional calculations on the oxygenation reaction of propene by a model for taurine/alpha-ketoglutarate dioxygenase (TauD) enzyme are presented. The oxo-iron active species of TauD is shown to be a powerful and aggressive oxidant, which is able to hydroxylate C-H bonds and epoxidize C=C bonds with low barriers. In the case of propene oxygenation, the hydroxylation and epoxidation mechanisms are competitive on a dominant quintet spin state surface. We have compared the mechanism and thermodynamics of TauD with oxo-iron heme catalysts, such as the cytochromes P450, and found some critical differences. The TauD model is found to be much more reactive toward oxygenation of substrates than oxo-iron complexes in a heme environment with much lower reaction barriers. We have analyzed this and assigned this to the strength of the O-H bond formed after hydrogen abstraction from a substrate, which is at least 10 kcal mol(-)(1) stronger in five-coordinated oxo-iron nonheme complexes than in six-coordinated oxo-iron heme complexes. Since, the metal in TauD enzymes is five-coordinated, whereas in heme-enzymes it is six-coordinated there are some critical differences in the valence molecular orbitals. Thus, in oxo-iron heme catalysts one of the antibonding pi orbitals is replaced by a low-lying nonbonding delta orbital resulting in a lower overall spin state. Moreover, heme-enzymes have an extra oxidation equivalent located on the heme, which is missing in non-heme oxo-iron catalysts. As a result, the oxo-iron species of TauD reacts via single-state reactivity on a dominant quintet spin state surface, whereas oxo-iron heme catalysts react via two-state reactivity on competing doublet and quartet spin states.

Theoretical Study of the Mechanism of Acetaldehyde Hydroxylation by Compound I of CYP2E1

Recent experimental studies revealed that cytochrome P450 2E1 (CYP2E1) could metabolize not only ethanol but also its primary product, acetaldehyde, accompanying the well-known acetaldehyde dehydrogenases (ALDH) in the metabolism of acetaldehyde. Mechanistic aspects of acetaldehyde hydroxylation by Compound I model active species of CYP2E1 were investigated by means of B3LYP DFT calculations in the present paper. Our study results demonstrate that acetaldehyde hydroxylation by CYP2E1 is in accord with the effectively concerted mechanisms both on the high quartet spin state (HS) and on the low doublet spin state (LS). The rate-limiting step is H-abstraction, and the activation energy is about 11.7 approximately 14.0 kcal/mol on the quartet (doublet) reaction route, which is about one-half to one-third of that needed by methane hydroxylation. The phenomenon that the HS and LS reaction routes are both effectively concerted was shown for the first time to occur in trans-2-phenyl-iso-propylcyclopropane hydroxylation by Kumar et al. (see Figure 7 in the paper of Kumar, D.; de Visser, S. P.; Sharma, P. K.; Cohen, S.; Shaik, S. J. Am. Chem. Soc. 2004, 126, 1907) and was confirmed in our work of acetaldehyde hydroxylation by cytochrome P450. Theoretical exploration of the HS O-rebound barrier degradation is also presented in the present paper on the basis of Shaik's valence bond (VB) model.

A hollow tetrahedral cage of hexadecagold dianion provides a robust backbone for a tuneable sub-nanometer oxidation and reduction agent via endohedral doping

We show, via density functional theory calculations, that dianionic Au16(2-) cluster has a stable, hollow, Td symmetric cage structure, stabilized by 18 delocalized valence electrons. The cage maintains its robust geometry, with a minor Jahn-Teller deformation, over several charge states (q = -1,0,+1), forming spin doublet, triplet and quadruplet states according to the Hund's rules. Endohedral doping of the Au16 cage by Al or Si yields a geometrically robust, tuneable oxidation and reduction agent. Si@Au16 is a magic species with 20 delocalized electrons. We calculate a significant binding energy for the anionic Si@Au16/O2- complex and show that the adsorbed O2 is activated to a superoxo-species, a result which is at variance with the experimentally well-documented inertness of Au16- anion towards oxygen uptake.

Spin Crossover in Cobalt(II) Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Expectation values of thee+He(Se3)system

Close to converged energies and expectation values for e+He(3Se) are computed using a ground state wave function consisting of 1500 explicitly correlated Gaussians. The best estimate of the e+He(3Se) energy was –2.250 595 08 hartree, which has a binding energy of 0.000 595 08 hartree against dissociation into Ps+He+. The 2gamma annihilation rate for the spin doublet state was 5.713×109 s–1. The estimated annihilation rate with the core He+ electron was 2.506×106 s–1. The derived enhancement factor for annihilation with the He+(1s) core was 2.29, just over 10% smaller than the enhancement factor derived from analyses of annihilation during e+-He+ scattering. The diffuse nature of the wave function, with well separated He+ and Ps subsystems, is demonstarted to be on the threshold of satisfying the formal criteria that define a quantum halo state.

Two States and Two More in the Mechanisms of Hydroxylation and Epoxidation by Cytochrome P450

Past studies have shown that oxidation reactions by P450 Compound I (Cpd I) can be described by two competing quartet and doublet spin states, which possess three unpaired electrons, hence tri-radicals. One electron excitation from the delta orbital to sigma* xy generates two states that possess five unpaired electrons, so-called penta-radicals, in sextet and quartet situations, and which were shown by theory to lie only approximately 12-14 kcal/mol higher in energy than the tri-radical ground states (ref 7). The present study focuses on the C-H hydroxylation and C=C epoxidation of propene by these penta-radical states. It is shown that the initial energy differences, between the penta-radical and tri-radical states, diminish along the reaction pathway, due to the favorable and cumulative exchange stabilization of the more open-shell species. Furthermore, theory suggests that hydrogen bonding to the thiolate ligand, and general polarity of the environment, reduce these gaps further, thereby making the penta-radical states accessible to ground-state reactivity. The interconversion between the tri-radical and penta-radical states along the reaction coordinate will depend on the dynamics of spin-flips and energy barriers between the states. Especially interesting should be the region of the reaction intermediates; for both epoxidation and hydroxylation, this region is typified by a dense manifold of spin states and electromeric states (that differ by the oxidation state of iron), such that the total reactivity would be expected to reflect the interplay of these states, giving rise to multistate reactivity.

Metal−Metal Interactions in Mixed-Valence [M2Cl9]2- Species: Electronic Structure of d1d2 (V, Nb, Ta) and d4d5 (Fe, Ru, Os) Face-Shared Systems

The molecular and electronic structures of mixed-valence d1d2 (V, Nb, Ta) and d4d5 (Fe, Ru, Os) face-shared [M2Cl(9)]2- dimers have been calculated by density functional methods in order to investigate metal-metal bonding in this series. General similarities are observed between d1d2 and d4d5 systems and can be considered to reflect the electron-hole equivalence of the individual d1-d5 and d2-d4 configurations. The electronic structures of the dimers have been analyzed using potential energy curves for the broken-symmetry and other spin states resulting from the d1d2 and d4d5 coupling modes. In general, a spin-doublet (S = 1/2) state, characterized by delocalization of the metal-based electrons in a metal-metal bond with a formal order of 1.5, is favored in the systems containing 4d and 5d metals, namely, the Nb, Ta, Ru, and Os dimers. In contrast, the calculated ground structures for [V2Cl9]2- and [Fe2Cl9]2- correspond to a spin-quartet (S = 3/2) state involving weaker coupling between the metal centers and electron localization. In the case of [Ru2Cl9]2-, both the spin-doublet and spin-quartet states are predicted to be energetically favored suggesting that this species may exhibit double-minima behavior. A comparison of computational results across the (d1d1, d1d2, d2d2) [Nb2Cl9]z- and [Ta2Cl9]z- and (d4d4, d4d5, d5d5) [Ru2Cl9]z- and [Os2Cl9]z- series has revealed that, in all four cases, the shortening of the metal-metal distances correlates with an increase in formal metal-metal bond order.

Sulfoxidation Mechanisms Catalyzed by Cytochrome P450 and Horseradish Peroxidase Models: Spin Selection Induced by the Ligand,

The sulfoxidation of dimethyl sulfide (DMS), by two different heme-type enzyme models (without the protein), namely, horseradish peroxidase (HRP) and cytochrome P450 (P450), was studied using density functional theory. The models differ from each other by the axial ligand of the iron, which is imidazole in the case of HRP and thiolate in the case of P450. The computational results reveal a concerted oxygen atom transfer to sulfur, with spin-state selection dependent upon the identity of the proximal ligand. In the case of thiolate, the mechanism prefers the high-spin quartet pathway; whereas in the case of imidazole, the mechanism involves two-state reactivity (TSR), with competing quartet and doublet spin states. Furthermore, with thiolate the high-spin transition state, (4)TS(P450), has an upright conformation with a large Fe-O-S(DMS) angle of 147 degrees , whereas the low-spin species, (2)TS(P450), has a small angle and its Fe-O moiety makes an O-N(Por) bond with one of the nitrogen atoms of the porphine macrocycle. By contrast, when the proximal ligand is imidazole, both transition states possess a bent Fe-O bond and an O-N(Por) bond. These spin-state selection patterns obey simple orbital-selection rules, which are manifestations of the electronic nature of the ligand, i.e., the electron-releasing effect of the thiolate vis-a-vis the electron-withdrawal effect of imidazole. Other possible reactivity expressions of the spin-selection patterns are discussed [Dowers, T. S., Rock, D. A., Rock, D. A., Jones, J. P. (2004) J. Am. Chem. Soc. 126, 8868-8869]. Theory shows that intrinsically, HRP should be as reactive as P450 toward sulfoxidation.

Three-Electron Systems in Quantum Nanostructures: Ground State Transition from a Spin-Doublet State to a Spin-Quartet State

The ground state and the lowest excited state of three electrons confined in quantum structures (quantum dots) are calculated. In general, the ground state is a spin-doublet one in a small dot, level crossing occurs as the dot size increases, and the ground state is a spin-quartet state in a large dot. It is found that the spin-quartet ground state is more likely to be realized in a quantum ring (QR) than in a harmonic confinement. In a narrow QR, two-electron exchanges are unlikely to occur because of the large Coulomb barrier while three-electron exchanges are little suppressed, leading to the fully spin-polarized (spin-quartet) ground state. The present calculation conforms to Herring's theorem that the ground state of three spin-1/2 fermions on a one-dimensional ring is a fully spin-polarized one if the repulsive interaction between fermions is sufficiently strong. The extensions of the present result to more complicated cases are also discussed.