Quartet Spin State Research Articles

Structure and property study of the O2 + O2− electron transfer system

The structures and the properties of the O 2 + O − 2 electron transfer system are studied with ab initio calculations at the UMP2(full)/6–311 + G ∗ basis set level for the five selected structures: two T-type, collinear, parallel, and crossing. The stability of the encounter complex, O 2···O − 2, in the five encounter structures is compared for the low (doublet) and the high (quartet) spin states. The activation barriers and the coupling matrix elements in the electron transfer process are also calculated for two different spin states. Results indicates that the interaction between the donor and the acceptor in the encounter complexes is weak and the structures are floppy. Although the encounter complexes in doublet states have lower activation barriers and greater coupling matrix elements than those in quartet state, the low spin coupling needs the change of spin state of an electron in the species O 2 and will result in the negative stabilization energies for the encounter complexes. The electron transfer is more likely to take place via the quartet state.

EPR investigation of endofullerenes in solution

and Lu@C82 were prepared by arc burning and subsequent HPLC purification. EPR spectra of Lu@C82 could be interpreted as arising from unresolved hyperfine interaction with the I=7/2 nuclear spin of 175Lu. At temperatures above 250 K, a thermally activated process, which is tentatively attributed to a hopping process of the encapsulated ion or to time-dependent population of close-lying electronic states, leads to pronounced line broadening. For the Ho@C82 sample, no EPR signals could be detected, indicating a high spin state of this molecule. Spin relaxation data of N@C60, which was prepared by ion bombardment, could be interpreted by assuming that collision-induced deformation of the carbon shell leads to a fluctuating zero-field splitting, sensed by the quartet spin state of the central encapsulated nitrogen atom.

On the Reactivity of Ti+(4F,2F). Reaction of Ti+ with OH2

The reaction of Ti+(4F,2F) + OH2 has been studied in detail for both doublet and quartet spin states. The only exothermic products are TiO+(2Δ) and H2; formation of several endothermic products is also examined. An in-depth analysis of the reaction paths leading to each of the observed products is given, including various singlet, triplet, and quartet minima, several important transition states, and a discussion of the two H2 elimination mechanisms proposed in the literature. The experimentally observed spin-forbidden crossing is given a possible explanation. Throughout this work comparison to experimental results in energetics, reaction products, and suggested mechanisms has been central.

Characterization of a Resting State Model of Peroxidases by ab Initio Methods: Optimized Geometries, Electronic Structures, and Relative Energies of the Sextet, Quartet, and Doublet Spin States

Ferric aquo heme complexes comprise the active site of three major families of heme proteins, the peroxidases, the cytochrome P450s, and metmyoglobin. There is ample evidence from a variety of spectroscopic studies of wild type (wt) and mutant enzymes that in these resting state complexes, the sextet, quartet, and doublet states are very close in energy and the predominant spin state observed is a sensitive function of the number, nature, and geometry of the axial ligands. One correlation that is very difficult to determine experimentally is the relationship between spin state and geometry in the same heme complex. This coupling of geometry and spin state in the same complex can be functionally important since spin state changes are often a key part of function, for example, in the enzymatic cycle of the cytochrome P450s. To further explore the relationship between geometry and spin state, we report here for the first time the use of ab initio methods to calculate optimized geometries and electronic struc...

Reaction Paths for the Conversion of Methane to Methanol Catalyzed by FeO+

AbstractWe propose possible theoretical reaction paths for the conversion of methane to methanol catalyzed by FeO+. The geometric and electronic structures for the reactant, product, intermediates, and transition states were calculated and analyzed in detail by means of a hybrid Hartree–Fock/density functional method. Sextet and quartet spin states were taken into consideration in the analysis of the reaction paths. The conversion of methane to methanol was shown to proceed through basic concerted hydrogen‐ and methyl‐shift reactions. A fragment molecular orbital analysis for the formation of the reactant complex, OFe+–CH4, which plays an important role in the initial stage of methane activation, was carried out in order to understand the nature of the interesting Fe–C bond. The five‐coordinate methane in the reactant complex was calculated to have a C3v‐type geometry. Each reaction path presented in this paper includes an important insertion species, HO–Fe+–CH3 or H–Fe+–OCH3, and two transition states. Thus, there are several kinds of reaction paths, if the high‐spin sextet and low‐spin quartet states are taken into consideration. A reaction towards the hydroxy intermediate, HO–Fe+–CH3, was found to be more favorable in both the sextet and quartet spin states from the viewpoint of activation energy, and this intermediate is extremely stable. It was found from intrinsic reaction coordinate (IRC) analyses that two basic reactions coexist, namely, hydrogen or methyl migration between the reactant and the methoxy intermediate, H–Fe+–OCH3. This transition state is interesting, because the two transition states resulting from C–H bond cleavage and methyl migration are located in the same region of space on the potential energy surfaces. IRCs are partially shown for the complicated first halves of the total reaction paths.

Fourier transform EPR study of N@C 60 in solution

Using pulsed EPR techniques, the electron spin relaxation properties of nitrogen atoms encapsulated in C 60 have been studied. Because of the high symmetry of the cage, most of the conventional relaxation mechanisms otherwise affecting electron spins in solution are absent, resulting in an exceptional homogeneous ERP linewidth of only 2.5 kHz. Apparently, solvent collision-induced deformations of the carbon shell which modulate the zero-field-splitting sensed by the quartet spin state of N@C 60 are the dominant spin relaxation mechanism.

Pulsed ESR/ENDOR and ESTN(Electron Spin Transient Nutation) study of polycationic high-spin states of one-and two-dimensional (diarylamino)benzenes

Polycationic high-spin states of 1,3-bis(diarylamino)benzenes, 1,3,5-tris(diarylamino)benzenes, and their homologues have been studied by cw and pulsed ESR/ENDOR spectroscopy. An electron spin transient nutation (ESTN) method based on pulsed ESR spectroscopy was applied to the dicationic and tricationic high-spin states of N,N,N′,N″-tetra(4-anisyl)-2,4-dimethyl-1,5phenylenediamine and N,N,N′,N′,N″,N″-hexa(4-anisyl)-1,3,5-triaminobenzene, respectively, in order to identify the spin multiplicity of the polycationic states. The observed 2D nutation spectra of the dicationic and tricationic states unequivocally showed that the dicationic and tricationic states are ground-state triplet and quartet, respectively. We have also carried out cw-and pulsed ENDOR measurements to the tricationic state of 1,3,5-tris(diarylamino)benzene, clarifying the electronic structure of the cationic quartet spin state. On the central phenyl ring there are carbon sites with large π-spin densities which are additively brought from each diarylamino moiety by spin polarization mechanism. In the triplet and quartet ground states, the topological spin polarization mechanism is dominant for forming the high-spin states.

Structure and bonding of Group 13 monocarbonyls

A. J. Bridgeman, J. Chem. Soc., Dalton Trans., 1997, 1323 DOI: 10.1039/A606474D

Excited state absorption of Fe3+in garnet crystal

Abstract The excited state absorption (ESA) from the metastable level 4T1 (4G) of Fe3+ in tetrahedral sites of garnets is reported. The spectra show strong lines due to spin allowed transitions to other excited spin-quartet states. It is also shown that this ESA could be strongly reduced if the metastable level could is used as donor level in the process of sensitaization of rare earths ions (such as Tm3+) by Fe3+.

The non-additive exchange energies of H3and He3

Non-additive SCF energies are calculated for H3 (quartet spin state) and He3 using Hartree-Fock quality basis sets containing only s functions. The results are analysed using a modification of an effective Gaussian model, replacing the explicit dependence on Gaussians by related dimer and/or trimer properties. A good fit to the SCF results is obtained using a model based on dimer and trimer charge density overlap integrals, and two adjustable parameters.

Ab initio multiconfiguration self-consistent-field calculations of the excited states of a Mn impurity in CaF2.

We analyze Mn absorption in ${\mathrm{CaF}}_{2}$:Mn by the employment of ab inito quantum-mechanical cluster calculations and ligand-field methods. The [${\mathrm{MnF}}_{8}$${]}^{6\mathrm{\ensuremath{-}}}$ ${\mathit{O}}_{\mathit{h}}$ cluster is chosen to represent the isolated ${\mathrm{Mn}}^{2+}$ substitutional impurity in an otherwise perfect crystal. The methods of unrestricted open-shell Hartree-Fock self-consistent field (SCF), Mo/ller-Plesset perturbation theory to second- and fourth-order, and singles and doubles configuration interaction are used to calculate the spin sextet and quartet ground states. With the active space consisting of the Mn 3d molecular orbitals, the spin quartet excited states are calculated by the method of multiconfiguration SCF. It was found that the presence of an external field designed to reproduce the Madelung potential difference within the cluster did not significantly affect the Mn d-to-d transitions. The crystal-field term splitting diagrams for the eight-coordinated ${\mathrm{Mn}}^{2+}$ impurity in ${\mathit{O}}_{\mathit{h}}$ symmetry are calculated. The results showed a narrowing of the multiplet terms in energy with respect to the six-coordinated ${\mathit{O}}_{\mathit{h}}$ result. This increases the crystal-field parameter Dq from the previously published value of 420--570 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Non-additive three-body interaction energies for H3(quartet spin state)

The results of an Unsold average energy calculation of the non-additive interaction energy for H3 (quartet spin state) are presented for equilateral triangular configurations. They are discussed in the context of the problems associated with the representation of non-additive energies for the interaction of closed-shell species.

Quenching rate constants of excited halogen atoms in quartet states

Excited halogen atoms in quartet spin states F*(2p43s, 4P5/2), Cl*(3p44s, 4P5/2), and Br*(4p45s, 4P5/2) were produced from helium sensitized radiation chemical decomposition of SF6, CF3Cl, CF3Br, and CF2Br2. Quenching rate constants of these excited halogen atoms by simple gas molecules such as O2, N2, H2, CO, CO2, NO, NO2, N2O, CH4, C2H6, and Xe including parent molecules were determined from absorption decay curves at 685.8 nm for F*, 837.5 nm for Cl*, and 827.4 nm for Br*. The optical densities were assumed to be proportional to (number of excited atoms per one cubic centimeter)0.9. The quenching rate constants obtained here were compared to those reported of metastable rare-gas atoms and an excited oxygen atom O*(2p33s, 5S2), and further discussed in terms of several theoretical kinetic models.

Configuration interaction calculations of the structure and energetics of the high symmetry isomers of the O +4 cation

The identification of the oxygen dimer cation, O + 4, by cryogenic matrix-isolation IR and ESR spectroscopies, and as the important core-cation in the ionization of gas-phase O 2 clusters, has led to theoretical investigations of the stable isomeric structures available to this species. The open-shell quartet spin state favoured by O + 4 together with its highly symmetric structures means that ab initio all-electron calculations are bedevilled by difficulties including spin contamination, symmetry breaking and con- figurational convergence. The QCISD(T) approximation used in the calculations reported here seems to account for these computational problems. The 4B u state of the trans-planar (C 2h) isomers is found to be the most stable structure, though the 4B 3g state of the rectangular (D 2h) isomer, calculated as of 4 kcal mol −1 higher energy, is so because of the influence of a large zero-point vibration energy.

Synthesis and characterization of new multi-radical compound 1,3,5-tri-6′-(1,3,5-triphenylverdazyl)-benzene

We report the synthesis of a new triradical material: 1,3,5,-tri-6′-(1,3,5,-tri-phenylverdazyl)-benzene. It is characterized by 1H-NMR in solution, static magnetic susceptibility measurement and EPR (electron paramagnetic resonance) in crystalline solid state and dilute liquid as well as frozen solution. Hyperfine coupling and fine structure splitting are analysed. In the solid state, the spin-quartet state is situated at ΔE Q-D/k B = 20K above the doublet ground-state. EPR-line exchange narrowing indicates substantial intermolecular exchange interaction. Ferromagnetic and antiferromagnetic exchange interactions cancel in the crystalline solid, however. We point to the influence of the molecular packing in the solid state on the molecular magnetic properties (doublet-quartet separation).

ESR of the cationic triradical of 1,3,5-tris(diphenylamino)benzene

The ESR spectrum of the cationic triradical of 1,3,5-tris(diphenylamino)benzene (TDAB) is discussed. The tricationic state of TDAB was observed by means of cyclic voltammetry. The orange cationic triradical was prepared by oxidation of TDAB with trifluoroacetic anhydride in the presence of tetra-n-butylammonium tetrafluoroborate in CH 2 Cl 2 . The ESR spectrum of the randomly oriented radicals in the CH 2 Cl 2 glass agrees well with the theoretical prediction of a quartet (S=3/2) spin state with a zero-field splitting parameter D' of 13.1 G (0.0012 cm -1 )

Electron depolarization by inelastic exchange scattering from Cr3+ magnetic moments.

Inelastic exchange scattering of low-energy electrons off disordered localized Cr{sup 3+} magnetic moments at a Cr{sub 2}O{sub 3} surface is studied by spin-polarized electron-energy-loss spectroscopy. The excitations from a spin-quartet ground state to doublet states via exchange scattering give rise to sharp energy-loss lines, which are predominantly spin flip, and provide an efficient electron depolarization mechanism.

Ab Intio Calculations on the Adsorption of H, O, and CO atop Cu2+ on MgO(001)

AbstractThe adsorption of hydrogen, oxygen, and carbon monoxide atop Cu2+ in a Mg‐vacancy on the MgO(001) surface is studied in a cluster Hartree‐Fock approximation. The H atom binds slightly atop Cu2+ with binding energy of 0.05 eV. The O atom binds with Cu2+ in a quartet spin state with binding energy of about 0.12 eV. In the doublet state it does not bind to Cu2+. The CO molecule in a Cu2+CO geometry perpendicular to the surface binds to Cu2+ with binding energy of about 0.2 eV. In the Cu2+OC geometry it binds directly with Cu2+ with about 0.33 eV. In both geomtrie, the atom of CO closer to the Cu2+ has some more electronic charge near the surface, than in free space. In all three substances, mutual exchanges of electrons occur between z‐directed orbitals. It seems that the adsorption of CO is rather electrostatic in nature (via the dipole moment) than covalent, where a donation and a back donation occurs. In both geometries the donation is mainly from the 5σ CO orbital. The back donation from Cu2+ occurs mainly with the 2σ* CO orbital in the Cu2+OC geometry. In the Cu2+CO geometry it occurs mainly with a σ orbital.

Optically detected magnetic-resonance studies of oxygen vacancy defects in the phosphorescent spin quartet state in CaO.

A new zero-phonon line emission at 638 nm is observed from photoexcited additively colored calcium oxide crystals. By means of optically detected magnetic resonance it is established that the emission arises from a localized spin-quartet state with the following spin Hamiltonian parameters \ensuremath{\Vert}D\ensuremath{\Vert}=1030\ifmmode\pm\else\textpm\fi{}5 MHz, \ensuremath{\Vert}E\ensuremath{\Vert}=463\ifmmode\pm\else\textpm\fi{}3 MHz, and g=2.00\ifmmode\pm\else\textpm\fi{}0.05. The fine-structure principal axes are parallel with the [001], [1\ifmmode\bar\else\textasciimacron\fi{}10], and [110] crystallographic directions. It is proposed that the emission is from defects which comprise three adjacent oxygen vacancies which have trapped three electrons.

Optically detected magnetic resonance and spin coherence of self-trapped hole centers in an excited spin-quartet state in calcium oxide.

A new defect in additively colored calcium oxide is studied by means of optically detected magnetic resonance and spin-coherence experiments. It is established that the defect shows emission from a spin-quartet state with spin-Hamiltonian parameters $|D|=1950\ifmmode\pm\else\textpm\fi{}2$ MHz, $|E|=550\ifmmode\pm\else\textpm\fi{}2$ MHz, and $g=2.00\ifmmode\pm\else\textpm\fi{}0.05$. The fine-structure principal axes are along [110], [110], and [001]. We argue the existence of a vacancy-interstitial hole, the $F$ center-${\mathrm{O}}_{2}^{\ensuremath{-}}$ pair, that can be photoexcited in the phosphorescent quartet state. Time-resolved microwave recovery experiments at zero field yielded the lifetimes of the radiative and nonradiative quartet sublevels to be 230 \ensuremath{\mu}s and 3.8 ms, respectively. From optically detected spin-coherence decay measurements at 1.4 K a phase memory time of 38 \ensuremath{\mu}s was found, showing that irreversible spin dephasing is not determined by population relaxation.