Limits of delocalization in through-conjugated dinitrenes: aromatization or bond formation?

[reaction: see text] Dinitrenes 4 and 5 both can form quinonoidal structures by conjugative bond formation. However, ESR spectroscopy detects a thermally populated, excited-state, triplet quinonoidal structure only for 4, with a zero-field splitting of |D/hc| = 0.0822 cm(-)(1), |E/hc| congruent with 0.0 cm(-)(1). The tendency to maintain aromaticity in the additional ring of 5 favors a dinitrene structure (with one less formal pi-bond) over a quinonoidal structure. The thermally populated quintet state of 5 has a zero-field splitting of |D/hc| = 0.287 cm(-)(1), |E/hc| 相似文献   

Investigations of the EPR zero-field splitting and the defect structure for Mn(2+) and Fe(3+) ions in anatase crystals

The EPR zero-field splittings D of Mn(2+) and Fe(3+) ions in anatase crystals at room and low temperatures are calculated from the high-order perturbation formula of zero-field splitting D for 3d(5) ions in tetragonal symmetry based on the dominant spin-orbit coupling mechanism. The calculated results are consistent with the observed values. From the calculations, Mn(2+) and Fe(3+) ions are suggested to substitute for Ti(4+) ions in anatase (in the previous paper, Mn(2+) ion was suggested at an interstitial site rather than substitutional site) and the defect structures (characterized mainly by the local oxygen parameter u) for both tetragonal Mn(2+) and Fe(3+) impurity centers are estimated. The different zero-field splitting at room and low temperatures are due mainly to the change of local oxygen parameter u with the temperature. These results are discussed.     

Sulphate ion dynamics in α-alums studied by esr at high pressures

The variation of zero-field splitting and linewidth of Cr³⁺ ion in KCr and KAI alums with hydrostatic pressure and with temperature is investigated. A model for the apparent phase transition is proposed on the basis of the reorientational motion of the SO^2?₄ groups.     

ESR TRIPLET SPECTRA OF ACRIDINE DYES

Abstract— The triplet states of some acridine dyes (proflavine, acriflavine, acridine yellow, acridine orange, ethacridine (Rivanol), benzoflavine), dissolved in a dimethylformamide/water matrix were investigated at 90°K by ESR. Zero-field splitting parameters were calculated from the ESR spectra. The relationship between the zero-field splitting and the distribution of the spin-density of the electrons of the triplet state of the acridine derivatives is discussed.     

EPR parameters and local geometry for Cr3+ and V2+ ions in HfS2 crystals

From the high-order perturbation formulas of EPR parameters (zero-field splitting D, g factors gparallel, gperpendicular and hyperfine structure constants Aparallel, Aperpendicular) based on the two spin-orbit coupling parameter model for 3d3 ions in trigonal symmetry, the EPR parameters of Cr3+ and V2+ ions in HfS2 crystals are calculated. From the calculations, it is found that the local trigonal distortion angle theta of impurity center in HfS2:Cr3+ is smaller than that in HfS2:V2+. The dominant cause of the small zero-field splitting |D| and g-anisotropy |Deltag|=|gparallel-gperpendicular| in HfS2:Cr3+ (compound to HfS2:V2+) is due to the small local trigonal distortion angle theta rather than to the small impurity-ligand distance R in HfS2:Cr3+.     

Studies of the g factors and zero-field splitting for the trigonal Ti(2+)(Ga)-S(p) defect center in GaP crystal

The EPR g factors g(parallel), g(perpendicular) and zero-field splitting D suggested to be caused by a donor-acceptor nearest-neighbour pair defect [Ti(Ga)(2+)-Sp] in GaP:Ti(2+) co-doped with sulphur are calculated from the high-order perturbation formulas based on a two spin-orbit coupling parameter model for the EPR parameters of 3d(2) ion in trigonal symmetry. The calculated results are close to the observed values. The suggestion of [Ti(Ga)(2+)-Sp] pair defect in GaP:Ti co-doped with sulphur is also confirmed from this calculation.     

Syntheses, optical and intramolecular magnetic properties of mono- and di-radicals based on nitronyl-nitroxide and oxoverdazyl groups appended to 2,6-bispyrazolylpyridine cores

This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-, and bi-radicals attached to 4-phenyl-2,6-bispyrazolylpyridine coupling unit, their optical, electron spin resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the axial zero-field splitting (zfs) parameter of the NN biradical (|D/hc| = 0.00719 cm(-1)) is larger than the OVZ biradical (|D/hc| = 0.00601 cm(-1)). Additionally both biradicals displayed forbidden half-field transitions (ΔM(s) = ±2; g(av) ~ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR measurements of the two biradicals showed a Curie magnetic behaviour of the ΔM(s) = ±2 signal intensities (χ(EPR)) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density distribution, zfs and the spin-spin exchange coupling (J) of both NN and OVZ based biradicals showed that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly degenerate singlet-triplet states with J(NN)?J(OVZ).     

Local tilting angles tau for Fe+ in Cd2+ site and Fe3+ in Si4+ site of CdSiP2 semiconductor

The EPR parameters (g factors, g(parallel), g(perdendicular) and zero-field splitting D) for Fe+ in Cd2+ site and Fe3+ in Si4+ site of CdSiP2 semiconductor are calculated from the distinct high-order perturbation formulas. From the calculations, the local tetragonal distortions and hence the local tilting angles tau (which are different from the corresponding host values) for both paramagnetic centers are estimated. The results are discussed.     

Dimethylsilyl- and methylene-bridged aromatic groups : II. Phosphorescent triplet states

The zero-field splitting parameters D and E, or D, were measured by electron spin spectroscopy for the triplet states of bis(4-biphenylyl)dimethylsilane, 4,4′-bis-(4-biphenylyldimethylsilyl)biphenyl, 1,2-diphenyltetramethyldisilane, several monosilanes, bis(4-biphenylyl)methane, 4,4′-bis(4-phenylbenzyl)biphenyl, and dimethyldiphenylmethane. The zero-field splitting values of compounds having biphenyl subunits bridged with either SiMe₂- or CH₂-groups, compared with those of mono- and disubstituted biphenyls, show that the triplet electrons are localized mainly on one biphenyl group in each of the bridged compounds on the time scale of the ESR observations.     

Calculation of the zero-field splitting tensor on the basis of hybrid density functional and Hartree-Fock theory

The zero-field splitting (ZFS) (expressed in terms of the D tensor) is the leading spin-Hamiltonian parameter for systems with a ground state spin S>12. To first order in perturbation theory, the ZFS arises from the direct spin-spin dipole-dipole interaction. To second order, contributions arise from spin-orbit coupling (SOC). The latter contributions are difficult to treat since the SOC mixes states of different multiplicities. This is an aspect of dominant importance for the correct prediction of the D tensor. In this work, the theory of the D tensor is discussed from the point of view of analytic derivative theory. Starting from a general earlier perturbation treatment [F. Neese and E. I. Soloman, Inorg. Chem. 37, 6568 (1998)], straightforward response equations are derived that are readily transferred to the self-consistent field (SCF) Hartree-Fock (HF) or density functional theory (DFT) framework. The main additional effort in such calculations arises from the solution of nine sets of nonstandard coupled-perturbed SCF equations. These equations have been implemented together with the spin-orbit mean-field representation of the SOC operator and a mean-field treatment of the direct spin-spin interaction into the ORCA electronic structure program. A series of test calculations on diatomic molecules with accurately known zero-field splittings shows that the new approach corrects most of the shortcomings of previous DFT based methods and, on average, leads to predictions within 10% of the experimental values. The slope of the correlation line is essentially unity for the B3LYP and BLYP functionals compared to approximately 0.5 in previous treatments.     

Determination of the hyperfine coupling constant and zero-field splitting in the ESR spectrum of Mn(2+) in calcite

The hyperfine coupling constant (A in units of energy or A' in units of magnetic field) and zero-field splitting (D in units of energy or D' in units of magnetic field) in the ESR spectrum of Mn(2+) in calcite were determined. The spreading of the non-central allowed transitions |3/2, m --> |5/2, m, |1/2, m --> |3/2, m, |-3/2, m --> |-1/2, m and |-5/2, m --> |-3/2, m was analysed and the experimental transitions were attributed. Particular relevance was given to the difference between the two types of resonances, shoulder or divergence, and to their origin (M, m). The analysis explains the presence of a weak shoulder between each of the five central doublets |-1/2, m --> |1/2, m. Six independent methods for calculating the hyperfine coupling constant and five methods for calculating the zero-field splitting, based on the analysis of the allowed and forbidden transitions, were provided. The values of the hyperfine coupling constant range from -93.9 to -94.6 G and those of the zero-field splitting range from -79.5 to -80.5 G. A critical evaluation of the advantages and drawbacks of the 11 methods is included: the best value for A' is -94.30 G and that for D' is -79.95 G.     

Studies of electron paramagnetic resonance parameters and defect structures for Ti2+ and V3+ ions in CdS crystals

By applying the high-order perturbation formulas based on the cluster approach for the EPR parameters of 3d2 ions in trigonal symmetry, the zero-field splitting D, g factors gparallel, gperpendicular, and hyperfine structure constants Aparallel, Aperpendicular for Ti2+ and V3+ ions in CdS crystals are studied. From the studies, the defect structures of these paramagnetic impurity centers are obtained and the EPR parameters are also explained reasonably.     

The zero-field splitting tensors of dimeric bis(dialkyldithiocarbamato)-copper(II) and -silver(II)

The zero-field splitting tensor of dimeric bis(dialkyldithiocarbamato)-copper(II) is separated into a dipolar and a spin—orbit part, which both have been calculated using the results of an extended Hückel molecular orbital calculation. It has been established that the zero-field splitting tensor of the corresponding Ag dimer is determined by spin—orbit interactions.     

Investigations of EPR parameters for the trigonal Ti3+-Ti3+ pair in beryl crystal

By using the complete diagonalization of energy matrix of 3d1 ions in trigonal symmetry, the EPR parameters (g factors g( parallel), g( perpendicular) and zero-field splitting D) of the trigonal Ti3+-Ti3+ pair in beryl crystal are calculated. In the calculations, the exchange interaction in the Ti3+-Ti3+ pair is taken as the perturbation and the local trigonal distortion in the defect center is considered. The results (which are in agreement with the experimental values) are discussed.     

Theoretical determination of the zero-field splitting in copper acetate monohydrate

The zero-field splitting of the copper acetate monohydrate complex is studied using wave function based calculations. The anisotropy parameters extracted from highly correlated methods are in excellent agreement with the most accurate experimental results; in particular, the negative sign of the axial anisotropy parameter D is reproduced. During several decades, the interpretation of experimental data based on an analytical expression derived from perturbation theory led to a positive D-value. Although the validity of this expression is confirmed, it is explained that the incorrect attribution of a positive D is related to the assumption of an antiferromagnetic coupling between excited states. We have found in the present work that this coupling is actually ferromagnetic. The analysis of the various contributions to the anisotropy parameters shows that both spin-spin and spin-orbit couplings participate in the magnetic anisotropy of this complex. Although the anisotropy arising from the spin-spin coupling is essentially independent of the level of calculation, the zero-field-splitting parameters resulting from the spin-orbit coupling are strongly sensitive to the effects of dynamic correlation. This works provides important new insights into the physical origin of the zero-field-splitting parameters in copper dimers.     

Rotational and translational dynamics of lysozyme in water-glycerol solution

In this paper, we report a study of the effect of solvent viscosity on both translational and rotational dynamics of a simple model protein: the egg white lysozyme. For this, we investigated the dynamical properties of lysozyme in mixtures water–glycerol by means of parallel measurements of photon correlation spectroscopy (PCS) and dielectric spectroscopy at radiofrequencies (DS). In the framework of the Debye–Stokes–Einstein theory, the translational and rotational coefficients allow an estimation of hydrodynamic radius of the protein. A decoupling between translational and rotational dynamics, observed as a different estimation of hydrodynamic radius, is reported in the literature for some systems. In order to ascertain if this effect is present also in our sample, we performed PCS and DS measurements on lysozyme–water–glycerol solutions. The content of glycerol was in the range of 0–70% w/w, with a solvent viscosity from 0.9 to about 10 cpoise, and the protein concentration was up to 20 mg ml⁻¹. The average sizes of lysozyme, obtained by the two methods, are remarkably different at high protein concentrations. However, the values of hydrodynamic radius extrapolated to infinite dilution are coincident and independent of glycerol. These results indicate that the diffusive behavior of lysozyme in the water–glycerol mixture is coherent with the Debye–Stokes–Einstein hydrodynamic model.     

Investigations of zero-field splitting (ZFS) and local distortion parameters of Fe3+ ions doped into three oxi-spinels (ZnAl2O4, MgAl2O4, and ZnGa2O4) by theoretical analysis

The local environment around the paramagnetic centers formed by the Fe(3+) ions doped into three oxi-spinel crystals (ZnAl(2)O(4), MgAl(2)O(4), and ZnGa(2)O(4)) is investigated utilizing the fourth-order perturbation formula of the axial zero-field splitting parameter D on the basis of the dominant spin-orbit coupling mechanism. In order to fix a plausible cubic space group for B-sites located by Al(3+)/Ga(3+) cations, several modeling approaches are used and the results are discussed in detail.     

Theoretical studies of lattice distortions around the impurity ions in V2+ doped CdCl2, CdI2 and PbI2

The lattice distortions around the impurity ions in V(2+) doped CdCl(2), CdI(2) and PbI(2) are theoretically studied from the perturbation formulas of the spin Hamiltonian (SH) parameters zero-field splitting, g factors and the hyperfine structure constants for a 3d(3) ion in trigonal symmetry based on the cluster approach. In these formulas, the contributions from the s-orbitals of the ligands are taken into account. Based on the studies, it is found that the local angles beta (between the impurity-ligand bonding lengths and the C(3) axis) in the impurity centers are smaller than the angles beta(H) in the hosts. The calculated SH parameters based on the above local angles beta show better agreement than those on neglecting of the ligand s-orbital contributions (and those on the host angles beta(H)) with the experimental data.