Analysis of the covalent bonding and EPR parameters of dendrimeric liquid-crystal copper(II) complexes

Experimental EPR data for copper(II) complexes with liquid-crystal poly(propylenimine) den-drimers were analyzed. The influence of the pseudotetrahedral distortion of the nearest environment of the copper ion on the parameters of the EPR spectra was considered. The covalent bond parameters were calculated from the EPR data. The dependence of the delocalization of the unpaired electron of the copper ion on the pseudotetrahedral distortion of its coordination unit was determined. The behavior of the Zeeman coupling parameters was discussed. Various contributions to the components of the hyperfine coupling (HFC) tensor were calculated. The causes of the changes in the HFC parameters in the distorted complexes were dis cussed.     

On the hyperfine coupling in tetragonal copper(II) complexes

The vibronic effect on the hyperfine coupling (HFC) parameters in tetragonal copper(II) complexes was studied. The parameters of the vibronic contributions from the - and 4s-AOs of the Cu²⁺ ion to the ground-state function were estimated. The anomalous behavior of the isotropic HFC was analyzed in terms of the static model with consideration of the vibronic coupling. The static and vibronic contributions to the anisotropic portions of the components of the HFC tensor were estimated.Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 3, 2005, pp. 204–206.Original Russian Text Copyright © 2005 by Muravev.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Features of EPR spectra of complexes of ortho-benzoquinones with Tl(I) and Tl(III) diethyldithiocarbamates

A study has been made of the EPR spectra of Tl(l) and Tl(III) o-semiquinolates, obtained by the interaction of o-benzoquinones with thallium amalgam and thallium(III) diethyldithiocarbamate, respectively. A strong temperature dependence has been found for the constant of hyperfine coupling (HFC) with the metal nucleus in thallium(III) 3,5-di-tert-butyl-o-semiquinolate; this is explained by the presence of competing mechanisms of spin transfer from the o-semiquinone ligand to the metal. Cation exchange and effective complexation of the Tl(I) o-semiquinolate with thallium diethyldithiocarbamate have been observed. The kinetic parameters of exchange have been determined.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 786–790, April, 1990.     

Radical cations of branched alkanes as observed in irradiated solutions by the method of time-resolved magnetic field effect

Spin dynamics in radical ion pairs formed under ionizing irradiation of n-hexane solutions of two branched alkanes 2,3-dimethylbutane and 2,2,4-trimethylpentane has been studied by the method of time-resolved magnetic field effect in recombination fluorescence. Experimental curves of the magnetic field effect are satisfactorily described by assuming that the spin dynamics is determined by the hyperfine interactions in the radical cation (RC) of branched alkane under study with hyperfine coupling (HFC) constants averaged by internal rotations of RC fragments. The HFC constants determined from the magnetic field effect curves are close to those estimated within DFT B3LYP approach. Analysis of the results indicates that at room temperature the lifetimes of the RC of the studied branched alkanes amount to, at least, tens of nanoseconds.     

New metal chelates of 5-amino-4-azopyrazoles: The crystal and molecular structure of bis{1-phenyl-3-methyl-4-(<Emphasis Type="Italic">p</Emphasis>-tolyl)azo-5-(<Emphasis Type="Italic">p</Emphasis>-carboxymethoxyphenyl)pyrazolamidato}copper(II)

The reactions of 1,3-alkyl(aryl) derivatives of 5-amino-4-azopyrazoles (HL) with copper(2+) acetate afford metal chelates CuL₂. Complexes with the CuN₄ chromophore containing a substituted amino group (X = NR) as a donor, like their azomethine analogues, have a pseudotetrahedral structure. Copper chelates with X = NH are planar. The X-ray diffraction data and an additional hyperfine structure (ahfs) from ¹⁵N atoms of the azo group in the labeled complexes provide evidence for the formation of six-membered metallocycles. The coordination polyhedron in bis{1-phenyl-3-methyl-4-(p-tolyl)azo-5-(p-carboxymethoxyphenyl)pyrazolamidato}Cu(II) can be described as a pseudotetrahedron.     

ESR study of spin-adducts of dialkoxyphosphoryl radicals with di(p-methoxyphenyl)methanofullerene

The addition of phosphoryl radicals to C₆₀C(p-C₆H₄OMe)₂ was shown (ESR) to result in the formation of at least seven isomers differing in their hyperfine coupling constants (HFC) with the phosphorus nuclei and in their kinetic behavior and thermodynamic stability.     

Carboxylate binding in copper histidine complexes in solution and in zeolite Y: X- and W-band pulsed EPR/ENDOR combined with DFT calculations

The complexes of copper with histidine exhibit a wide variety of coordination modes in aqueous solution. This stems from the three potential coordination sites of the histidine molecule and the existence of mono- and bis-complexes. The present work concentrates on the determination of the carboxylate binding mode, via the (13)C hyperfine coupling of the carboxyl, in a number of copper complexes in frozen solutions. These are then used as references for the determination of the coordination mode of two zeolite encapsulated complexes. The (13)C hyperfine coupling (sign and magnitude) was determined by a variety of advanced pulsed EPR and electron-nuclear double resonance (ENDOR) techniques carried out at conventional and high magnetic fields. These showed that while the carboxyl (13)C isotropic hyperfine coupling of an equatorially coordinated carboxylate is negative with a magnitude of 3-4 MHz, that of a free carboxylate is small ( approximately 1 MHz) and positive. To rationalize the experimentally determined ligand hyperfine couplings ((1)H and (13)C) and further understand their dependence on the coordination mode and degree of protonation, density functional theory (DFT) calculations were carried out on a number of model complexes, representing the different Cu-histidine complexes studied experimentally. The exchange-correlation functional used for the calculation of the EPR parameters was B3LYP with triple-zeta plus polarization (TZP) quality basis sets. While the polarization agreement between the magnitudes of the calculated and experimental values varied among the various nuclei, sometimes exhibiting deviations of up to 40%, an excellent agreement was found for the sign prediction. This shows the unique advantage of combining high field ENDOR, by which the sign of the hyperfine can often be determined, with DFT predictions for structure determination.     

Car-Parrinello molecular dynamics simulations and EPR property calculations on aqueous ubisemiquinone radical anion

Car-Parrinello molecular dynamics (CPMD) simulations have been performed on ubisemiquinone radical anion in aqueous solution. The different types of hydrogen bonding formed between the semiquinone and the solvent were studied in terms of frequency and directionality, in comparison with the parent benzosemiquinone radical anion. The EPR parameters (g-tensors and hyperfine coupling constants) were obtained from quantum chemical property calculations performed on snapshots along the MD trajectory, and the contributions of different solvation effects to the EPR parameters have been evaluated. The influence of the anion’s conformational behaviour was examined, including the orientation-dependent effects of hyperconjugation on side-chain hyperfine coupling.     

An analysis of the EPR spectral parameters and the electronic state of the low-symmetry complexes of 67Zn+, 111Cd+, and 205Tl2+ns1 ions in crystals with the potassium sulfate structure

The electronic structures of the low-symmetry complexes of 4s ¹-⁶⁷Zn⁺, 5s ¹-¹¹¹Cd⁺, and 6s ¹-²⁰⁵Tl²⁺ ions in isostructural crystals of potassium sulfate, rubidium sulfate, and potassium selenate were studied by the EPR data. The sp-mixing of the atomic orbitals of the paramagnetic ion in the molecular orbital of the ground state was established. The parameters of the EPR spectra were analyzed. The sp-mixing was shown to result in negative shifts of the Zeeman interaction parameters. The signs of components of the hyperfine coupling tensor (A tensor) were considered. The determining role of the contribution from the isotropic hyperfine coupling to the components of the A tensor was found. The anomalously low observed values of the hyperfine coupling parameters of the thallium complexes were interpreted.     

Pulsed ENDOR study of Cu(I)-NO adsorption complexes in Cu-L zeolite

The local environments of Cu(I)-NO adsorption complexes formed in zeolites Cu-L and Cu-ZSM-5 were studied by electron spin resonance (ESR), pulsed electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE). Cu(I)-NO complexes have attracted special interest because they are important intermediates in the catalytic decomposition of nitric oxide over copper exchanged zeolites. Recently, detailed structures of the complexes in Cu-ZSM-5 zeolites, O2-Al-O2-Cu(I)-NO, have been proposed on the basis of quantum chemical calculations (Pietrzyk, et al. J. Phys. Chem. B 2003, 107, 6105. Dedecek, et al. Phys. Chem. Chem. Phys. 2002, 4, 5406). 27Al pulsed ENDOR and HYSCORE experiments allowed the hyperfine coupling parameters of an aluminum nuclei found in the vicinity of the Cu(I)-NO complex formed in zeolite Cu-L to be estimated. The data indicate that the aluminum atom is located in the third coordination sphere of the adsorbed NO molecule in agreement with the suggested geometry of the adsorption sites. Broad distributions of aluminum nuclear quadrupole and hyperfine coupling parameters and short electron spin relaxation times of the Cu(I)-NO species prevented the determination of the 27Al hyperfine couplings for zeolite Cu-ZSM-5.     

Preparative-scale one-pot syntheses of hexafluoro-1,3-butadiene

Hexafluoro-1,3-butadiene, with its negligible global warming potential, is required in quantities for application in plasma dielectric etching in semiconductor industry and as gaseous microbubble suspension contrast agents in diagnostic ultrasound imaging. Three efficient protocols for the preparation of perfluoro-1,3-butadiene in 62–70% overall yields have been described. They involve the coupling of (1) iodotrifluoroethylene (ITFE) with activated copper, (2) trifluorovinylzinc bromide in the presence of copper (II) or iron (III) salts and (3) trifluorovinylzinc chloride, prepared from 1,1,1,2-tetrafluorethane (HFC 134a) in the presence of copper (II) or iron (III) salts.     

Density Functional Calculations of 55Mn, 14N and 13C Electron Paramagnetic Resonance Parameters Support an Energetically Feasible Model System for the S2 State of the Oxygen‐Evolving Complex of Photosystem II

Metal and ligand hyperfine couplings of a previously suggested, energetically feasible Mn₄Ca model cluster ( SG2009^?1 ) for the S₂ state of the oxygen‐evolving complex (OEC) of photosystem II (PSII) have been studied by broken‐symmetry density functional methods and compared with other suggested structural and spectroscopic models. This was carried out explicitly for different spin‐coupling patterns of the S=1/2 ground state of the Mn^III(Mn^IV)₃ cluster. By applying spin‐projection techniques and a scaling of the manganese hyperfine couplings, computation of the hyperfine and nuclear quadrupole coupling parameters allows a direct evaluation of the proposed models in comparison with data obtained from the simulation of EPR, ENDOR, and ESEEM spectra. The computation of ⁵⁵Mn hyperfine couplings (HFCs) for SG2009^?1 gives excellent agreement with experiment. However, at the current level of spin projection, the ⁵⁵Mn HFCs do not appear sufficiently accurate to distinguish between different structural models. Yet, of all the models studied, SG2009^?1 is the only one with the Mn^III site at the Mn_C center, which is coordinated by histidine (D1‐His332). The computed histidine ¹⁴N HFC anisotropy for SG2009^?1 gives much better agreement with ESEEM data than the other models, in which Mn_C is an Mn^IV site, thus supporting the validity of the model. The ¹³C HFCs of various carboxylates have been compared with ¹³C ENDOR data for PSII preparations with ¹³C‐labelled alanine.     

Hyperfine coupling tensors of the benzosemiquinone radical anion from Car-Parrinello molecular dynamics.

Based on Car-Parrinello ab initio molecular dynamics simulations of the benzosemiquinone radical anion in both aqueous solution and the gas phase, density functional calculations provide the currently most refined EPR hyperfine coupling (HFC) tensors of semiquinone nuclei and solvent protons. For snapshots taken at regular intervals from the molecular dynamics trajectories, cluster models with different criteria for inclusion of water molecules and an additional continuum solvent model are used to analyse the HFCs. These models provide a detailed picture of the effects of dynamics and of different intermolecular interactions on the spin-density distribution and HFC tensors. Comparison with static calculations allows an assessment of the importance of dynamical effects, and of error compensation in static DFT calculations. Solvent proton HFCs depend characteristically on the position relative to the semiquinone radical anion. A point-dipolar model works well for in-plane hydrogen-bonded protons but deviates from the quantum chemical values for out-of-plane hydrogen bonding.     

ESR SPECTRA OF THE SQUARE PLANAR COPPER(II) COMPLEXES WITH VARIOUS N4-MACROCYCLIC LIGANDS

Abstract

The ESR parameters were determined for copper(II) complexes with various 14-, 15- and 16-membered N₄-macrocyclic ligands. Of these ESR parameters g^‖ has the most simple relation to the covalency of the coordination bonds in the present compounds, i.e., the smaller the g^‖ value the larger the covalency. The order of covalencies derived from the g^‖ values nicely matches with that supposed from the size of macrocyclic rings and the charge of the ligands. The approach to the estimation of covalency of the coordination bonds by use of copper hyperfine coupling constant was examined, but it was found that this approach gives rather unreliable results for the present compounds.     

EPR Spectra of complexes of 3,5-di-tert-butyl-o-benzoquinone with lead diethyldithiocarbamate,ethylxanthate, and o,o-diethyldithiophosphate and reaction with bases

We have observed the solvent dependence of the hyperfine coupling constant for hyperfine coupling with the metal nucleus in ortho-semiquinone complexes of lead(II) diethyldithiocarbamate, ethylxanthate, and O,O-diethyldithiophosphate. This is connected with the process of solvation of the metal cation with the solvent molecules. We observe a correlation between the hyperfine coupling constant (due to the²⁰⁷Pb isotope) and the basicity factor of the solvent. Using the EPR method, we have studied the dynamics of solvation of the 3,5-di-tert-butyl-o-semiquinone complex of lead diethyldithiocarbamate by amines. We have obtained the thermodynamic and kinetic parameters of the solvation process. We observe a compensation effect for the kinetics of the forward and backward solvation reactions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 52–58, January, 1990.     

ESR single-crystal study of radical and radical-pair formation in some γ-irradiated vinyl monomers

Single crystals of methyl methacrylate (MMA), methyl acrylate (MA), and acrolein (A) have been prepared by a low-temperature technique. After irradiation with γ-rays at 77°K the paramagnetic species were identified by ESR spectroscopy. MMA gave a seven-line single spectrum from radicals formed by hydrogen addition. The hyperfine coupling constants are slightly anisotropic with a mean value of 22 G. Radical pairs were observed as ΔM_s = 1 and ΔM_s = 2 transitions; the hyperfine coupling was 11 G. From the strongly anisotropic dipolar interaction, upper limits for the distances between the pair components were calculated to be 5.45 Å and 6.3 Å. MA gave a five-line main spectrum with the same hyperfine coupling values and two radical pairs, one with a distance 5.9 Å between the components. In a there was also a strongly anisotropic interaction. The hyperfine coupling of the ΔM_s = 2 transition was 9.8 G. The number of radical pairs compared to the total number of radicals increases only slightly with the radiation dose. This makes it likely that pair formation occurs in the spurs and blobs formed by the γ-radiation. At an increased temperature the radical pairs disappeared; the spectrum of MMA changed to that characteristic of propagating polymer radicals.     

Synthesis and ESR spectra of [4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzosemiquinonato]thallium(i)

[4,6-Di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzosemiquinonato]thallium(i) was synthesized and characterized by IR and ESR spectroscopy. The hyperfine coupling constants with ^203/205Tl nuclei were found to depend strongly on the nature (solvating ability) of solvents. At 298 K, the HFC constant a _Tl changes from 3.09 mT in n-hexane up to 19.70 mT in tetramethylethylenediamine. The coordination number of solvation was found to be 1 for DMF—benzene and pyridine—hexane systems. The thermodynamic characteristics of solvation in the pyridine—hexane system were determined.     

EPR-ENDOR of the Cu(I)NO complex of nitrite reductase

With limited reductant and nitrite under anaerobic conditions, copper-containing nitrite reductase (NiR) of Rhodobacter sphaeroides yielded endogenous NO and the Cu(I)NO derivative of NiR. (14)N- and (15)N-nitrite substrates gave rise to characteristic (14)NO and (15)NO EPR hyperfine features indicating NO involvement, and enrichment of NiR with (63)Cu isotope caused an EPR line shape change showing copper involvement. A markedly similar Cu(I)NONiR complex was made by anaerobically adding a little endogenous NO gas to reduced protein and immediately freezing. The Cu(I)NONiR signal accounted for 60-90% of the integrated EPR intensity formerly associated with the Type 2 catalytic copper. Analysis of NO and Cu hyperfine couplings and comparison to couplings of inorganic Cu(I)NO model systems indicated approximately 50% spin on the N of NO and approximately 17% spin on Cu. ENDOR revealed weak nitrogen hyperfine coupling to one or more likely histidine ligands of copper. Although previous crystallography of the conservative I289V mutant had shown no structural change beyond the 289 position, this mutation, which eliminates the Cdelta1 methyl of I289, caused the Cu(I)NONiR EPR spectrum to change and proton ENDOR features to be significantly altered. The proton hyperfine coupling that was significantly altered was consistent with a dipolar interaction between the Cdelta1 protons of I289 and electron spin on the NO, where the NO would be located 3.0-3.7 A from these protons. Such a distance positions the NO of Cu(I)NO as an axial ligand to Type 2 Cu(I).     

Density functional restricted-unrestricted approach for nonlinear properties: application to electron paramagnetic resonance parameters of square planar copper complexes

The density functional restricted-unrestricted approach for treatments of spin polarization effects in molecular properties using spin restricted Kohn-Sham theory has been extended from linear to nonlinear properties. It is shown that the spin polarization contribution to a nonlinear property has the form of a quadratic response function that includes the zero-order Kohn-Sham operator, in analogy to the lower order case where the spin polarization correction to an expectation value has the form of a linear response function. The developed approach is used to formulate new schemes for computation of electronic g-tensors and hyperfine coupling constants, which include spin polarization effects within the framework of spin restricted Kohn-Sham theory. The proposed computational schemes are in the present work employed to study the spin polarization effects on electron paramagnetic resonance spin Hamiltonian parameters of square planar copper complexes. The obtained results indicate that spin polarization gives rise to sizable contributions to the hyperfine coupling tensor of copper in all investigated complexes, while the electronic g-tensors of these complexes are only marginally affected by spin polarization and other factors, such as choice of exchange-correlation functional or molecular structures, will have more pronounced impact on the accuracy of the results.