Masamoto Iwaizumi

Anisotropic hyperfine interaction in the electron spin resonance spectrum of the methyl radical trapped in CH3COONa⋅3D2O crystal at low temperatures

The electron spin resonance spectrum of the methyl radical produced by γ irradiation of CH3COONa⋅3D2O single crystal at 77 K has been measured at low temperatures. The spectrum observed at below 24 K consisted of eight lines with approximately equal intensities, instead of the four lines (1:3:3:1) at higher temperatures, due to the quantum tunneling of the rotational states. The transitions at low temperatures were angular dependent upon the applied field. From the analyses of the angular dependence of the spectrum with the aid of the theoretical calculation of the transition field for the tunneling methyl radical, the following anisotropic hyperfine tensor for the proton was determined (MHz): Axx=−31.4, Ayy=−93.7, Azz=−60.3, a (iso) =−61.8. These parameters provide the first experimental assessment of the well‐known theoretical results on the proton hyperfine anisotropy in the free methyl radical.

A time-resolved electron nuclear double resonance (ENDOR) study of the photo-excited triplet state

Abstract A time-resolved ENDOR (TRENDOR) apparatus was constructed and applied to observe hyperfine couplings in the photo-excited triplet (T 1 ) state. The TRENDOR spectra of the T 1 states were observed both in mixed crystals and glass. The hyperfine coupling constants were obtained a 4–100 K for quinoxaline in single crystals of durene and p -dibromobenzene and for tetraphenyporphine (H 2 TPP) in toluene glass. The time profile of the ENDOR signal was analyzed with population kinetics using a four-level system.

Interplanar interactions in the triplet dimers of Zn and metal free complexes of crowned porphyrin and phthalocyanine studied by time-resolved electron paramagnetic resonance

Abstract The intermolecular interactions were analyzed by the time-resolved EPR technique for crown ether substituted porphyrins and phthalocyanines. The contributions of charge resonance-type(CR) and exciton-type(EX) interactions were directly estimated on the basis of the zero field splitting parameters, D, of the excited triplet (T 1 ) states of the monomers and their crown ether bridged dimers. In the case of T 1 porphyrin, 3 (a 2u , e g ), it was found that the central metal(Zn) plays an important role on the interplanar interaction in contrast to a small effect observed in T 1 , phthalocyanine, 3 (a 1u , e g ). These results are interpreted in terms of the interaction between the 4p z orbital of Zn and a 2u of porphyrin. The magnitude of CR contribution is correlated well with those of the red shifts in the lowest excited singlet and triplet states of the dimers.

EPR studies of copper(II) and cobalt(II) complexes of adriamycin

Cu2+ and Co2+ complexes of adriamycin (ADM) in aqueous solutions have been examined using EPR spectroscopy. An appreciable amount of Cu2+ and Co2+ complexes formed in the solutions were found to be in the EPR silent associated form, where the metal ions are antiferromagnetically coupled. The associated form of the Cu2+ complex may be neither a simple dimer nor coordination polymer but aggregates of a stacked type. Formation of a complex having Cu2+-ADM stoichiometry of 1:2 was observed for the solutions containing excess of ADM as an EPR observable species. The complex having Cu2+-ADM stoichiometry of 1:1 was not observed directly by EPR, but the presence of the complex is undeniable, especially at low pH range so far as large excessive ADM is not present. The Co2+ complex of ADM observed by EPR is in the high-spin (S = 3/2) state and may have a coordination structure of tetragonal symmetry. The EPR spectra of these complexes apparently show that the Cu2+ and Co2+ ions are bound at the carbonyl and phenolate oxygen in the 1,4-dihydroxyanthraquinone moiety and the amino nitrogen in the sugar part does not seem to participate in the coordination to the metal ions.

THE LOWEST EXCITED TRIPLET STATES OF LANTHANIDE BISPHTHALOCYANINE COMPLEXES STUDIED BY TIME RESOLVED ELECTRON PARAMAGNETIC RESONANCE

Interplanar interactions were analysed for metal bridged phthalocyanine dimers, [NBu4] [M(III)Pc2] (M = Y, Yb and Lu), by using zero field splitting (ZFS) parameters in the T1 states. The observed ZFS D and a triplet lifetime decreased remarkably due to dimerization. From a comparison of D between the monomer and the dimer the interplanar interactions are divided into two types: exciton and charge resonance. It is found that nearly 50% of the charge resonance contribution (b 2) is involved in the T1 state of LuPc- 2. The correlations between b 2, the triplet lifetime and the ionic radius of the central metal are discussed in terms of the charge resonance interaction.

EPR and ENDOR studies of charge transfer interaction of cobalt(II) and copper(II) porphyrins with π donors and acceptors

Abstract EPR spectra of cobalt(II) complexes of octaethylporphyrin and tetraphenylporphyrin, CoOEP and CoTPP, are strongly affected by interactions with π electron donors or acceptors, the effects of the interaction being generally larger in CoOEP than in CoTPP. Much smaller but still significant effects of charge transfer complex formations were observed also on EPR and ENDOR spectra of copper(II) porphyrins, CuOEP and CuTPP. Direct charge transfer interactions between the metal d orbitals and the π donors or acceptors make important contributions to the perturbation of the metal d orbital states by the charge transfer complex formation.

Observation of spin‐correlated radical pairs via two‐dimensional electron paramagnetic resonance nutation spectroscopy

Two‐dimensional (2D) Fourier‐transform (FT) electron paramagnetic resonance (EPR) nutation experiments were done in order to verify anomalous flip‐angle dependence of a FT–EPR spectrum of a photochemically generated spin‐correlated radical pair (SCRP). It was demonstrated by both the experiment and theory that the signals of SCRP have additionally higher and lower nutation frequencies than that of a free radical. This spectroscopy provides a direct spectroscopic evidence of spin‐correlation within SCRP, and enables one to partially separate SCRP signals from free radical signals in the 2D EPR spectrum.

Fourier transform EPR studies of metallofullerene (La@C82) in CS2 solution

Abstract Electron spin relaxation times T 1 and T 2 of La@C 82 in CS 2 were measured by Fourier transform electron paramagnetic resonance at temperatures from 283 to 173 K. The obtained relaxation times show a clear m 1 dependence of La and its features change with temperature. Analysis of the temperature dependence of the electron spin relaxation times indicates that there is a thermal equilibrium between two configurations having different magnitudes for the anisotropy parameters in La@C 82 .

A time-resolved electron paramagnetic resonance study on the photoinduced electron transfer reactions from metalloporphyrins to quinones

Abstract The photoinduced electron transfer reactions in the systems of zinc tetraphenylporphyrin (ZnTPP) and magnesium tetraphenylporphines (MgTPPs) and 1,4-benzoquinone were investigated by means of a time-resolved electron paramagnetic resonance (EPR) technique and deuteration of the compounds. Three kinds of reaction intermediate - the porphyrin cation, the quinone anion and their ion pair - were definitely assigned by reference to their EPR parameters. From simultaneous observations of the separated ions (SIs) and the ion pair, it is established that the rates from a contact ion pair to a solvent separated ion pair (SSIP) and from an SSIP to an SI are accelerated by axial ligation and addition of the salt respectively. The magnitudes of these effects were different for the ZnTPP and MgTPP systems, and their origins are briefly discussed.

Free-radical mechanism by which triphenyltetrazolium chloride stimulates aerobic oxidation of NADPH by microsomes.

Abstract 1. 1. Free-radical intermediate of 2,3,5-triphenyltetrazolium chloride (TTC) (TTC • ) in the NADPH-microsomes-TTC system has been detected by means of ESR spectroscopy. 2. 2. In contrast to reduced TTC (triphenylformazan), TTC • is easily autoxidizable. The product of reduced O 2 is H 2 O 2 . 3. 3. The mechanism by which TTC stimulates the aerobic oxidation of NADPH by microsomes has been interpreted based on the above findings. 4. 4. Autoxidizability of TTC • may also shed some light on the controversies about the mechanisms of enzymatic reduction of TTC.