Faculty Profiles - KUMAGAI, Jun

Papers - KUMAGAI, Jun

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Decay of H₂-anions in solid parahydrogen via two-stage quantum tunneling diffusion processes Reviewed

T. Kumada, N. Kitagawa, S. Mori, J. Kumagai, Y. Aratono, T. Miyazaki

J. Low Temp. Phys. Vol. 114 ( 5-6 ) page： 413-429 1999.5

Language：English Publishing type：Research paper (scientific journal)

Decay mechanism of H-2(-) anions in X(gamma)-irradiated solid para-H-2 (p-H-2) has been studied using high-resolution ESR spectroscopy in the temperature range between 1.3-6.6 K. The results are summarized as follows. First, the decay rate constant of the H-2(-) anions is not proportional to initial yields of reactive species such as cations and H atoms but proportional to the concentration of less-reactive species of HD molecules in p-H-2. This result shows that the decay of the H-2(-) anion is due to the reaction neither with cations nor H atoms but the HD molecules originally contained in the p-H-2 sample at natural abundance. Second, the decay rate constant of the H-2(-) anions increases proportionally with the increase in temperature below 3 K and decreases with the increase in temperature between 3-5 K, although it increases exponentially with the increase in temperature above 5 K. This result indicates that the decay rate constant of the H-2(-) anions is controlled by the rate constant for the diffusion of the H-2(-) anions via one-phonon assisted quantum tunneling below 3 K, two-photon scattered or assisted quantum tunneling between 3-5 K, and thermally-activated process above 5 K.

DOI： 10.1023/A:1021850103903

突然変異や癌化を引き起こす長寿命ラジカルを電子スピンエコー法から探る Invited Reviewed

熊谷　純

放射線化学 Vol. 67 page： 20-25 1999

An ENDOR spectrum of H atoms in solid H2 Reviewed

T. Kumada, N. Kitagawa, T. Noda, J. Kumagai, Y. Aratono, T. Miyazaki

Chem. Phys. Lett. Vol. 288 ( 5-6 ) page： 755-759 1998.5

Language：English Publishing type：Research paper (scientific journal)

An ENDOR spectrum of H atoms produced in the gamma-ray irradiated solid H-2 was measured at 4.2 K in order to elucidate the structure of the local environment of the H atoms in solid H-2. We found that the H atoms were not trapped in interstitial sites but in substitutional sites of the solid, and almost all the ortho-H(2 )molecules at the first nearest sites from the H atoms converted into para-H-2 molecules. This result shows that the ortho-para conversion is induced by electron spins of the H atoms

DOI： 10.1016/S0009-2614(98)00296-6

Temperature Effect on Selective Tunneling Abstraction Reaction by H Atoms in Neopentane-Alkane Mixtures 4-100K Reviewed

J. Kumagai, T. Kumada, N. KItagawa, N. Morishita, T. MIyazaki

J. Phys. Chem. A Vol. 102 ( 17 ) page： 2842-2846 1998.4

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

When H atoms are produced by the photolysis of HI or the radiolysis of neo-C5H12 in the neo-C5H12-i-C4H10(2 mol %) mixtures above 40 K, the H atoms react selectively with the solute i-C4H10. The selective reaction is suppressed below 30 K. The reactions have been studied by electron spin resonance and electron spin echo measurements at 4-100 K. The drastic temperature effect on the reaction by H atoms suggests that the H atoms concerned are not hot atoms, but thermal atoms. The thermal H atoms react with the solute alkane by quantum tunneling at low temperature. Temperature effects on the ESR spectra and the spin-lattice relaxation times of the solvent neo-C5H11 radicals and the solute t-C4H9 radicals indicate that local motions of the solute i-C4H10 molecules take place above 40 K, while motions of the solvent neo-C5H12 molecules are severely restricted even at 100 K. It is concluded that the local motions of the solute i-C4H10 assist the tunneling abstraction reaction by H atoms, resulting in the selective reaction with the solute above 40 K.

DOI： 10.1021/jp9801020

Structure of Conduction Electrons on Polysilanes As Studied by Electron Spin-Echo Envelope Modulation Analysis Reviewed

T. Ichikawa, H. Koizumi, J. Kumagai

J. Phys. Chem. B Vol. 101 ( 50 ) page： 10698-10703 1997.12

Language：English Publishing type：Research paper (scientific journal)

The orbital structures of conduction electrons on permethylated oligosilane, Si-8(CH3)(18), and poly(cyclohexylmethylsilane) have been determined by analyzing the electron spin-echo envelope modulation signals of the radical anions of these silanes in a deuterated rigid matrix at 77 K. The conduction electron on permethylated oligosilane is delocalized over the entire main chain, whereas that on poly(cyclohexylmethylsilane) is localized on a part of the main chain composed of about six Si atoms, Quantum-chemical calculations suggest that Anderson localization due to fluctuation of sigma conjugation by conformational disorder of the main chain is responsible for the localization of both the conduction electron and the hole.

DOI： 10.1021/jp972068f

Stabilization of Hydrogen Radical Anions in Solid Hydrogen Reviewed

T.Ichikawa, H. Tachikawa, J. Kumagai, T. Kumada, T. MIyazaki

J. Phys. Chem. A Vol. 101 ( 40 ) page： 7315-7317 1997.10

Language：English Publishing type：Research paper (scientific journal)

The electron spin resonance g-factor of H-2(-) in gamma-irradiated crystalline H-2 was found to be not 2.003 but 2.0023, A novel model has been proposed for explaining the formation and stabilization of Hz-in crystalline H-2. The model assumes H-2(-) to be located at the center of a cavity that has previously been occupied by a trapped electron. The excess electron of H-2(-) is forced to be attached on one H-2 in the cavity due to strong repulsive interactions between the excess electron and H-2 molecules surrounding the cavity. A quantum mechanical calculation shows that an electron trapped in the vacancy of crystalline Hz automatically converts to H-2(-) by drawing one of the surrounding H-2 molecules into the center of the vacancy.

DOI： 10.1021/jp971923s

Direct observation of radical formation by charge recombination

T. Ichikawa, K. Kagei, J. Kumagai, H. Koizumi, H. Yoshida, J. Kubo

Radiat. Phys. Chem. Vol. 49 ( 4 ) page： 433-436 1997.4

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

Formation of the 2,3,4-trihydronaphtyl radical from the radical cation of tetralin by charge recombination has been studied by electronic absorption spectroscopy. Illumination of the gamma-irradiated glassy alkane solution of tetralin with visible light at 77 K causes electron ejection from the radical anion of tetralin, and the ejected electron recombines with the radical cation to generate the excited tetralin, which then converts to the neutral 2,3,4-trihydronaphtyl radical. The quantum yield of the formation of the neutral radical is approximately the same as that for UV-excited tetralin, which suggests that highly excited states of tetralin are not important for the formation of the neutral radical. The highly excited tetralin molecules generated by charge recombination relax to the first excited state without any chemical reaction, and the scission of a C-H bond takes place under the first excited state.

DOI： 10.1016/S0969-806X(96)00164-8

Electronic Structure of Charge Carriers in a Polysilane Quantum Wire Reviewed

J. Kumagai, H. Yoshida, T. Ichikawa

Nucleonica Vol. 42 page： 465 1997

Interchain Migration of Electrons and Holes in Polysilanes Reviewed

J. Kumagai, H. Tachikawa, H. Yoshida, T. Ichikawa

J. Phys. Chem. Vol. 100 ( 42 ) page： 16777-16778 1996.10

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

The measurements of the ESR and optical absorption spectra of polysilane radical ions revealed that the hole in the radical cation is delocalized onto the side chains, whereas the excess electron in the radical anion is confined within the silicon main chain. This observation suggests that the side chains hinder the interchain hopping of the charge for the anion but not the cation. Therefore the hole is the primary charge carrier in polysilanes.

DOI： 10.1021/jp961004m

Radical ions of oligosilane and polysilane Reviewed

J. Kumagai, T. Ichikawa, H. Yoshida

Radiat. Phys. Chem. Vol. 47 ( 3 ) page： 497-499 1996.5

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

In order to elucidate electronic properties of polysilane Si-Si skeleton, the radical ions generated by gamma-irradiation of polycyclohexylmethylsilane and polymethylphenylsilane were studied by ESR and optical absorption methods. The ion radicals of permethyloligosilanes (Si-4 similar to Si-8) have also been studied for comparison. The observed results suggest that the unpaired electron is localized on a limited region of Si-Si skeleton in the polymers and it is delocalized on the whole Si-Si skeleton of the oligomers.

DOI： 10.1016/0969-806X(95)00147-P

Effect of ionizing radiation on polysilane Reviewed

J. Kumagai, K. Oyama, H. Yoshida, T. Ichikawa

Radiat. Phys. Chem. Vol. 47 ( 4 ) page： 631-636 1996.4

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

ESR and gel permeation chromatographic measurements of poly(diethylsilane) and poly(cyclohexylmethylsilane) gamma-irradiated at temperatures between 77 K and 300 K were carried out to elucidate the mechanism of radiation-induced degradation of polysilanes. Radical species observed after the irradiation are not silyl-type radicals generated by the cleavage of the Si-Si bond but alkyl radicals generated by the scission of the C-H bond. The G value of the main-chain scission depends strongly on the irradiation temperature and dose. It is proposed that sigma-conjugation along the main chain allows the migration of the radiation energy to the weakest bond in the main chain where the direct scission of the bond is induced.

DOI： 10.1016/0969-806X(95)00066-7

Formation and isomerization mechanisms of M⁺CO₂^-(M=Li, Na)ion-pair complexes. An Abinitio MO and RPKM study Reviewed

H. Tachikawa, Y. Takatori, A. Ohtake, J. Kumagai, H. Yoshida

Theochem. Vol. 342 page： 1-7 1995.10

Language：English Publishing type：Research paper (scientific journal)

Structures and electronic states of the M(+)CO(2)(-) (M = Li and Na) ion-pairs have been investigated by means of ab initio MO calculations. The potential energy surfaces (PESs) for the ion-pair formation reaction (M + CO2 --> M(+)CO(2)(-)) show that the ground state MCO(2) complex has a van der Waals (vdW) character (M . CO2) at the entrance region in the reaction and an ionic one (M(+)CO(2)(-)) at short M-C distances and that the activation barrier is found at the intermediate region between the vdW and ion-pair states. The wavefunctions of the ion-pair for the low-lying excited states are obtained by the multireference single and double excitation configuration interaction (MR-SD-CI) method. The first and second electronic excitations are characterized by charge-transfer transition from CO2- to Na+ and local excitation within the CO2- moiety in M(+)CO(2)(-) respectively. In order to elucidate the isomerization mechanism of the Li+CO2- ion-pair from C-s to C-2v structures, which was observed by Kafafi et al. (J. Am. Chem. Sec., 105 (1983) 3886), the Rice-Ramsperger-Kassel and Marcus (RRKM) theory is applied to calculate the microcanonical reaction rate along the reaction coordinate. The calculations strongly support their experimental results in which the reaction Li+CO2-(C-s) --> Li+CO2- (C-2v) occurs preferentially on IR irradiation.

Electronic Structure of Oligosilane and Polysilane Radical Cations as Studied by Electron Spin Resonance and Electronic Absorption Spectroscopy Reviewed

J. Kumagai, H. Yoshida, T. Ichikawa

J. Phys. Chem. Vol. 99 ( 20 ) page： 7965-7969 1995.5

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

The electron spin resonance (ESR) and the electronic absorption spectra of oligosilane (Si(n)R(n+1)R(n+1)', n = 2 similar to 6) radical cations in a cryogenic solid of a Freon mixture have been compared with those of poly(cyclohexylmethylsilane) and poly(methylphenylsilane) radical cations for elucidating the structure of the SOMO orbital of the radical cations or the HOMO orbitals of the neutral polysilanes, The absorption and ESR spectra of the polymer radical cations are similar to each other and also similar to those of the oligosilane radical cations, which suggests that the unpaired electron of the polymer radical cations is not delocalized all over the Si-Si main chain. The absorption spectra of the polysilane radical cations are composed of a UV and near IR bands which correspond to SOMO --> LUMO and SOMO - 1 --> SOMO, respectively. The oligosilane and the polysilane radical cations show broad isotropic ESR spectra due to hyperfine interactions with side-chain protons. Analysis of the ESR spectra shows that the unpaired electron of the polysilane radical cation is confined to only a part of the polymer chain composed of six Si atoms.

DOI： 10.1021/j100020a019

Electronec Structure of Oligosilane and Polysilane Radical Anions As Studied by Electron Spin Resonance and Electronic Absorption Spectroscopy Reviewed

J. Kumagai, H. Yoshida, H. Koizumi, T. Ichikawa

J. Phys. Chem. Vol. 98 ( 50 ) page： 13117-13122 1994.12

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

The electron spin resonance (ESR) and the electronic absorption spectra of poly(cyclohexylmethylsilane) and poly(methylphenylsilane) radical anions in a cryogenic 2-methyltetrahydrofuran solid have been compared with those of permethyloligosilanes [Si-n(CH3)(2n+2), n = 4-6] for elucidating the electronic structure of polysilane radical anions. The spectra of the polysilane radical anions are very similar to each other and also to those of the oligosilane radical anions in which an unpaired electron occupies not an antibonding sigma(not asymptotic to)(sp) orbital but a pseudo-pi(p) orbital composed of antibonding Si-Si and antibonding Si-C orbitals. The polysilane radical anions show axially symmetric ESR spectra with g(perpendicular to) = 2.0023 and g(parallel to) = 2.0064 and the electronic absorption spectra composed of UV and near-IR bands. Analysis of the ESR and the absorption spectra shows that the unpaired electron of the polysilane radical anions is not delocalized all over the polymer chain but is confined in only a part of the polymer chain, probably around the elongated Si-Si bond of a branched polymer chain.

DOI： 10.1021/j100101a005

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