Showing papers by "Takashi Kubo published in 2009"

Halo structure of the island of inversion nucleus 31Ne.

Abstract: The cross sections for single-neutron removal from the very neutron-rich nucleus $^{31}\mathrm{Ne}$ on Pb and C targets have been measured at $230\text{ }\text{ }\mathrm{MeV}/\mathrm{\text{nucleon}}$ using the RIBF facility at RIKEN. The deduced large Coulomb breakup cross section of 540(70) mb is indicative of a soft $E1$ excitation. Comparison with direct-breakup model calculations suggests that the valence neutron of $^{31}\mathrm{Ne}$ occupies a low-$\ensuremath{\ell}$ orbital (most probably $2{p}_{3/2}$) with a small separation energy (${S}_{n}\ensuremath{\lesssim}0.8\text{ }\text{ }\mathrm{MeV}$), instead of being predominantly in the $1{f}_{7/2}$ orbital as expected from the conventional shell ordering. These findings suggest that $^{31}\mathrm{Ne}$ is the heaviest halo system known.

Resonance Balance Shift in Stacks of Delocalized Singlet Biradicals

Abstract: Recently we succeeded in the isolation of delocalized singlet biradicals utilizing the spin-delocalizing character of the phenalenyl radical. We demonstrated that the singlet biradical 1 has strong spin–spin interactions between molecules through the overlap of phenalenyl rings in the onedimensional (1D) chain even though the closed-shell Kekul structure 1 can be drawn as a resonance contributor (Scheme 1). Huang and Kertesz gave further insight into the spin–spin interactions from a theroretical point of view and showed that the spin–spin interaction between the molecules was predicted to be stronger than that within the molecule. These experimental and theoretical findings are associated with very fundamental issues: Do delocalized singlet biradicals actually have open-shell character? Are the electrons coupled within a molecule involved in covalent bonding between molecules? In this study we will demonstrate that intraand intermolecular spin–spin interactions strongly correlate and can be altered in magnitude by an applied external field. Our proposal is based on the experimentally determined molecular structure of 2, a temperature-dependent reflection spectrum of 2, and a pressuredependent reflection spectrum of 1. Methyl groups at the 2and 10-positions in 2, where the frontier molecular orbital has very small coefficients, are expected to alter the distance between the overlapping phenalenyl rings with respect to the analogous separation in 1, and as a result, the magnitude of the intermolecular spin–spin interaction should be affected. The synthesis of 2 is outlined in Scheme 2. The 3,10and 3,11-bis(bromomethyl) compounds 3 were synthesized according to the previously reported procedures. The individual isomers were not isolated because both were expected to lead to the single compound 2. Bis(2-methylpropionic acid) derivatives 5 were obtained in three steps by standard methods. Friedel–Crafts cyclization of the acyl chloride derivatives of 5 with AlCl3 afforded diketones 6. These were reduced with NaBH4 and subsequently dehydrated with a catalytic amount of p-toluenesulfonic acid to afford the dihydro compounds 8. Dehydrogenation of 8 with p-chloranil afforded the hydrocarbon 2 as green prisms. Compound 2 was found to be stable in the solid state at room temperature. The small HOMO–LUMO gap of 2, which is an essential factor for a singlet biradical electronic structure, was confirmed by electrochemical and optical methods. The cyclic voltammogram of 2 shows four reversible redox waves: E 2 = + 0.51, E 1 =+ 0.11, E red 1 = 1.09, and E 2 = 1.62 (V vs. ferrocene/ferrocenium couple (Fc/Fc), see Figure S1 in the Supporting Information), which led to an electrochemical HOMO–LUMO gap of 1.20 eV. The electronic absorption spectrum of 2 in CH2Cl2 shows an intense low-energy band at 756 nm (13200 cm = 1.64 eV, e= 115000, f= 0.605, see Scheme 1. Resonance structures of 1 and 2. The arrows in the biradical structure represent antiparallel spins.

An Extremely Simple Dibenzopentalene Synthesis from 2‐Bromo‐1‐ethynylbenzenes Using Nickel(0) Complexes: Construction of Its Derivatives with Various Functionalities

Abstract: Nice and easy: A very simple synthesis for dibenzopentalenes, which starts from 1-bromo-2-ethynylbenzenes, has been developed. It uses Ni(0) complexes (see scheme), from which a relatively stable Ni(II) complex as an important intermediate has been isolated. Dibenzopentalenes with various functional groups can be prepared by the procedure, and their electronic properties are consistent with theoretical calculations.An extremely simple dibenzopentalene synthesis from readily available 2-bromo-1-ethynylbenzenes using a nickel(0) complex is described. Although the yields are moderate, the formation of three C-C bonds in a single process and the high availability of the starting materials are important advantages of this reaction. The corresponding aryl-nickel(II) complex as an important intermediate was isolated as relatively stable crystals, and the structure was confirmed by X-ray crystallographic analysis. The high stability of this complex should play a key role in this reaction. The reaction is applicable to the preparation of dibenzopentalenes bearing various functional groups. Their electronic properties are consistent with theoretical calculations. The cyclic voltammograms of these compounds reveal highly amphoteric redox properties. In particular, the electron-donating property of a tetramethoxy derivative is greater than that of oligothiophenes and dibenzodithiophenes and almost comparable to that of pentacene.

Remarkable two-photon absorption in open-shell singlet systems

Abstract: Remarkable enhancement of two-photon absorption (TPA) peak is theoretically predicted in symmetric open-shell singlet diradical systems with intermediate diradical character as compared to closed-shell and pure diradical systems It is revealed that the largest TPA peak intensities occur for open-shell singlet diradicals having a ferromagnetically coupled ground state and strongly depend on the ratio between damping factors of the excited states This result confirms that open-shell singlet conjugated molecules with intermediate diradical characters have precedence over conventional closed-shell conjugated systems in resonant third-order nonlinear optical properties

Development of large deformation in 62Cr.

Abstract: The structure of neutron-rich isotopes 60Cr and 62Cr was studied via proton inelastic scattering in inverse kinematics. The deformation lengths (delta) for 60Cr and 62Cr were extracted as 1.12(16) and 1.36(14) fm, respectively, providing evidence for enhanced collectivity in these nuclei. An excited state at 1180(10) keV in 62Cr was identified for the first time. We adopted 4;{+} as its spin and parity, leading to the rapid increase of the Ex(4;{+})/E_{x}(2;{+}) ratio, which indicates the development of large deformation in 62Cr near N=40. Importance of the admixture of the gd-shell component above N=40 is also discussed by comparing with a modern shell model calculation.

Low-lying states inMg32studied by proton inelastic scattering

Abstract: Proton inelastic scattering on the neutron-rich nucleus $^{32}\mathrm{Mg}$ has been studied at 46.5 MeV/nucleon in inverse kinematics. Populated states were identified by measuring de-excitation $\ensuremath{\gamma}$ rays, in which five new states were found by $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence analyses. By analyzing the angular differential cross sections via coupled-channel calculations, their spins and parities were constrained and the amplitudes for each transition were extracted. The spin and parity of the 2321-keV state was assigned as ${4}_{1}^{+}$. The ratio between the energies of the ${2}_{1}^{+}$ and ${4}_{1}^{+}$ states indicates that $^{32}\mathrm{Mg}$ is a transitional nucleus rather than an axially deformed rigid rotor. The collectivities in the nucleus $^{32}\mathrm{Mg}$ with $N=20$ are discussed based on the results obtained in the present experiment.

Scanning Tunneling Microscopy Study of a Phenalenyl-Based Singlet Biradical on Graphite

Abstract: The adsorption of a phenalenyl-based singlet biradical, 8,16-diphenyl-s-indaceno[1,2,3-cd:5,6,7-c′d′]diphenalene (Ph2-IDPL), on a highly oriented pyrolytic graphite surface has been studied using scanning tunneling microscopy (STM), tunneling spectroscopy, and ultraviolet photoelectron spectroscopy (UPS) under ultra-high-vacuum conditions. Ph2-IDPL molecules form a highly ordered monolayer on the graphite surface. The STM images of Ph2-IDPL monolayers show a strong bias-dependence in the range from −1.8 to +1.6 V. The observed image contrast may be associated with the distortion of the adsorption configuration or structural deformation of the molecules. UPS and tunneling spectroscopy reveal the electronic structure of Ph2-IDPL. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was determined to be 2.7 eV from the measured peak-to-peak energy difference between the HOMO and LUMO in the tunneling spectrum.

Magnetic properties of iron(II) and cobalt(II) complexes of tetrakis(2-pyridyl)methane. Spin-crossover behavior in the cobalt(II) complex

Abstract: Iron(II) and cobalt(II) mononuclear metal complexes with tetrakis(2-pyridyl)methane have been synthesized and characterized. A gradual spin-crossover to the high-spin state above 300 K is observed ...

β Decay of the proton-rich nucleus Si24 and its mirror asymmetry

Abstract: $\ensuremath{\beta}$-decay spectroscopy of the proton-rich nucleus $^{24}\mathrm{Si}$ was performed. The decay scheme was reconstructed from results of delayed $\ensuremath{\gamma}$-ray and proton measurements. We observed two $\ensuremath{\beta}$ branches to bound states in $^{24}\mathrm{Al}$ for the first time. The branching ratios were determined to be $31(4)%$ and $23.9(15)%$ for the ${1}_{1}^{+}$ state at $0.426$ MeV and the state at $1.090$ MeV, respectively. The observation of an allowed transition to the $1.090$-MeV state enabled us to firmly determine its spin-parity as ${1}^{+}$. In the proton measurements performed with the $\ensuremath{\Delta}E\text{\ensuremath{-}}E$ method, we observed a new unbound level at $6.735$ MeV. The branching ratios to three unbound states, including the new level, were also determined for the first time. Based on the decay scheme, the $B(\mathrm{GT})$ values of $^{24}\mathrm{Si}$ were deduced. The $B(\mathrm{GT})$ values were smaller than those of the mirror nucleus $^{24}\mathrm{Ne}$ by $22%$ and $10%$ for the ${1}_{1}^{+}$ and ${1}_{2}^{+}$ states, respectively. The mirror asymmetries of $B(\mathrm{GT})$, observed in both the ${1}_{1}^{+}$ and the ${1}_{2}^{+}$ states, indicate changes in configuration in the wave function associated with the Thomas-Ehrman shift. To clarify the mechanism of this asymmetry, a comparison with shell-model calculations is also discussed. The calculations attribute the changes in configuration to the lowering of the $1{s}_{1/2}$ orbital.

Yrast spectroscopy in 49-51Ti via fusion-evaporation reaction induced by a radioactive beam

Abstract: In-beam \( \gamma\) -ray spectroscopy of high-spin states in 49-51Ti was performed via the fusion-evaporation reaction using a radioactive beam. By excitation function and \( \gamma\) -\( \gamma\) coincidence analysis, yrast high-spin levels up to I = (21/2-),(11+),(17/2-) in 49-51Ti were determined. The levels were compared with full-pf -shell model calculation. The level structure indicates the persistency of the N = 28 shell gap at yrast states in 49-51Ti .

Third‐Order Nonlinear Optical Properties of Open‐Shell Systems: Diradical Character and Spin State Dependences

Abstract: The third‐order nonlinear optical (NLO) properties of open‐shell singlet molecular systems are investigated using model and real molecular systems by the ab initio molecular orbital (MO) and density functional theory (DFT) approaches. The diradical character dependence of the second hyperpolarizability (γ)—the microscopic origin of the third‐order NLO properties—is elucidated as well as its spin state dependence: γ values for the singlet systems with intermediate diradical character are remarkably enhanced as compared to those of closed‐shell and pure diradical systems, and the γ values for intermediate diradical systems are significantly reduced by changing from singlet to triplet state. Such features are exemplified by two‐site diradical model systems, diphenalenyl diradicaloids, and nanographenes. The γ values of multi‐radical systems including one‐dimensional diphenalenyl diradical clusters are also examined, while the intermolecular interaction effects and the size dependence of γ per unit are discus...