Tokuko Watanabe

Evidence for surface Ag+ complexes as the SERS-active sites on Ag electrodes

Abstract Evidence is given that SERS-active sites at Ag electrodes are associated with Ag + ions, forming sparingly soluble surface complexes with ligands such as pyridine molecules and halide ions. Such surface Ag + complexes contribute a factor of >800 to the overall (10 7 -fold) enhancement, possibly via a resonance Raman effect.

The network structure of gellan gum hydrogels based on the structural parameters by the analysis of the restricted diffusion of water

Abstract Gellan gum gels formed with potassium and calcium ions were investigated by the pulsed field gradient (PFG) stimulated echo NMR method. The structure of the gel network of the gellan gum gel was discussed on the basis of the interbarrier distance, a, the permeability of the barrier, p, and the actual diffusion coefficient of water, D0. These structural parameters were estimated by the analysis of the restricted diffusion of water in the gel network. Three different types of gel, namely, weak gel, true gel and brittle gel, were obtained at various concentrations of the gel-promoting cation. Interestingly, the topological structure of the network in each type of gel as reflected by the structural parameters is identical in K+-bridged gels and Ca2+-bridged gels, although the absolute concentration of the cation needed to form the different network types is quite different. It is assumed that the binding structures at the crosslink and at the bridge between the helices are different between the type of cation. Nevertheless, structural properties of the network such as the average distance between junction zones and the permeability of the junction zone were independent of the type of gel-promoting cation. The change in the overall gel architecture formed by the junction zones in the gelation process seems to be identical in the cases of monovalent and divalent cations and the difference is only the gel-forming efficacy of cations.

Effects of underpotentially deposited TI and Pb submonolayers on the surface-enhanced Raman scattering (SERS) from pyridine at Ag electrodes

Abstract SERS from pyridine on an Ag electrode is almost completely quenched by underpotential deposition of TI to a surface coverage of ≈ 3%. Removal of TI by anodic stripping does not restore the original SERS intensity. These results suggest that only a small fraction of adsorbed pyridine molecules is SERS active, and TI is specifically deposited on such a SERS active site, which then undergoes irreversible destruction. Pb appears to be non-specifically deposited on the Ag surface, but exhibits a similar irreversible quenching of SERS.

ESR spectra on single crystals of alkali metal fulleride complexes by means of wet-chemical synthesis

Abstract The facile and simple wet-chemical synthesis of single crystals of alkali metal doped fullerenes was developed by means of diffusion in organic solvents of C 60 and alkali metal tetraphenylborate. The single crystals M x C 60 (THF) y (M = Na, K) posess a hexagonal unit cell and ESR showed the existence of a typical broad C 60 anion radical signal, in addition to a small sharp signal which is amenable to microwave power saturation. The existence of the spike signal is almost always involved in fullerene anion radical systems. The solid-state 23 Na and solid-state 13 C NMR on Na x C 60 (THF) y also revealed the homogeneity of the single crystal.

Gel structure and water diffusion phenomena in starch gels studied by pulsed field gradient stimulated echo NMR

Abstract Water diffusion phenomena in potato starch gels (starch concentrations in the range of 17.1–42.0 w/w%) were observed as restricted diffusion by the pulsed field gradient stimulated echo (PFGSTE) NMR method. The size of water compartments, a , permeability of the barriers, p , and self diffusion coefficient of compartmentalized water between the barriers, D 0 , were estimated using a theoretical equation. These parameters changed with both starch concentration and duration of storage time. The structure of the starch gels and the structural change during retrogradation are discussed with respect to these parameters.

Hydration structure and dynamics in pullulan aqueous solution based on 1H NMR relaxation time

Abstract Dynamic properties of water and polymer chain proton in pullulan/H2O systems in aqueous solutions and in frozen states were analyzed based on T2 relaxation times in 1H-NMR and DSC. Two relaxing species with different T2 detected in the CPMG pulse sequence were assigned to inert polymer protons with the shorter T2 and to water protons with the longer T2 by using deuterated pullulan solutions in D2O. It has been proved that hydration water and free water undergoes rapid exchange in pullulan aqueous solutions. In the frozen state at −11°C, protons in ice crystals (T 2 ∼17 μs ), protons in mobile water (T2>ms) and inert and labile protons in polymer chains (T 2 ∼0.1 ms ) were distinguished in FID curves measured by using the solid echo pulse sequence. With increase in temperature, the inert protons and the labile protons show different mobility, and the inert protons are separately observable from the labile protons in the Carr–Percell–Meiboom–Gill (CPMG) method at temperatures higher than 0°C and the labile protons become mixed with water protons by rapid chemical exchange. These findings indicate the extremely high flexibility of the pullulan chain in aqueous solution.

ELECTRON SPIN RESONANCE OF γ -IRRADIATED COBALT(III) COMPLEXES; ELECTRONIC AND SPIN STATES OF HOT COBALT(II) IONS

Abstract ESR investigation of the γ -ray irradiated Co(III) complexes have been carried out. The identification and the classification of the irradiation products on the basis of the ESR signal patterns, and the electronic and the spin states of the products are discussed. Co(III) complexes are reduced by the γ irradiation into Co(II) complexes, where the produced Co(II) ions have low spin configurations, S = ½. Authors refer to these particular spin states as hot ions.

Origin of the resistivity anomalies of (EDT–TTF)[M(dmit)2](M = Ni, Pd)

α-(EDT–TTF)[Ni(dmit)2] comprises crossing columns of EDT–TTF and Ni(dmit)2 and exhibits unique metallic behaviour with a characteristic resistivity peak at around 14 K. The possibility of the development of the superstructure was examined down to 9 K by using a low-temperature imaging plate X-ray system, but no extra X-ray reflections could be found below 14 K. The 14 K anomaly is, therefore, thought not to originate from charge density wave (CDW) instability. New phases of γ-(EDT–TTF)[Pd(dmit)2] and γ-(EDT–TTF)[Ni(dmit)2] were found. γ-(EDT–TTF)[Pd(dmit)2] has parallel columns of EDT–TTF and Pd(dmit)2 and shows a resistivity anomaly around 80 K. Below 80 K, X-ray satellite reflections indicating CDW formation were observed. γ-(EDT–TTF)[Ni(dmit)2] undergoes a metal–insulator transition at around 100 K.

Structural change of polymer chains of gellan monitored by circular dichroism

The random-coil-to-helix transition of gellan gum with various counter monocations, such as Li+, Na+, K+, Rb+, Cs+ and tetramethyl ammonium (TMA+) in aqueous solution has been studied by circular dichroism (CD) and NMR methods in the absence of any additional salts. CD data show a sharp decrease in the molar ellipticity at 202 nm at the transition point, the temperature of which is defined as T CD, i.e., the transition temperature determined by CD. T CD depends on the concentration of gellan and on the countercation, for example, from 0.65 to 21.2 °C for 0.1–0.92% of Na-type gellan solution and from 6.7 to 26.1 °C for 0.13–1.0% Rb-type gellan solution. The molar ellipticity changes reversibly over the whole temperature range investigated for 1% Na, Rb, and TMA-type gellan solutions, while it shows a temperature delay in the recovery to random coil for 1% Li- and K-type gellan solutions. This phenomenon becomes more remarkable in higher concentration solutions. The water proton spin-spin relaxation time, T 2, drastically decreases around the transition temperature. The temperature at which the decrease of T 2 begins, however, is higher than T CD in some cases. This observation indicates that a change in the state of water (mobility, hydrogen bonding, hydration structure and so on) may occur preceding the helix formation of the polymer chain. The T 2 value shows the irreversibility of the transition in 1 % Li, K, and TMA gellan solutions. In comparison to the result obtained from CD, the aggregate formed at low temperature still remains as an aggregation of random coils at higher temperatures in the heating process.

CHEMICAL SHIFT, CHEMICAL SHIFT ANISOTROPY, AND SPIN-LATTICE RELAXATION TIME IN 89Y-MAS AND -STATIC NMR OF YTTRIUM COMPOUNDS

The chemical shift, the range of chemical shift anisotropy, and the spin-lattice relaxation times (T1) in Y2O3, Y3Al5O12, and Y2O2S have been measured by solid state 89Y (nuclear spin 1/2)-MAS and -static NMR. The static NMR data and T1 are reported for the first time. The range of chemical shift anisotropy was 1500—2400 Hz. This range was influenced more by the nature of the atom bound to Y than by the coordination number or the crystallographic symmetry of Y. Very long T1 values were obtained for Y2O2S (6.61 h) and Y2O3 (3.92 h at 24d site and 3.81 h at 8b site). On the other hand, the T1 value (1.10 h) of Y3Al5O12 is much shorter compared with those of Y2O2S and Y2O3. The next-nearest neighboring atom of Y in Y3Al5O12 is identified as Al, which has the nuclear spin 5/2 of 100% natural abundance. The most likely origin of significantly shorter T1 of Y3Al5O12 is a dipole–dipole interaction between 89Y and 27Al.