Shunzo Yamamoto

Solvent and Pressure Effects on the Tautomeric Equilibrium of 4-Phenylazo-1-naphthol

The azo-hydrazone tautomerism of 4-phenylazo-1-naphthol has been studied in ethanol-water and acetone-water systems and at high pressures. The hydrazone form is favoured by adding water to organic cosolvents and by applying pressure. The effects of solvent polarity and pressure on the equilibrium constant (KT) between the tautomers were examined by using Kirkwood-type equations applied to chemical equilibrium. Remarkable deviations were observed in the Kirkwood plots for both types of mixtures. Solvent-induced polarization, especially of the hydrazone form in a high polarity region, was suggested. In ethanol-water mixtures, correlation is fair between ln KT and ET, while in acetone-water system the ln KT against ET plot was curved. The above findings, together with the difference in the volume data for the two systems, suggest an existence of special interactions between acetone molecules and hydrophobic parts of 4-phenylazo-1-naphthol.

Photoinduced hydride transfer reaction between methylene blue and leuco crystal violet

Abstract The photoinduced hydride transfer reaction between methylene blue (MB+) and leuco crystal violet (CVH) was investigated spectrophotometrically under conditions of direct excitation of MB+ with steady-illumination of visible light and of photosensitization by benzophenone (BP) and α-nitronaphthalene (NNP) with UV light. It was found that upon the irradiation of the solution of MB+ and CVH with visible light, MB+ disappeared and crystal violet (CV+) formed simultaneously. The ratio of the CV+ concentration formed to that of MB+ consumed was unity. The photosensitized reactions by NNP gave similar results. From the findings that the fluorescence of MB+ was not quenched by CVH and the triplet photosensitizations were observed, it was concluded that the hydride transfer reaction in acetonitrile occurs in the triplet state of MB+. The quantum yield for the disappearance of MB+ in acetonitrile was estimated to be 1.1×10−4 and it was independent of CVH concentration. The electron transfer from CVH to triplet methylene blue ( 3 MB +∗ ) and the back electron transfer within the geminate radical pair must occur effectively. A spin adduct of a radical formed during the reaction was detected by the ESR method with 2-methyl-2-nitrosopropane (MNP) as a radical trap. This radical was attributed to CV . The results can be explained by a mechanism with stepwise electron–proton–electron transfers. In tert-butyl alcohol, however, it was observed that the fluorescence of MB+ was quenched by CVH and the quantum yield of the disappearance of MB+ depended on CVH concentration. Different mechanism from that in acetonitrile was proposed for the reaction in tert-butyl alcohol.

Mechanism of hydride transfer reaction from 4-(dimethylamino)phenyl methane derivatives to 2,3-dichloro-5,6-dicyano-p-benzoquinone

The kinetics of the hydride-transfer reactions from bis[4-(dimethylamino)phenyl] methane (MH2), bis[4-(dimethylamino)phenyl] methoxy methane (MHOMe), Leuco Malachite Green (MGH) and Leuco Crystal Violet (CVH) to 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) have been investigated spectrophotometrically by means of the stopped-flow technique. The negative activation enthalpies were obtained for the hydride-transfer reactions from MH2, MHOMe and MGH to DDQ in dichloromethane. These negative activation enthalpies suggest that the charge–transfer (CT) complexes are true intermediates in the hydride-transfer reactions from these donors to DDQ, as previously pointed out for MH2–DDQ system in trichloroethane. The formation of charge–transfer complexes between MH2, MHOMe, MGH and CVH and p-chloranil (CA) were observed in EPA (ethylether–pentane–ethanol) at low temperatures. The oxidation potential (Eox°) of MHOMe, MGH and CVH were estimated using the linear correlation between the CT transition energy and the Eox° value for the known CT complexes between CA and N,N-dimethylaniline derivatives. The rates were found to be correlated to the Eox° values of the donors.

Cd(5 3P1)-photosensitized luminescence of N,N,N′,N′-tetramethyl-1,3-propanediamine

Cd(5 3P1)-Photosensitized luminescence of N,N,N′,N′-tetramethyl-1,3-propanediamine has been studied under the conditions of steady-state illumination and pulsed laser excitation. There are two emission bands for the sensitized luminescence, the quantum yields of which were determined as well as the overall quenching rate constant of Cd(5 3P1) by the steady-state experiment. The temporal profiles of the two emission bands and that of the resonance fluorescence were measured under pulsed excitation conditions. It is suggested that both the emitters of the two bands are formed directly from Cd(5 3P1) and there are collision-induced mixing processes between these two kinds of emitters. The energy and entropy differences between these emitters were estimated.

Analysis of the effects of protic, aprotic, and multi-component solvents on the fluorescence emission of naphthalene and its exciplex with triethylamine

The emission spectra of naphthalene (NP)–triethylamine (TEA) systems were measured under steady-state illumination conditions in some protic and aprotic solvent-tetrahydrofuran (THF) mixtures. The fluorescence spectrum of the NP–TEA system in THF could be separated into two component bands (band A at 329 nm (fluorescence of NP) and band B at 468 nm (emission from an intermolecular exciplex)). The intensities of bands A and B decreased with increasing solvent polarity. The intensity of band B also decreased owing to the hydrogen-bonding interaction between TEA and protic solvents, but in this case the intensity of band A increased. The decrease in the intensity of band A with increasing solvent polarity is considered to be caused by the enhanced formation of an ion-pair parallel to the formation of an exciplex with increasing solvent polarity. The decrease in the intensity of band B is considered to be caused by the enhanced formation of ion-pair both parallel to and through the formation of the exciplex. The increase in the intensity of band A and the decrease in that of band B upon the addition of protic solvents is caused by the decrease in the concentration of free TEA. Acetonitrile only has a polar effect and trichloroacetic acid only has a hydrogen-bonding (protonation) effect, while alcohols have both the effects.

Effects of alcohols on emission spectra of toluene-triethylamine mixtures in THF: separation into polar and hydrogen-bonding interactions

The emission spectra of toluene (T)–triethylamine (TEA) systems were measured under conditions of steady-state illumination in some protic and aprotic solvent–THF mixtures. The fluorescence spectrum of the T–TEA system in THF could be separated into three component bands (band A at 279 nm (fluorescence of T), band B at 336 nm (fluorescence of TEA) and band C at 373 nm (emission from an intermolecular exciplex)). The intensities of bands B and C decreased with increasing solvent polarity. The decrease in the intensities of bands B and C is considered to be caused by the enhanced conversion of the exciplex to an ion-pair with increase in solvent polarity. The intensities of bands B and C also decreased owing to the hydrogen-bonding interaction between TEA and protic solvents, but in this case the intensity of band A increased. Acetonitrile only has a polar effect and trichloroacetic acid only has a hydrogen-bonding (or protonation) effect, while alcohols have both effects. The equilibrium constants for the formation of intermolecular hydrogen-bonded complexes of TEA with alcohols were estimated from the changes in the intensity of band A. The hydrogen-bonding and polar effects of alcohols on the intensities of bands B and C could be evaluated separately. The ratio of the hydrogen-bonding effect to the polar effect of alcohols was observed to increase with increasing vol.% of alcohol.

Kinetic studies on oxidations of leucomethylene blue and leucothionine by iron (III) in aqueous solution

The reaction kinetics and mechanism of the oxidations of leucomethylene blue (MBH) and leucothionine (TH) by iron (III) sulfate in aqueous solution were studied spectrophotometrically by the stopped-flow technique. MBH and TH, which were freshly prepared by photoreduction of methylene blue and thionine, respectively, with ascorbic acid were used in the kinetic measurements. The pseudo-first-order rate constants (kobsd) show kinetic saturation at high initial concentrations of iron (III) sulfate for MBH and TH. It was found that the reciprocal of kobsd increases linearly with increase in the reciprocal of [Fe3+]0. A broad absorption band was observed on mixing MBH and Fe3+ solutions at low temperatures, and this was attributed to a charge-transfer complex between MBH and Fe3+. The effects of Fe2+ ion and L-(+)-ascorbic acid on the rates of oxidation were also investigated. A small kinetic isotope effect on the oxidation rate for MBH was observed. The results can be explained by a general mechanism with stepwise electron–proton–electron transfers with the formation of a complex between reactants. Copyright

Effect of the size of the chelate ring on the stability of cyclic exciplexes between the cadmium(5 3P1) atom and diamines

The cadmium(5 3P1)-sensitized luminescence of some aliphatic diamines [R2N(CH2)nNR2; R = H or CH3, n= 1–6] has been studied under the conditions of steady-state illumination at temperatures between 493 and 609 K. It was found that the diamines of n= 2, 3, 4, and 5 have two emission bands (wavelengths at the peaks of these bands are ca. 450 and 550 nm), while the diamines of n= 1 and 6 have only a short-wavelength band. These two bands were assigned to two kinds of 1 : 1 exciplex (cyclic and acyclic ones) between an excited cadmium atom and a diamine molecule. From the temperature dependence of the equilibrium constants between two exciplexes, the enthalpy and entropy changes of the equilibrium were evaluated. The stability of the cyclic exciplexes assigned to the long-wavelength band was discussed in view of the ring size of cyclic exciplexes.

Pressure-resisting glass cell for high-pressure ESR measurements

The authors have described a pressure-resisting glass cell with which high-resolution ESR measurement of liquid samples under pressures up to 900 bar can be carried out on an ordinary high-resolution spectrometer. Recent investigations at high pressures have been presented.

Effects of Binary Mixed Surfactants on the Hydride Transfer Reaction of Methylene Blue and 2,5‐Dihydroxyl‐1,4‐benzoquinone in Aqueous Solution

Abstract The hydride transfer reaction between methylene blue (MB+) and 2,5‐dihydroxyl‐1,4‐benzoquinone (BQ) in mixed surfactant solutions has been studied at 25°C using a stopped‐flow spectrometer. The results indicate that the mixed surfactant systems of cationic or anionic with zwitterionic or nonionic surfactants (LSS/CTAB, LSS/TTAB, LSS/DTAB, LSS/SDS, Triton X‐100/CTAB, Triton X‐100/SDS, Brij35/CTAB, Brij35/TTAB, Brij35/DTAB, Brij35/SDS) have inhibition effects on the hydride transfer reactions, while a Brij35/TritonX‐100 mixed surfactant system shows no appreciable effect. These effects are attributed to the effect of the electrostatic interactions and compared with that of single surfactants. Above the cmc of the mixed surfactant systems, the effects are treated quantitatively using a kinetic model.