Hiroshi Fukutomi

Nuclear magnetic resonance study of the kinetics of the water exchange process in the equatorial positions of uranyl complexes

Abstract The kinetics of the water exchange process in the equatorial positions of the aqua uranyl ion, mono-DMSO, mono-chloro, and mono-bromo complexes, have been studied by the NMR line broadening method in water-acetone mixtures. The measurements were made in the temperature region from 25 to −100°C and it was found that the relaxation process was controlled by water exchange below −50°C. The first-order rate constants for the exchange of the coordinated water molecules measured at −70°C for the various complexes are: k aq = 2.99 × 10 2 sec −1 , k DMSO = 4.55 × 10 2 sec −1 , k Cl = 1.63 × 10 2 sec −1 , and k Br = 2.65 × 10 2 sec −1 . The values of the activation parameters are 9.9 ± 0.5 kcal mol −1 for ΔH ≠ and 2.1 ± 2.6 e.u. for ΔS ≠ for the aqua complex; considerably smaller values for ΔH ≠ and ΔS ≠ are obtained in the mono-substituted complexes.

Kinetics and mechanism of the uranium(IV)-uranium(VI) electron exchange reaction in hydrochloric acid

Abstract The rate of the electron exchange reaction between U(IV) and U(VI) has been studied in high concentrations of hydrochloric acid and was found to be accelerated by large concentrations of chloride ion. From the rates at different temperatures, ranging from 30° to 70°C, an activation energy and entropy of 32·2 kcal/mole and 25·5 e.u., respectively, were calculated. Possible mechanisms consistent with the observed rate law are proposed.

Kinetic study of photoaccelerated U(IV)–U(VI) electron exchange reaction in hydrochloric acid

Abstract A kinetic study of the photoaccelerated U(IV)–U(VI) electron exchange reaction has been carried out in hydrochloric acid. The variation of the quantum yield as a function of wavelength of the incident light indicates that the exchange reaction is accelerated only by the light absorbed by uranium(VI). The rates and quantum yields were determined as a function of uranium(IV) and uranium(VI) concentrations, light intensity and temperature under light irradiation of 365 and 436 nm, which correspond to the main absorption bands of uranium(VI). For light absorption by the weak spin-forbidden visible band of uranium(VI), the quantum yield of the exchange reaction is larger than that for light absorption by the spin-allowed intense UV band. The reaction appears to be complex and the quantum yield for both visible and UV bands follows virtually the same kinetic rate law. The proposed mechanism involves formation of uranium(V) from uranium(IV) and excited uranium(VI), a one electron transfer reaction between uranium(V) and uranium(IV), and uranium(V) and uranium(VI) and the disappearance of uranium(V) through the disproportionation to uranium(IV) and uranium(VI). The respective quantum yields at 365 and 436 nm had essentially the same activation energy for the overall exchange reaction; 14.8±0.4 and 15.6±0.8 kcal/mole, respectively. The photoacceleration mechanism seems to involve the reaction from the lowest triplet excited state of uranium(VI).

A nuclear magnetic resonance study of the kinetics of ligand exchange reactions in uranyl complexes. VII: Acetylacetonate exchange in bis(acetylacetonato)(N, N-dimethylformamide)-dioxouranium(VI)

The exchange reaction of acac(acetylacetonate) in UO2(acac)2dmf (dmf=N,N-dimethylformamide) in o-dichlorobenzene has been studied by the NMR line-broadening method. The exchange rate depends on the concentration of the enol isomer of acetylacetone in its low region, and approaches the limiting value in its high region. It is proposed that the exchange reaction proceeds through the mechanism in which the dissociation of one end of the chelated acac is the rate-determining step. The kinetic parameters for this step are as follows: k (25 °C)=5.03 × 10−3 s−1, ΔH‡=91.6 ± 3.8 kJ mol−1, and ΔS‡ =17.2 ± 10.5 JK−1 mol−1. The exchange rate becomes slower by the addition of free DMF. This may be due to the competition of DMF with the enol isomer of acetylacetone in attacking a four-coordinated intermediate in the equatorial plane.

Equilibria and kinetics of dissociation of triphenylphosphine from [N,N′-ethylenebis(salicylideneiminato)]bis(triphenylphosphine)ruthenium(II) in benzene

The reactions (i) and (ii)[H2salen =N,N′-ethylenebis(salicylideneimine)] in benzene solution have been studied by both equilibrium and kinetic spectrophotometric methods. At 25 °C, k1= 1.61 ± 0.02 [Ru(salen)]+ PPh3 [graphic omitted] [Ru(salen)(PPh3)](i), [Ru(salen)(PPh3)]+ PPh3 [graphic omitted] [Ru(salen)(PPh3)2](ii) dm3mol–1s–1, k–1=(2.7 ± 0.4)× 10–4s–1, k2=(1.18 ± 0.05)× 10–1dm3mol–1s–1, k–2=(1.20 ± 0.09)× 10–3s–1, K1=(0.60 ± 0.08)× 104 dm3 mol–1, and K2= 98 ± 6 dm3 mol–1.

The first identification of hydrolysis species of uranyl ions by 17O n.m.r. spectroscopy

The main hydrolysis species of uranyl ions, [(UO2)2(OH)2]2+ and [(UO2)3(OH)5]+, were identified by 17O n.m.r. spectroscopy, signals being observed at 2.0 and 6.7 p.p.m. relative to the uranyl ion, respectively.

Kinetics and mechanism of malonate replacement in bis(malonato)oxovanadate(IV) by oxalate

The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal)2H2O]2−(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps (k1 and k2: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)]2−. The rates for each step depended linearly on the concentrations of free oxalate species, Hox− and ox2−. The second-order rate constants for the replacement by ox2− were much larger in the k1 step than in the k2 step and the activation parameters were determined as follows: ΔH≠= 43.5 ± 5.6 kJ mol−1, ΔS±-53 ± 19 J K−1 mol−1 and ΔH≠= 43.6 ± 0.5 kJ mol−1, δS≠ = -62 ± 2 J K−l mol−1 for the k1 and k2 steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm3 mol−1 at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants.