Koji Ishihara

High‐pressure stopped‐flow apparatus for the study of fast reactions in solution

A high‐pressure stopped‐flow apparatus with spectrophotometric detection has been exploited, which enables one to follow fast reactions in various media at pressure up to 2000 kg cm−2. The rate of the following reactions was successfully measured at various pressures: (1) reaction of nickel (II) nitrate with 1‐(2‐pyridylazo)‐2‐naphthol in glacial acetic acid and (2) reaction of nickel (II) perchlorate with isoquinoline in N,N‐dimethylformamide. The apparatus allows the measurement of activation volumes for reactions with half‐lives longer than several milliseconds.

Activation volume as evidence for a dissociative-interchange mechanism of nickel(II) ion complexation with isoquinoline in water, N,N-dimethylformamide, acetonitrile, methanol, and ethanol

Activation volumes for the complexation of nickel(I1) ion with isoquinoline in various solvents were determined by a high-pressure stopped-flow technique. Values of activation volume for formation and dissociation of the (isoquinoline)nickel(II) complex are respectively 7.4 f 1.3 and 8.9 i 0.8 cm3 mol- in water, 9.3 f 0.3 and 12.2 f 0.3 cm3 mol- in N,N-dimethylformamide, 12.8 i 0.6 and 9.9 f 0.5 cm3 mol- in methanol, and 12.6 f 0.5 and 15.7 f 1.1 cm3 mol-I in ethanol. The activation volume of the complex formation in acetonitrile is 9.4 i 0.1 cm3 mol-. All the positive values of the activation volume strongly indicate that the ligand substitution reactions on nickel( 11) ion proceed via a dissociative-interchange mechanism in these solvents.

High‐pressure stopped‐flow nuclear magnetic resonance apparatus for the study of fast reactions in solution

A high‐pressure proton nuclear magnetic resonance (NMR) probe for a stopped‐flow method has been developed, which enables us to follow fast reactions at various pressures up to 200 MPa. The high‐pressure stopped‐flow NMR (HP‐SF‐NMR) method has been applied to the study of several reactions at high pressures: the reaction of tetramethyltin with iodine in chloroform and the solvent exchange reaction on aluminum(III) ion in N, N‐dimethylformamide. The volumes of activation for the iodomerization of tin and the solvent exchange reaction on aluminum(III) were determined to be −24.9±3.8 cm3u2009mol−1 and −2.2±0.8 cm3u2009mol−1, respectively. The apparatus permits measurement of activation volumes for reactions with half lives longer than a few seconds if the nuclear relaxation time of the measured proton is short enough. Several requirements for the applicability of HP‐SF‐NMR are discussed.

Formation kinetics of peroxovanadium(V) complexes in strongly acidic media as studied by a high-pressure stopped-flow technique

Abstract A high-pressure stopped-flow vessel was made of acid-proof tantalum block which enabled us to follow reactions of vanadium(V) ion with hydrogen peroxide in strongly acidic media. The rate for the formation of the monoperoxo complex (VO(O 2 ) + ) is expressed as d [VO(O 2 ) + ]/d t = ( k 1 [H + ] −1 + k 2 + k 3 [H + ])[VO 2 + ]− [H 2 O 2 ] where k 1 (s −1 ) (25 °C)=60.2 [Δ H ≠ (kJ mol −1 = 21.1 ± 2.6, ΔS ≠ (J mol −1 K −1 ) = − 140 ± 9, Δ V ≠ (cm 3 mol −1 ) = 9.9 ± 1.7], k 2 (mo1 −1 dm 3 s −1 ) (25 °C) = 3.47 × 10 3 [Δ H ≠ = 46.5 ± 1.3, Δ S ≠ = −21 ±4, Δ V ≠ = 2.8 ± 1.0] and k 3 (mol -2 dm 6 s −1 ) (25 °C) = 1.63 × 10 3 [Δ H ≠ = 20.1 ± 4.7, Δ [tl]S ≠ = −116 ± 16, Δ V ≠ = 14.2 ± 3.2]. The rates of formation and dissociation of the diperoxo complex are described as d [VO(O 2 ) 2 − ]/d t = k f [VO(O 2 ) + ] [H 2 O 2 ] and − d [VO(O 2 ) 2 − ]/d t = k d [VO(O 2 ) 2 − ] [H + ] 2 , respectively, where k f (mol −1 dm 3 s −1 ) (25 °C) = 3.49 × 10 3 [Δ H ≠ (kJ mol −1 ) = 40.2 ± 0.8, Δ S ≠ (J mol −1 K −1 ) = −42±3, Δ V ≠ (cm 3 mol −1 ) =0.0±0.2]. and k d (mol −2 dm 6 s −1 ) (25 °C) = 3.79 × 10 3 [Δ H ≠ = 45.7 ± 1.7, Δ S ≠ = −23 ∓ 6]. Positive volumes of activation for the monoperoxo complex formation have been attributed to an expanded transition state with the distorted pentagonal-bipyramidal structure.

Activation volume for the cobalt(II) ion complexation with N-methyltetraphenylporphine. Evidence for a dissociative interchange mechanism

A positive activation volume (8.0 cm3 mol–1) for cobalt(II) ion incorporation into N-methyltetraphenylporphine in N,N-dimethylformamide, obtained by a high-pressure stopped-flow technique, is consistent with a dissociative interchange mechanism