D. A. Palmer

Spectrophotometric stopped‐flow apparatus suitable for high‐pressure experiments to 200 MPa

A modified high‐pressure, stopped‐flow apparatus has been developed to enable kinetic experiments to be monitored spectrophotometrically to 200 MPa, with efficient mixing of the reactant solutions. This apparatus is compact, relatively inexpensive to construct, and the solutions are only exposed to inert materials, such as Kel‐F, Teflon, quartz (low‐pressure windows), and glass or Viton. The drive‐syringe pistons are propelled by a step motor housed on top of the stopped‐flow unit inside the pressure vessel, resulting in a dead time for the system of ∼10 ms at 25 °C.

Volumes of activation for the anation of Pd(II) substituted dien complexes by chloride ion in aqueous solution. A high pressure stopped-flow instrument for studying the kinetics of fast reactions under pressure

Abstract The kinetics of anation of Pd(1,1,7,7-Et 4 dien)OH 2+ 2 and Pd(1,1,4-Et 3 dien)OH 2+ 2 by chloride ion were studied under pressures up to 1 kbar using a high pressure stopped-flow system. The construction and testing of the latter system are discussed in detail. The rate and activation parameters for the above reactions are: k(25 °C) = 4.2 ± 0.1 and 1558 ± 68 M −1 sec −1 ; ΔH≠ = 13.4 ± 0.4 and 10.5 ± 0.7 kcal mol −1 ; ΔS≠ = −10.7 ± 1.2 and −8.7 ± 2.3 cal K −1 mol −1 ; and ΔV≠ = −3.0 ± 0.2 and −2.7 ± 0.2 cm 3 mol −1 , respectively. These are considered as evidence for an associative type of anation process.

Activation volumes for the linkage isomerization reactions of nitritopentaammine complexes of cobalt(III), rhodium(III) and iridium(III) in aqueous solution

Abstract The pressure and temperature dependencies of the rates of linkage isomeration of the complexes, M(NH 3 ) 5 ONO 2+ , where M represents Co(III), Rh(III) and Ir(III), were mesured in aqueous solution at an ionic strength of 0.1 M . The volumes of activation were found to be −6.7 ± 0.4, −7.4 ± 0.4 and −5.9 ± 0.6 cm 3 mol −1 , resp. Partial molar volumes of the nitrito- and nitro-pentaamminecobalt(III) cations were determined at 82.3 ± 1.4 and 69.4 ± 0.1 cm 3 mol −1 . The volume profile for this reaction indicates the mechanism to be intramolecular in agreement with previous work [1, 2].

Effect of pressure on the substitution reactions of [Pt(dien)X]+ complexes in aqueous solution

Abstract The effect of pressure on the substitution reactions of [Pt(dien)X] + complexes (dien = diethylenetriamine) with a number of nucleophiles, Y, have been studied in aqueous solution at 25°C and an ionic strength of 0.2M, where X = Cl − , Br − , I − , N 3 − and Y = Oh − , I − , N 3 − , NO 2 − , SCN − and pyridine. The volumes of activation are all negative with the mean value for the nucleophile independent path being –18±2 ml mol −1 . Partial molar volumes of the respective substrates and products were measured at 25°C. The data are discussed in terms of an associative mechanism for the nucleophile dependent path, while for the independent path both associative and dissociative mechanisms are considered.

Activation volumes and the mechanism of aquation of trans-CoN4X+2 and trans-Co(NH3)4CN(X)+ complexes

Abstract The pressure, temperature and pH dependencies of the rate of aquation of a range of trans-complexes with the general formula CoN4X+2 were studied where X = Cl and N4 ≡ (NH3)4; (en)2; R,S-(2,3,2-tet); RR,SS-(2,3,2-tet); (Meen)2; (Eten)2; (Pren)2; (3,2,3-tet); (2,3,2-tet); (cyclam); and X= Br, N4 ≡ (en)2; (2,3,2-tet); (3,2,3-tet). The ΔVne values for all the dichloro complexes, including cis-Co(en)2Cl+2 averaged −1.4 ± 1.0 cm3 mol−1 which was interpreted in terms of an Id mechanism. The slightly more positive ΔV≠ values for the dibromo species are also consistent with this mechanism. The aquation of the trans-Co(NH3)4CN(X)+ complexes, where X = Cl, Br, I, N3 and DMSO, was also considered to involve a similar mechanism, although in this case it possesses significantly more associative character.

Temperature and pressure dependencies of the anation of Pd(MeEt4dien)OH2+2 by chloride ion in acidic aqueous solution. A mixing system for studying the kinetics of moderately fast reactions under pressure

Abstract The kinetics of anation of Pd(MeEt 4 dien)OH 2+ 2 by chloride ion were studied under the following conditions: 0,011 ≤ [Cl − ] ≤ 0,081 M ; 15 ≤ temperature ≤ 35°C; 1 ≤ pressure ≤ 1000 bar; [H + ] = 2.5 × 10 −3 M (HClO 4 ) and ionic strength 0,1 M (NaClO 4 ). The [Cl − ] and temperature dependencies of the reaction rate were investigated at 500 bar. A high pressure rapid mixing system was used in these studies and is described in detail. Mixing of the pressurized reactant solutions can be accomplished within five seconds so that reactions with half-lives in the order of ten seconds can be measured accurately. The activation parameters for the title reaction are considered as evidence for an associative mechanism, possibly of the I a type.

Mechanistic information from the effects of pressure on the kinetics of the reduction of Co(NH3)5F2+, Co(NH3)5Cl2+ and Co(NH3)5Br2+ by iron(II) in dimethylsulphoxide

Abstract The kinetics of reduction of Co(NH 3 ) 5 F 2+ , Co(NH 3 ) 5 Cl 2+ and Co(NH 3 ) 5 Br 2+ by Fe(II) have been studied at 35°C as a function of pressure up to 1,7 kbar. The volumes of activation were independent of pressure and found to be +10,3 ± 0,4; +3,8 ± 0,7 and 0,0 ± 0,4 cm 3 mol −1 for the three systems, respectively. The results are discussed with reference to earlier work reported in literature. Changes in solvation and possible steric requirements can account for the observed kinetic parameters.

Volumes of activation for the linkage isomerization reaction of thiocyanatopentaamminecobalt(III) ion in aqueous solution and in dimethylsulphoxide

Abstract Pressure and temperature dependncies of the rate of linkage isomerization of Co(NH 3 ) 5 SCN 2+ in acidic aqueous solution yielded the activation parameters: ΔH‡ = 102.8 ± 0.8 kJ mol −1 ; ΔS‡ = −14 ± 3J K −1 mol −1 ; ΔV‡ = −5.3 ± 0.8 cm 3 mol −1 . A similar study in DMSO resulted in the following parameters: ΔH‡ = 105 ± 2 kJ mol −1 ; ΔS‡ = −2 ± 4J K −1 mol −1 ; ΔV‡ exp = −1.0 ± 1.2 cm 3 mol −1 . These values emphasize that the rearrangement are intramolecular. They exclude the possibility that an “intimate ion-pair” it involved and rather suggest that the reactions proceed via a !abile intermediate in which cobalt is bound to thiocyanate through a filled π orbital.

Volumes of activation for the mercury(II) induced aquation reactions of halopentaamminecobalt(III), rhodium(III) and chromium(III) ions in acidic aqueous solution

Abstract The pressure dependencies of the rates of Hg(II) induced aquation of Co(NH 3 ) 5 Cl 2+ , CoNH 3 5 Br 2+ Cr(NH 3 ) 5 Cl 2+ , and Rh(NH 3 ) 5 Cl 2+ were studied at 15°C, with the exception of the latter which was investigated at 25°C, and a [H + ] of 0.31 M with μ = 0.6 M . The respective volumes of activation were found to be −1.7 ± 1.0, +0.8 ± 0.5, +0.7 ± 0.4, and −1.0 ± 0.4 cm 3 mol −1 . These results are discussed with reference to the numerous conventional kinetic data in the literature, further establishing a D-mechanism for the rate-determining step.

Volumes of activation for the solvolysis of [Pd(Et4dien)I]+ in different solvents

Abstract The pressure dependencies of the rates of solvolysis of [Pd(Et4dien)I]+ in the presence of 0.05 M Br− were measured in methanol, ethanol, dimethyl sulfoxide, N,N-dimethylformamide and acetonitrile at 40 °C. The respective volumes of activation are −13.4 ± 0.5, −12.9 ± 0.4, −10.1 ± 0.2, −9.3 ± 0.3 and −7.9 ± 0.5 cm3 mol−1. The temperature depen- dencies of the rate constants were also determined for the solvents acetonitrile, acetone and N,N- dimethylacetamide to supplement the existing data in the remaining solvents. An associative interchange mechanism is favored to explain these results.