Kinetics and mechanism of substitution reactions of (dimethylsulfoxide)pentaamminecobalt(III) ion in dimethylsulfoxide solvent: complications due to redox and conjugate base processes

1976 ◽  
Vol 54 (23) ◽  
pp. 3685-3692 ◽  
Author(s):  
S. T. Danny Lo ◽  
Evelyn M. Oudeman ◽  
Jean C. Hansen ◽  
Thomas W. Swaddle
Keyword(s):  
Ion Pairs ◽  
Activation Parameters ◽  
Rate Coefficients ◽  
Solvent Exchange ◽  
Exchange Reactions ◽  
Substitution Reactions ◽  
Enthalpy Of Activation ◽  
A Minor ◽  
Conjugate Base ◽  
Anation Rate

Simple anation and solvent exchange reactions of Co(NH3)5DMSO3+ in DMSO are accompanied by reactions in which the conjugate base of this complex undergoes either internal redox to cobalt(II) or relatively rapid substitution. These conjugate base reactions are eliminated by addition of H+, although a minor redox pathway persists within ion pairs of Co(NH3)5DMSO3+ with chloride or bromide. The latter pathway is evidently not mechanistically related to the solvent exchange or anation reactions, which proceed by dissociative interchange (Id) according to the activation parameters (volume of activation = +10 cm3 mol−1 for DMSO exchange; enthalpy of activation = 123, 121, and ∼126 kJ mol−1 for DMSO exchange and bromide and chloride anation respectively). Enthalpies of activation for solvent exchange and for the limiting anation rate are shown to be better criteria of mechanism than the corresponding rate coefficients.

scholarly journals Electron and hydrogen self-exchange of free radicals of sterically hindered tertiary aliphatic amines investigated by photo-CIDNP

2013 ◽  
Vol 9 ◽  
pp. 437-446 ◽  
Author(s):  
Martin Goez ◽  
Isabell Frisch ◽  
Ingo Sartorius
Keyword(s):  
Ion Pairs ◽  
Activation Parameters ◽  
Radical Cations ◽  
Outer Sphere ◽  
Exchange Reactions ◽  
Radical Pairs ◽  
Time Resolved ◽  
Alkyl Radicals ◽  
Acid Base Equilibrium ◽  
Reorganization Energies

The photoreactions of diazabicyclo[2,2,2]octane (DABCO) and triisopropylamine (TIPA) with the sensitizers anthraquinone (AQ) and xanthone (XA) or benzophenone (BP) were investigated by time-resolved photo-CIDNP (photochemically induced dynamic nuclear polarization) experiments. By varying the radical-pair concentration, it was ensured that these measurements respond only to self-exchange reactions of the free amine-derived radicals (radical cations DH • + or α-amino alkyl radicals D • ) with the parent amine DH; the acid–base equilibrium between DH • + and D • also plays no role. Although the sensitizer does not at all participate in the observed processes, it has a pronounced influence on the CIDNP kinetics because the reaction occurs through successive radical pairs. With AQ, the polarizations stem from the initially formed radical-ion pairs, and escaping DH • + then undergoes electron self-exchange with DH. In the reaction sensitized with XA (or BP), the polarizations arise in a secondary pair of neutral radicals that is rapidly produced by in-cage proton transfer, and the CIDNP kinetics are due to hydrogen self-exchange between escaping D • and DH. For TIPA, the activation parameters of both self-exchange reactions were determined. Outer-sphere reorganization energies obtained with the Marcus theory gave very good agreement between experimental and calculated values of ∆G ‡ 298.

An Electron Spin Resonance Investigation of the Mechanism of the Intramolecular Cation Migration of Alkali Metal Semiquinone Ion-pairs

1971 ◽  
Vol 49 (14) ◽  
pp. 2412-2417 ◽  
Author(s):  
T. E. Gough ◽  
P. R. Hindle
Keyword(s):  
Exchange Process ◽  
Ion Pairs ◽  
Ion Pair ◽  
Kinetic Characteristics ◽  
Exchange Reactions ◽  
Ionic Interaction ◽  
Free Energy Of Activation ◽  
Enthalpy Of Activation ◽  
Cation Migration ◽  
Spin Resonance

A systematic survey of the kinetic characteristics of the intramolecular cation exchange reactions of semiquinone ion-pairs is described. A linear correlation exists between the free energy of activation for the exchange and the strength of the ionic interaction within the ion pair; however, this correlation does not extend to the enthalpy of activation. The significance of the kinetic parameters is discussed in terms of the role solvation plays in determining the course of the exchange process.

scholarly journals Rate and Product Studies with 1-Adamantyl Chlorothioformate under Solvolytic Conditions

2021 ◽  
Vol 22 (14) ◽  
pp. 7394
Author(s):  
Kyoung Ho Park ◽  
Mi Hye Seong ◽  
Jin Burm Kyong ◽  
Dennis N. Kevill
Keyword(s):  
Rate Constants ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Carbonyl Sulfide ◽  
Activation Parameters ◽  
Chloride Ions ◽  
Reaction Channels ◽  
Decomposition Pathway ◽  
Solvent Isotope ◽  
Solvent Isotope Effects

A study was carried out on the solvolysis of 1-adamantyl chlorothioformate (1-AdSCOCl, 1) in hydroxylic solvents. The rate constants of the solvolysis of 1 were well correlated using the Grunwald–Winstein equation in all of the 20 solvents (R = 0.985). The solvolyses of 1 were analyzed as the following two competing reactions: the solvolysis ionization pathway through the intermediate (1-AdSCO)+ (carboxylium ion) stabilized by the loss of chloride ions due to nucleophilic solvation and the solvolysis–decomposition pathway through the intermediate 1-Ad+Cl− ion pairs (carbocation) with the loss of carbonyl sulfide. In addition, the rate constants (kexp) for the solvolysis of 1 were separated into k1-Ad+Cl− and k1-AdSCO+Cl− through a product study and applied to the Grunwald–Winstein equation to obtain the sensitivity (m-value) to change in solvent ionizing power. For binary hydroxylic solvents, the selectivities (S) for the formation of solvolysis products were very similar to those of the 1-adamantyl derivatives discussed previously. The kinetic solvent isotope effects (KSIEs), salt effects and activation parameters for the solvolyses of 1 were also determined. These observations are compared with those previously reported for the solvolyses of 1-adamantyl chloroformate (1-AdOCOCl, 2). The reasons for change in reaction channels are discussed in terms of the gas-phase stabilities of acylium ions calculated using Gaussian 03.

Steroids and steroidases. IX. Activation parameters and the mechanism of base- and enzyme-catalyzed isomerizations of androst-5-ene-3,17-dione. The nature of the active center of the Δ5 → Δ4-3-ketosteroid isomerase of Pseudomonas testosteroni

1969 ◽  
Vol 47 (23) ◽  
pp. 4459-4466 ◽  
Author(s):  
J. Bryan Jones ◽  
Donald C. Wigfield
Keyword(s):  
Amino Acids ◽  
Active Center ◽  
Activation Parameters ◽  
Circumstantial Evidence ◽  
Kcal Mole ◽  
Acid Base ◽  
Ketosteroid Isomerase ◽  
Enzyme Action ◽  
Enthalpy Of Activation

Determination of the activation parameters for the acid-, base-, and enzyme-catalyzed isomerizations of androst-5-ene-3,17-dione has revealed that the facility of the enzymic process is mainly due to an extremely low enthalpy of activation of 5.0 kcal mole−1. Further circumstantial evidence regarding the nature of the reacting groups at the active center has also been obtained, and a mechanism of enzyme action is proposed employing tyrosine and histidine as the principal amino acids responsible for catalyzing the isomerization.

Isotope effects and activation parameters for the proton transfer reaction from 1-(4-nitrophenyl)-1-nitroethane to free anions and ion pairs of cesium n-propoxide in n-propanol solvent

1986 ◽  
Vol 64 (6) ◽  
pp. 1021-1025 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Przemyslaw Pruszynski ◽  
Kenneth T. Leffek
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Activation Parameters ◽  
Solvation Shell ◽  
Ion Pair ◽  
Initial State ◽  
Free Ion ◽  
Deuteron Transfer

Rate constants for the proton and deuteron transfer from 1-(4-nitrophenyl)-1-nitroethane to cesium n-propoxide in n-propanol have been measured under pseudo-first-order conditions with an excess of base for four temperatures between 5 and 35 °C. Using literature values of the fraction of cesium n-propoxide ion pairs that are dissociated into free ions, separate second-order rate constants for the proton and deuteron transfer to the ion pair and to the free ion have been calculated. The cesium n-propoxide ion pair is about 2.8 times more reactive than the free n-propoxide ion. The primary kinetic isotope effects for the two reactions are the same (kH/kD = 6.1–6.3 at 25 °C) within experimental error. The enthalpy of activation is smaller for the ion-pair reaction and the entropy of activation more negative than for the free-ion reaction. For proton transfer, ΔH±ion pair = 8.3 ± 0.2 kcal mol−1, ΔH±ion = 9.6 ± 1.0 kcal mol−1, ΔS±ion pair = −12.3 ± 0.6 cal mol−1 deg−1, ΔS±ion = −10.1 ± 3.4 cal mol−1 deg−1. The greater reactivity of the ion pair relative to the free ion is interpreted in terms of the weaker solvation shell of the ion pair in the initial state.

Rate and equilibrium studies of the deprotonation of benzylic ketones bearing pyridinium cations

1989 ◽  
Vol 67 (3) ◽  
pp. 428-432 ◽  
Author(s):  
John W. Bunting ◽  
Dimitrios Stefanidis
Keyword(s):  
Activation Parameters ◽  
Kinetic Studies ◽  
Similar Data ◽  
Equilibrium Studies ◽  
Hydroxide Ion ◽  
Pyridinium Cation ◽  
Carbon Acids ◽  
Entropy Of Activation ◽  
Pyridinium Cations ◽  
Conjugate Base

Rates and equilibria for the deprotonation of four benzylic ketones containing pyridinium substituents (1, 2, 5, and 6) have been investigated in basic aqueous solution (ionic strength 0.1) over the range 15–45 °C, and thermodynamic and activation parameters have been evaluated. Similar data are also reported for the deprotonation of nitroethane. The kinetic preference for hydroxide ion addition to the carbonyl group in competition with the thermodynamically preferred enolate ion formation, which was previously reported for the 1-methyl-4-phenylacetylpyridinium cation (1) and its 3-phenylacetyl isomer (2), is also found for the 1-(1-methyl-2-oxo-2-phenylethyl)pyridinium cation (6). Rates of equilibration of the 1-(2-oxo-2-phenylethyl)-pyridinium cation (5) with its enolate ion conjugate base are too rapid to allow investigation by stopped-flow spectrophotometry. For the hydroxide ion catalyzed deprotonation of each of 1, 2, 6, and nitroethane, [Formula: see text] is more negative than ΔS0. This difference, which represents the entropy of activation for protonation of the conjugate base of each of these carbon acids by water, is approximately constant at [Formula: see text] = −4.9 ± 0.5 cal deg−1 mol−1. Keywords: carbon acids, kinetic studies, deprotonation, activation parameters, pKa values.

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