Spectroscopic studies of the interaction of cobalt(II) N,N',N″,N‴-tetramethyltetra-3,4-pyridinoporphyrazine with amino acids and nitrogen oxides

2001 ◽  
Vol 05 (12) ◽  
pp. 839-845 ◽  
Author(s):  
MAMOTHIBE A. THAMAE ◽  
TEBELLO NYOKONG
Keyword(s):  
Nitric Oxide ◽  
Amino Acids ◽  
Nitrogen Oxides ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Kinetic Data ◽  
Spectroscopic Studies

The interaction of histidine, cysteine, NO and nitrite with cobalt(II) N,N',N″,N‴-tetramethyltetra-3,4-tetrapyridinoporphyrazine ([ Co II tmtppa ]4+) is reported. Metal-based autoreduction of [ Co II tmtppa ]4+ occurs with the formation of the [ Co I tmtppa (-2)]3+ species in the presence of histidine and cysteine. Kinetic data for the auto reduction of [ Co II tmtppa ]4+ in the presence of these amino acids gave the rate constants k f = 2.1 × 101 and 2.8 dm3 mol-1 s-1, for cysteine and histidine, respectively. One molecule of NO or nitrite was found to coordinate to the [ Co II tmtppa ]4+ species. The equilibrium and rate constants for the coordination of the nitric oxide were K = 2.3 × 104 dm 3 mol -1 and k f = 7.5 dm 3 mol -1 s -1, respectively. The coordination of nitrite to [ Co II tmtppa ]4+ occurred with an equilibrium constant of K = 2.0 × 102 dm 3 mol -1 and a rate constant of k f = 4.0 × 10-3 dm 3 mol -1 s -1. There was no evidence for the coordination of two molecules of nitrite to the [ Co II tmtppa ]4+ species.

THE REACTION OF NITROGEN ATOMS WITH OXYGEN ATOMS IN THE ABSENCE OF OXYGEN MOLECULES

1961 ◽  
Vol 39 (8) ◽  
pp. 1601-1607 ◽  
Author(s):  
C. Mavroyannis ◽  
C. A. Winkler
Keyword(s):  
Nitric Oxide ◽  
Nitrogen Atom ◽  
Rate Constant ◽  
Rate Constants ◽  
Flow System ◽  
Titration Method ◽  
Active Nitrogen ◽  
Reaction Tube ◽  
Fast Flow ◽  
Oxygen Atoms

The reaction has been studied in a fast-flow system by introducing nitric oxide in the gas stream with excess active nitrogen. The nitrogen atom consumption was determined by titrating active nitrogen with nitric oxide at different positions along the reaction tube. The rate constant is found to be k1 = 1.83(± 0.2) × 1015 cc2 mole−2 sec−1 at pressures of 3, 3.5, and 4 mm, and with an unheated reaction tube.The homogeneous and surface decay of nitrogen atoms involved in the above system were studied using the nitric oxide titration method, and the rate constants were found to be k3 = 1.04 ± 0.17 × 1016 cc2 mole−2 sec−1, and k4 = 2.5 ± 0.2 sec−1 (γ = 7.5 ± 0.6 × 10–5), respectively, over the range of pressures from 0.5 to 4 mm with an unheated reaction tube.

Electron-Transfer Reactions in Non-Aqueous Media. IV. A Temperature-Dependence Study of the Reduction of CoCl(NH3)52+ by Iron(II) in N,N-Dimethylformamide

1979 ◽  
Vol 32 (7) ◽  
pp. 1425 ◽  
Author(s):  
KR Beckham ◽  
DW Watts
Keyword(s):  
Electron Transfer ◽  
Temperature Dependence ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Iron Atom ◽  
Rate Constants ◽  
Aqueous Media ◽  
Transfer Reactions ◽  
Complex Functions ◽  
Temperature Dependences

A detailed study has been made of the temperature dependence of the rate of reduction of CoCl-(NH3)52+ by iron(II) in N,N-dimethylformamide. The observed rate constants (kobs) for this reaction are complex functions of an equilibrium constant (K) for the formation of a bridged intermediate, the rate constant for electron transfer in this bridged intermediate (k), and the iron(II) concentration. From studies of the dependence of kobs on iron(II) concentration at five temperatures the temperature dependences of both K and k have been resolved, yielding respectively ΔH� -20k�12 kJ mol-1, ΔS� -44�40 J K-1 mol-1 and ΔH* 107�4 kJ mol-1, ΔS* 57�16 J K-1 mol-1. The results are interpreted in terms of a bridged intermediate in which the iron atom is tetrahedrally coordinated.

Kinetics of oxidation of Cu(I) complexes of cysteine and penicillamine: nature of intermediates and reactants at pH 10.0

1989 ◽  
Vol 67 (4) ◽  
pp. 736-745 ◽  
Author(s):  
Stephen P. Mezyk ◽  
David A. Armstrong
Keyword(s):  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Slow Component ◽  
Mixed Valence ◽  
Planar Geometry ◽  
Published Data ◽  
Diffusion Controlled ◽  
Square Planar ◽  
Thiolate Ligand

The Cu(I)•L2 complex with cysteine ligands at total Cu(I) concentrations of 10–30 μM was shown to be oxidised by cysteinyl radicals (RS•) with a diffusion-controlled rate constant k11a = 1.8 × 109 M−1 s−1. The corresponding reaction with the cysteine disulphide anion (RS•—SR−) proceeded at a slower rate, k11b = 2.7 × 108 M−1s−1. At higher Cu(I) concentrations, a slow and a fast component of absorption growth was observed. The slow component rate was independent of Cu(I) concentration, but it became more intense as the Cu(I) concentration rose. The yields and kinetic data were shown to be consistent with the presence of an equilibrium between the Cu(I)•L2 species and a second Cu(I) complex, Cu(I)2•L3, with an equilibrium constant of K1 = 162.[Formula: see text]This finding is consistent with the earlier work of Bagiyan etal. The rate constant of the oxidation of Cu(I)2•L3 by the cysteinyl radical was k12 = 1.0 × 109 M−1 s−1. Similar results were obtained with penicillamine, except the rate constants and equilibrium constant were smaller, (k11a = 4.5 × 108 M−1 s−1, k11b < 2 × 108 M−1 s−1, k12 = 5.5 × 108 M−1 s−1 and K1 = 113). This was attributed to the presence of the β-methyl groups in penicillamine, which exert a large steric effect.The ultraviolet spectra of the long-lived products, which are stable on a millisecond timescale, was consistent with a Cu(II)•L2 structure with a square planar geometry. The oxidation of the Cu(I)2.L3 species proceeded via intermediates, which relaxed to the final product spectra with rate constants of k13b = 2.6 and 1.1 × 104 s−1 for cysteine and penicillamine, respectively. Comparison of the spectra of the intermediates with published data showed that they were consistent with the presence of a bridging thiolate ligand between Cu(I) and Cu(II). Keywords: oxidation, copper, mixed valence, cysteine, penicillamine, complexation.

Studies in solvolysis. Part II. Some comments on the ion-pair mechanism for displacements at a primary carbon atom

1970 ◽  
Vol 48 (24) ◽  
pp. 3807-3818 ◽  
Author(s):  
John M. W. Scott
Keyword(s):  
Carbon Atom ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Ion Pairs ◽  
Ion Pair ◽  
Experimental Basis ◽  
Methyl Halides ◽  
Bimolecular Rate Constant ◽  
Substitution Process

The implication of Sneen and co-workers (2–5) that all SN2 substitutions at a primary carbon atom probably proceed via the intermediate production of an intimate ion-pair is examined with respect to the reactions of the methyl halides (MeX; X = F, Cl, Br, I) with various nucleophiles (H2O, OH−, F−, Cl−, Br−, I−, CN−, CSN−, S2O32−) in water. By establishing certain rules concerning the behavior of derived reactivity scales (essentially ρ values) as contrasted with absolute reactivity scales (observed rate constants), it is concluded that Sneen and Larsen's mechanistic description is consistent with the experimental facts, and that in such cases the substitution process involves a pre-equilibrium constant, Ke, which is independent of the attacking nucleophile. This is followed by a rate determining bimolecular rate constant, kn, which depends on the ion pair and the nature of the nucleophile. The observed rate (k°) is given by k = Kekn. A method of calculating Ke is described and values of kn for nine nucleophiles attacking the four methyl halide ion-pairs are reported along with a number of confirmatory calculations. It is concluded that the classical Hughes–Ingold SN2 Heitler–London description of these reactions is inadequate. Some further suggestions to place the new mechanistic description on a firmer experimental basis are made.

Rate of addition of bromine atoms to ethylene

1970 ◽  
Vol 48 (15) ◽  
pp. 2426-2429 ◽  
Author(s):  
K. T. Wong ◽  
D. A. Armstrong
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Kinetic Data ◽  
Addition Reactions

The rate of combination of bromine atoms in the photosensitized hydrobromination of ethylene has been determined by the rotating sector technique. From this and kinetic data reported previously the rate constant for the addition of bromine atoms to ethylene in the presence of 380 Torr of propane as deactivating gas has been calculated to be 1.5 ± 0.3 × 10−14 cm3 molecule−1 s−1, independent of temperature in the range 15 to 35 °C. This result is discussed and compared with rate constants for other addition reactions to ethylene.

scholarly journals Unreliability of rate constants derived from a linear transformation of kinetic data, with special reference to cholesterol equilibration between phospholipid vesicles

1992 ◽  
Vol 283 (2) ◽  
pp. 537-539 ◽  
Author(s):  
N Gains
Keyword(s):  
Linear Regression ◽  
Special Reference ◽  
Correlation Coefficient ◽  
Rate Constant ◽  
Rate Constants ◽  
Kinetic Data ◽  
Time Dependent ◽  
Phospholipid Vesicles ◽  
Zero Time ◽  
Made In

In the time-dependent transfer of a lipid from a donor to an acceptor vesicle population a(t) is the amount transferred to the acceptor vesicles at time t, a infinity is the equilibrium transfer value and a0 is the value at zero time. In order to plot kinetic data (a(t) as ln[(a infinity - a(t))/(a infinity - a(t))] against time and to fit these with a linear regression, it is necessary to know the equilibrium value, a infinity, or to choose one. Here it is shown that even if a very larger error is made in the choice of a infinity, the resulting plot can still be acceptably linear and the correlation coefficient of the regression acceptably high. When a infinity is overestimated the rate constant derived from the slope of such a plot is underestimated. In extreme cases a 10-fold error can occur.

Effect of the acyl substituent on the equilibrium constant for hydration of esters

1980 ◽  
Vol 58 (13) ◽  
pp. 1281-1294 ◽  
Author(s):  
J. Peter Guthrie ◽  
Patricia A. Cullimore
Keyword(s):  
Aqueous Solution ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Acid Catalysis ◽  
Methyl Esters ◽  
Equilibrium Constants ◽  
Free Energies ◽  
General Acid ◽  
Aldehydes And Ketones

Heats of hydrolysis have been measured for the trimethyl orthoesters of isobutyric, propionic, benzoic, methoxyacetic, chloroacetic, and cyanoacetic acids using aqueous acid with an organic cosolvent where necessary, and of the corresponding esters in alkaline solution. Solubilities or free energies of transfer from gas to aqueous solution have been measured, permitting calculation of the free energies of formation of the aqueous orthoesters, and by methods which we have published previously, calculation of the free energies of formation of the covalent hydrates of the esters, and the free energy changes for hydration of these esters.Using estimated pKa values equilibrium constants were calculated for the addition of hydroxide to the esters. The data are in good agreement with the appropriate Marcus equation relating rate and equilibrium constants with a value for b of 8.99 ± 0.17. This line was used to estimate the equilibrium constant for addition of hydroxide, and thence of water, to some additional esters where only the rate constant was available. Rate constants for hydrolysis of methyl esters in aqueous solution at 25 °C were calculated from literature data, correcting for the effect of other conditions as necessary. From the equilibrium constants for addition of water we could estimate the rate constants for uncatalyzed hydrolysis; for the cases where this rate constant has been measured, the agreement was satisfactory. For acid catalyzed hydrolysis the data permit a test of the two alternative mechanisms considered previously, namely specific acid catalysis and general acid catalysis with hydronium ion acting as a general acid. For esters the mechanism is clearly specific acid catalysis, but for aldehydes and ketones it appears very likely that the mechanism is general acid catalysis.

Determination of rate constants of redox reactions of second order from current-less potential-time curves by a simplified graphical-numerical method

1983 ◽  
Vol 48 (5) ◽  
pp. 1358-1367 ◽  
Author(s):  
Antonín Tockstein ◽  
František Skopal
Keyword(s):  
Numerical Method ◽  
Explicit Form ◽  
Rate Constant ◽  
Optimization Algorithm ◽  
Rate Constants ◽  
Redox Reactions ◽  
Second Order ◽  
Wide Region ◽  
Recurrent Formula

A method for constructing curves is proposed that are linear in a wide region and from whose slopes it is possible to determine the rate constant, if a parameter, θ, is calculated numerically from a rapidly converging recurrent formula or from its explicit form. The values of rate constants and parameter θ thus simply found are compared with those found by an optimization algorithm on a computer; the deviations do not exceed ±10%.

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