The kinetics of complexation of nickel(II) and cobalt(II) with cinchomeronate in aqueous solution

The pressure-jump method has been used to determine the rate constants for the formation and dissociation of nickel(II) and cobalt(II) complexes with cinchomeronate in aqueous solution at zero ionic strength. The forward and reverse rate constants obtained are k_f = 2.27 × 10⁶ M^?1 s^?1 and k_r = 3.81 × 10¹ s^?1 for the nickel(II) complex and k_f = 1.23 × 10⁷ M^?1 s^?1 and k_r = 2.66 × 10² s^?1 for the cobalt(II) complex at 25°C. The activation parameters of the reactions have also been obtained from the temperature variation study. The results indicate that the rate determining step of the reaction is a loss of a water molecule from the inner coordination sphere of the cation for the nickel(II) complex and the chelate ring closure for the cobalt(II) complex. The influence of the pyridine ring nitrogen atom of the cinchomeronate ligand on the complexation of cobalt(II) ion is also discussed.     

THE KINETICS OF ELECTRON TRANSFER FOR A SERIES OF IRON(II) CYCLIDENE COMPLEXES

Abstract

The kinetics of oxidation of a series of iron(II) cyclidene complexes by tris(1,10-phenanthroline)cobalt(III) in methanol have been measured by stopped flow spectrophotometry. The reactions obey a first order rate law when the cobalt(III) complex is present in large excess. The corresponding second order rate constants fall in the range 5.0–130 × 10⁵ M^?1s^?1 (25°C). A linear correlation between the logarithm of the rate constant and the iron(III)/(II) redox potential indicates that the reactions behave as simple outer-sphere electron transfer processes. The self-exchange rate constants for the iron cyclidene complexes have been estimated from the Marcus equation and found to vary between 0.7 × 10⁷ M¹s¹ and 9 × 10⁷ M¹s¹. The dependence of the self-exchange rate constant on ligand structure is discussed.     

Determination by the phase shift method of the absolute rate constants of reactions of O(3P) atoms with olefins at 25°C

Technical improvements have been made in the application of the phase shift method, recently developed and used in this laboratory to measure the absolute values of the rate constants of the reactions of O(3P) atoms with ethylene, propylene, 1-butene, and isobutene. More accurate rate constants have now been obtained for these olefins and themeasurements have been also extended to three additional olefins (cis-2-butene, trimethylethylene, and tetramethylene).The mean least squares values of the rate constants of the reactions of O(³P) with olefins at 26°C, expressed as k₂ × 10^?9 (1./mole²·sec), and their standard deviations are ethylene 0.43 ±0.05, propelene 2.02 ± 0.17, 1-butene 2.40 ± 0.32, isobutene 9.85 ± 1.34, cis-2-butene 9.00 ± 1.76, trimethylethylene 31.1 ± 3.0, and tetramethylethylene 44.4 ± 4.1. A Value of (5.78 ± 0.08) × 10¹⁰l.²/mole²·sec was obtained for the rate constant of the reaction O(³P) + NO + M, with M = N₂O. Where comparison with the absolute rate constants in the literature is possible, agreement is very good, especiallywith some recent absolute values obtained by entirely different experimental techniques. Consistency with the relative rate constant of the O(³P) reactions with olefins previously determined in this laboratory, is excellent.     

The flash photolysis of aqueous solutions of rhodizonic and croconic acids

The flash photolysis of aqueous solutions of rhodizonic and croconic acids has been studied in the presence and absence of electron acceptors. No transient absorption which could be identified with an excited state was observed with either anion. The rate of recovery of the ground state in the absence of additives was a first-order process with both acids and gave rate constants for deactivation of the excited state, k_D, of 2.4 × 10⁵ s^?1 for rhodizonate and 2.8 × 10⁵ s^?1 for croconate. With croconate dianion in the presence of three acceptors, 4-nitrobenzylbromide, methylviologen, and biacetyl, a transient absorption was detected, with a maximum absorbance at 500 nm, and was tentatively identified with the monoanion radical, formed following electron transfer to the acceptor. From the rate of growth of the transient, rate constants for the rate of electron transfer to the acceptor were measured as follows: 4-nitrobenzylbromide: 2.8 × 10⁹ M^?1 s^?1; methyl viologen: 3.7 × 10¹⁰ M^?1 s^?1; and biacetyl: 2.0 × 10⁸ M^?1 s^?1. The significance of the measurements is discussed in relation to the mechanism proposed for the photochemical reactions of these dianions. © 1994 John Wiley & Sons, Inc.     

Absolute rate constants for the reaction of O(3P) aroms with n-butane and NO(M  Ar) over the temperature range 298–439 K

Absolute rate constants for the reaction of O(³P) atoms with n-butane (k₂) and NO(M  Ar)(k₃) have been determined over the temperature range 298–439 K using a flash photolysis-NO₂ chemiluminescence technique. The Arrhenius expressions obtained were k₂ = 2.5 × 10^?11exp[-(4170 ± 300)/RT] cm³ molecule^?1 s^?1, k₃ = 1.46 × 10^?32 exp[940 ± 200)/ RT] cm⁶ molecule^?2 s^?1, with rate constants at room temperature of k₂ = (2.2 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 and k₃ = (7.04 ± 0.70)×10^?32 cm⁶ molecule^?2 s^?1. These rate constants are compared and discussed with literature values.     

Rate constants for the reactions of conjugated olefins with NO2 in the gas phase

Gas-phase rate constants for the reaction of NO₂ with 16 conjugated olefins were determined at room temperature by either conventional methods for bimolecular processes or by competitive reactions. It was found that the rate constants for conjugated olefins were larger than those for simple mono-olefins by factors of 10³–10⁴. Temperature dependence studies reveal that the difference in the rate constants for the two types of reactions can primarily be attributed to differences in their activation energies: k_{1,3-cyclohexadiene} = 5.8 × 10^?14 exp[?(6.1 ± 1.6)/RT] cm³ molecule^?1 s^?1; k_cis-2-butene = 4.68 × 10^?14 exp(?11.2/RT) cm³ molecule^?1 s^?1 [2]. A linear free energy relationship between the reactions of OH and NO₂ with conjugated diolefins was observed.     

The relaxation spectra of nickel(II) and cobalt(II) arginine complexes

The temperature-jump method has been used to determine the nickel(II)- and cobalt(II)-arginine complexation kinetics. In the pH range studied, the neutral form of the ligand, HL, is the attacking, as well as the complexed, ligand species. The reactions reported on are of the type where n = 1, 2, 3 and M is Ni or Co. At 25° and ionic strength 0.1M the association rate constants are: for nickel(II) k₁ = 2.3 × 10³(±20%), k₂ = 2.4 × 10⁴(±20%), k₃ = 3.5 × 10⁴(±40%) M^?1 sec^?1; for cobalt(II) k₁ = 1.5 × 10⁵(±20%), k₂ = 8.7 × 10⁵(±20%), k₃ = 2.0 × 10⁵(±40%) M^?1 sec^?1. Arginine binds to metal ions less well than homologous chelating agents due to the electrostatic repulsion arising from the positively charged terminus of the zwitterion. Kinetically, the effect appears in the association rate constants with nickel reactions more strongly influenced than cobalt.     

Rate constants for hydrogen abstraction reactions of the sulfate radical,SO4−. Alkanes and ethers

Rate constants have been determined for the reactions of SO₄^? with a series of alkanes and ethers. The SO₄^? radical was produced by the laser-flash photolysis of persulfate, S₂O₈^2?. For methane, only an upper limit of 1 × 10⁶ M^?1 s^?1 could be determined. For ethane, propane, and 2-methylpropane, rate constants of 0.44, 4.0, and 10.5 × 10⁷ M^?1 s^?1 were found. For ethyl and n-propyl ether, rate constants of 1.3 × 10⁸ and 2.2 × 10⁸ M^?1 s^?1 were found and for 1,4-dioxane and tetrahydrofuran, rate constants of 7.2 × 10⁷ and 2.8 × 10⁸ were obtained. The reaction of SO₄^? with allyl alcohol was also studied and found to have a rate constant of 1.4 × 10⁹ M^?1 s^?1.     

Decomposition of meta- and para-phenylphenol during ozonation process

The decomposition of meta-phenylphenol (m-PP) and para-phenylphenol (p-PP) in a heterogeneous gas-liquid system using ozone was investigated. The influence of different reaction parameters such as ozone and PP isomers concentration as well as pH and temperature of the reaction mixture on the PP decay rate was determined. The second-order rate constants for the direct reaction of molecular ozone, determined in a homogeneous system, were (5.85 ± 0.35) × 10² M^?1 s^?1 and (8.90 ± 0.33) × 10² M^?1 s^?1 for m-PP and p-PP, respectively. The rate constants for the reaction of m-PP and p-PP with ozone increased with increasing pH. The reaction rate constants with ozone were found to be (1.75 ± 0.02) × 10⁹ M^?1 s^?1 and (1.86 ± 0.02) × 10⁹ M^?1 s^?1 for m-PP and p-PP anions, respectively.     

Time-resolved investigation of the de-excitation of xenon excimer by SF6 and N2 excited with synchrotron radiation

An attempt has been made 'o measure the rate constant for the de-excitation of a state-specified excimer using the pulse character of synchrotron radiation. The rate constants have been obtained for de-excitation of a vibrationally relaxed excimer Xe₂^*(O_u⁺; low v) by SF₆ and N₂ as 9 × 10^?10 and 7 × 10^?12 cm³ molecule^?1 s^?1 respectively.     

Reactivity of selected volatile organic compounds (VOCs) toward the sulfate radical (SO4−)

Rate constants for a series of alcohols, ethers, and esters toward the sulfate radical (SO₄^?) have been directly determined using a laser photolysis set‐up in which the radical was produced by the photodissociation of peroxodisulfate anions. The sulfate radical concentration was monitored by following its optical absorption by means of time resolved spectroscopy techniques. At room temperature the following rate constants were derived: methanol ((1.6 ± 0.2) × 10⁷ M^?1 s^?1); ethanol ((7.8 ± 1.2) × 10⁷ M^?1 s^?1); tert‐butanol ((8.9 ± 0.3) × 10⁵ M^?1 s^?1); diethyl ether ((1.8 ± 0.1) × 10⁸ M^?1 s^?1); MTBE ((3.13 ± 0.02) × 10⁷ M^?1 s^?1); tetrahydrofuran (THF) ((2.3 ± 0.2) × 10⁸ M^?1 s^?1); hydrated formaldehyde ((1.4 ± 0.2) × 10⁷ M^?1 s^?1); hydrated glyoxal ((2.4 ± 0.2) × 10⁷ M^?1 s^?1); dimethyl malonate (CH₃OC(O)CH₂C(O)OCH₃) ((1.28 ± 0.02) × 10⁶ M^?1 s^?1); and dimethyl succinate (CH₃OC(O)CH₂CH₂C(O)OCH₃) ((1.37 ± 0.08) × 10⁶ M^?1 s^?1) where the errors represent 2σ. For the two latter species, we also measured the temperature dependence of the corresponding rate constants. A correlation of these kinetics with the bond dissociation energy is also presented and discussed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 539–547, 2001     

A kinetic study of the reactions of the hydrogen atom with the cyclohexadiene isomers in the gas phase

Absolute values of the rate constants for the reaction of hydrogen atoms with cyclic olefins in the gas phase have been measured in a discharge-flow system under 3.5, 16, and 22 torr Ar at 23°C. The attenuation of hydrogen atom concentration in the reaction tube in the presence of a large excess of olefin was measured with an ESR spectrometer, and the products were analyzed by gas chromatography. Cyclic C₆ hydrocarbons were the only significant products obtained when the hydrogen atom concentration was 2.6 × 10^?10 mole/1., the olefin concentration was in the range of 9 to 22 × 10^?8 mole/1., and the pressure was 16 torr Ar. The values for the rate constants for reaction with cyclohexadiene-1,3, cyclohexadiene-1,4, and cyclohexene are, respectively, (9 ± 2) × 10⁸, (12 ± 1) × 10⁸, and (6 ± 1) × 10⁸ l./mole-sec, and they are not changed significantly by a sixfold change in total pressure. The fraction of the total interaction that proceeds by addition is 84% in the cyclohexadiene-1,3 system, but only 18% in the cyclohexadiene-1,4 system, and the cyclohexadienyl radical is therefore the dominant radical species in the latter system. The pattern of interaction between the hydrogen atom and the cyclohexadienyl radical was determined, and comprises 65% of disproportionation, and 13% and 23% of combination to yield cyclohexadiene-1,3 and cyclohexadiene-1,4, respectively. These results are consistent with the general patterns of reactivity emerging from studies of the reactions between free radicals and olefins in related systems.     

Photocurrent kinetics during electron emission from metal into electrolyte solution: Part III. H3O+ solutions

The photocurrent kinetics in acid solutions have been investigated. The diffusion coefficients of atoms H?((7±2)×10^?5cm²s^?1) and D?((4±1)×10^?5cm²s^?1) and OH^? and OD^? radicals ((1±0.3)×10^?5cm²s^?1) are found. The rate constants of capture of solvated electrons by H₃O⁺ and D₃O⁺ ions are identical and equal to (8±1)×10⁹M^?1s^?1. From the shape of the kinetic curves it follows that electrochemical desorption of atomic hydrogen occurs from the adsorbed state. The rate constant of this process has been measured. It is shown that the rate constant of electrochemical desorption depends only slightly on the potential.     

Ion-Molecule reactions in methylamine and dimethylamine and trimethylamine systems

Fourier transform ion cyclotron resonance mass spectrometry has been used to measure the reaction rates for ions derived from methylamine with dimethylamine or trimethylamine. The use of the selective ion ejection technique greatly simplifies the elucidation of the ion-molecule reaction channels. The rate constants for proton transfer from protonated metwlamine, CH₃NH ₃ ⁺ (m/z 32), to dimethylamine and trimethylamine are 16.1 ± 1.6 × 10^?10 and 9.3 ± 0.9 × 10^?10 cm³ molec^?1s^?1, respectively. The rate constants for charge transfer from methylamine molecular ion, CH₃NH ₂ ⁺ (m/z 31), to dimethylamine and trimethylamine are 9.3 ± 1.8 x 10^?10 and 15.0 ± 5 × 10^?10 cm³molec^?1s^?1, respectively.     

Rate constants for the reactions of the OH radical with the propyl and butyl nitrates and 1-nitrobutane at 298 ± 2 k

Using a relative rate method, rate constants for the gas-phase reactions of the OH radical with 1- and 2-propyl nitrate, 1- and 2-butyl nitrate and 1-nitrobutane have been determined in the presence of one atmosphere of air at 298 ± 2 K. Using rate constants for the reactions of the OH radical with propane and cyclohexane of 1.15 × 10^?12 and 7.49 × 10^?12 cm³ molecule^?1 s^?1, respectively, following rate constants (in units of 10^?12 cm³ molecule^?1 s^?1) were obtained: 1-propyl nitrate, 0.62; 2-propyl nitrate, 0.41; 1-butyl nitrate, 1.78; 2-butyl nitrate, 0.93; and 1-nitrobutane, 1.35. These rate constants are compared and discussed with the literature data.     

Rate constants for Hydrogen abstraction reactions of NO3 in aqueous solution

Rate constants have been measured by pulse radiolysis for the reactions of the NO₃ radical with five cyclic ethers and a series of alcohols. Rate constants ranged from 3.5 × 10⁴ M^×1 s^×1 for deuterated methanol to 1.1 × 10⁷ M^?1 s^?1 for tetrahydrofuran. The rate constants for the reactions of NO₃ with the alcohols 1-propanol to 1-heptanol were found to be linearly dependent on the number of CH₃ groups with a group reactivity of 6.4 × 10⁵ M^?1 s^?1.     

Inclusion complexation of warfarin withβ-cyclodextrins and its influence on absorption kinetics of warfarin in rat

The inclusion complexes of warfarin withβ-cyclodextrin, 2-hydroxypropyl-β-CD and methyl-β-CD have been investigated in aqueous solution. The apparent binding constants of warfarin are found to be 542±19, 442±18 and 112±6M^?1 respectively, calculated from the increments in fluorescence emission of the drug. The influence of theβ-CDs on the absorption rate of the drug is investigated within situ experiments in a chronically isolated internal loop, in the small intestine of the rat. The first-order disappearance (absorption) rate constant decreases to 3.6×10^?4 min^?1 inβ-CD, to 5.0×10^?4 min^?1 in 2-hydroxypropyl-β-CD and to 1.4×10^?3 min^?1 in methyl-β-CD compared to 3.2×10^?3 min^?1 in isotonic phosphate buffer (pH=7.4) solution, all of them showing a good agreement with the percentage of free warfarin in their complexed solutions: 16%, 18% and 47% calculated, respectively.     

Rate constants for the reactions of O3 and OH radicals with a series of alkynes

Rate constants for the reactions of O₃ and OH radicals with acetylene, propyne, and 1-butyne have been determined at room temperature. The rate constants obtained at 294 ± 2 K for the reactions of O₃ with acetylene, propyne, and 1-butyne were (7.8 ± 1.2) × 10^?21 cm³/molecule · s, (1.43 ± 0.15) × 10^?20 cm³/molecule · s, and (1.97 ± 0.26) × 10^?20 cm³/molecule · s, respectively. The rate constants at 298 ± 2 K and atmospheric pressure for the reactions with the OH radical, relative to a rate constant for the reaction of OH radicals with cyclohexane of 7.57 × 10^?12 cm³/molecule · s, were determined to be (8.8 ± 1.4) × 10^?13 cm³/molecule · s, (6.21 ± 0.31) × 10^?12 cm³/molecule · s, and (8.25 ± 0.23) × 10^?12 cm³/molecule · s for acetylene, propyne, and 1-butyne, respectively. These data are discussed and compared with the available literature rate constants.     

A relative rate study of the reaction of bromine atoms with a variety of organic compounds at 295 K

The relative rate technique has been used to determine rate constants for the reaction of bromine atoms with a variety of organic compounds. Decay rates of the organic species were measured relative to i-butane or acetaldehyde or both. Using rate constants of 1.74 × 10^?15 and 3.5 × 10^?12 cm³ molecule^?1 s^?1 for the reaction of Br with i?butane and acetaldehyde respectively, the following rate constants were derived, in units of cm³ molecule^?1 s^?1: 2, 3?dimethylbutane, (6.40 ± 0.77) × 10^?15; cyclopentane, (1.16 ± 0.18) × 10^?15, ethene, (≤2.3 × 10^?13); propene, (3.85 ± 0.41) × 10^?12; trans-2-butene, (9.50 ± 0.76) × 10^?12, acetylene, (5.15 ± 0.19) × 10^?15; and propionaldehyde, (9.73 ± 0.91) × 10^?12. Quoted errors represent 2σ and do not include possible systematic errors due to errors in the reference rate constants. Experiments were performed at 295 ± 2 K and atmospheric pressure of synthetic air or nitrogen. The results are discussed with respect to the mechanisms of these reactions and their utility in serving as a laboratory source of alkyl and alkyl peroxy radicals.     

Absolute rate constants for the gas-phase reaction of OH radicals with cyclohexane and ethane at 295 K

The absolute rate constants for the gas-phase H-atom abstraction by hydroxyl radicals from cyclohexane and ethane have been determined at room temperature. OH radicals were produced by pulse radiolysis of an H₂O-Ar mixture, and the decay of OH was followed by monitoring the transient light absorption around 309 nm. The rate constants were found to be k = (5.24±0.36) × 10⁻¹² and (2.98±0.21) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ for cyclohexane and ethane, res- pectively. These results are compared with literature data.