Dale W. Margerum

Water chlorination chemistry: nonmetal redox kinetics of chloramine and nitrite ion.

The first step in NH 2 Cl oxidation of NO 2 - is the general acid-assisted formation of nitryl chloride, where k HA is 7.6×10 6 M -2 s -1 for HaO + and is 46 M -2 s -1 for H 2 PO 4 - (25.0 o C, μ=0.50 M). This is the rate-limiting step at very low [NH 3 ] and high [NO 2 - ] concentrations: HA+NH 2 Cl+ NO 2 - →A - +NH 3 +NO 2 Cl, where the subsequent reactions of NO 2 Cl are rapid

Kinetics and Mechanisms of Chlorine Dioxide Oxidation of Tryptophan

The reactions of aqueous ClO2 (*) and tryptophan (Trp) are investigated by stopped-flow kinetics, and the products are identified by high-performance liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry and by ion chromatography. The rates of ClO2 (*) loss increase from pH 3 to 5, are essentially constant from pH 5 to 7, and increase from pH 7 to 10. The reactions are first-order in Trp with variable order in ClO2 (*). Below pH 5.0, the reactions are second- or mixed-order in [ClO2 (*)], depending on the chlorite concentration. Above pH 5.0, the reactions are first-order in [ClO2 (*)] in the absence of added chlorite. At pH 7.0, the Trp reaction with ClO2 (*) is first-order in each reactant with a second-order rate constant of 3.4 x 10(4) M(-1) s(-1) at 25.0 degrees C. In the proposed mechanism, the initial reaction is a one-electron oxidation to form a tryptophyl radical cation and chlorite ion. The radical cation deprotonates to form a neutral tryptophyl radical that combines rapidly with a second ClO 2 (*) to give an observable, short-lived adduct ( k obs = 48 s(-1)) with proposed C(H)-OClO bonding. This adduct decays to give HOCl in a three-electron oxidation. The overall reaction consumes two ClO2 (*) per Trp and forms ClO2- and HOCl. This corresponds to a four-electron oxidation. Decay of the tryptophyl-OClO adduct at pH 6.4 gives five initial products that are observed after 2 min and are separated by HPLC with elution times that vary from 4 to 17 min (with an eluent of 6.3% CH 3OH and 0.1% CH 3COOH). Each of these products is characterized by mass spectrometry and UV-vis spectroscopy. One initial product with a molecular weight of 236 decays within 47 min to yield the most stable product, N-formylkynurenine (NFK), which also has a molecular weight of 236. Other products also are observed and examined.

Reactivity and aging in hydroxy-iron(III) polymers, analogs of ferritin cores

Abstract The aging of hydrolytic polymers of ferric iron, analogs of the hydroxy-iron cores of ferritin, has been monitored via increases in light absorption, sedimentation velocity, and resistance to attack by acid. Two concurrent, yet distinct aging processes are observed, which can be associated with structural change and with growth via particle accretion. The acid decomposition kinetics are first order in the number of polymerized iron(III) ions in solution and first order in nitric acid concentration. Moreover the polymer particles shrink in proportion to the amount of acid consumed. The kinetic evidence suggests that either the particles harden outward from the center, or that they are accessible to the solvent over much of their interior. The polymers can be annealed in solution, rapidly decreasing their reactivity.

Nonmetal redox kinetics: mono-, di-, and trichloramine reactions with cyanide ion.

Trichloramine reacts with excess CN- in a stepwise series of Cl + transfer reactions to give CICN and to generate in sequence dichloramine, monochloramine, and ammonia : NCl 3 + CN- + H 2 O→NHCl 2 + ClCN + OH- ; NHCl 2 + CN- + H 2 O→NH 2 Cl + CICN + OH- ; and NH 2 Cl + CN- + H 2 O→NH 3 + CICN + OH-. The reaction rates are first order in the corresponding concentrations of the chloramine species and in cyanide ion, with second-order rate constants (M -1 s -1 , 25.0°C, μ = 1.00 M) of 1.83 x 10 9 for NCl 3 , 576 for NHCl 2 , and 1.96 x 10 -2 for NH 2 Cl. The NH 2 Cl reaction is general-acid assisted with third-order rate constants (M -2 s -1 ) of 4.32 x 10 10 for H 3 O + and 84 for HCO 3 - . The reactions of NHCl 2 and of NCl 3 with CN- are not acid assisted. Since HCN (pK a 8.95) is not reactive, the rates of the NHCl 2 and NCl 3 reactions decrease with decrease of pH (below pH 9). Off-setting effects of acid assistance and HCN formation cause the NH 2 Cl rate to increase from pH 13 to pH 9 and to level off below pH 9. At pH 7, the relative reactivities are NCl 3 > HOCI >> NH 2 Cl >> NHCl 2 .

Chlorine dioxide oxidation of dihydronicotinamide adenine dinucleotide (NADH).

The oxidation of dihydronicotinamide adenine dinucleotide (NADH) by chlorine dioxide in phosphate buffered solutions (pH 6-8) is very rapid with a second-order rate constant of 3.9 x 10(6) M(-1) s(-1) at 24.6 degrees C. The overall reaction stoichiometry is 2ClO2(*) per NADH. In contrast to many oxidants where NADH reacts by hydride transfer, the proposed mechanism is a rate-limiting transfer of an electron from NADH to ClO2(*). Subsequent sequential fast reactions with H(+) transfer to H2O and transfer of an electron to a second ClO2(*) give 2ClO2(-), H3O(+), and NAD(+) as products. The electrode potential of 0.936 V for the ClO2(*)/ClO2(-) couple is so large that even 0.1 M of added ClO2(-) (a 10(3) excess over the initial ClO2(*) concentration) fails to suppress the reaction rate.

Role of halogen(I) cation-transfer mechanisms in water chlorination in the presence of bromide ion

Bromide ion is rapidly converted to HOBr via BrCl by reaction with HOCl. The subsequent slow reactions of (HOCl, OCl-)/(HOBr, OBr-) mixtures are monitored directly by multiwavelength UV-vis absorbance methods and simultaneously by ion chromatographic measurement of ClO2-, ClO3-, and BrO3- (p[H+] 5.6-7.6). A first-order loss of HOCl is observed which is catalyzed by trace concentrations of Br- and BrCl. Chlorite ion forms first and is subsequently oxidized to ClO3-. The loss of HOBr is slower and is second-order in HOBr, so that BrO3- formation takes longer than ClO3- formation. Under the conditions of this work, the relative yield of BrO3- increases with increase in pH. The decomposition of HOCI by bromide proceeds primarily by a series of halogen(I) cation-transfer reactions with subsequent halide release. The presence of HOCI increases the BrO3- yield three-fold from HOBr decay alone.

Dioxygen-induced decarboxylation and hydroxylation of [Ni II (glycyl-glycyl-L-histidine)] occurs via Ni III : X-ray crystal structure of [Ni II (glycyl-glycyl-?-hydroxy-D,L-histamine)]3H2O

Electrochemical and EPR studies show that the dioxygen-induced decarboxylation and hydroxylation of [NiII(GGH-H–2)]–, where GGH is glycyl-glycyl-L-histidine (HL), in aqueous solution occurs via a NiIII intermediate; the product [NiII(Gly-Gly-α-hydroxy-D,L-histamine-H2)]·3H2O is shown by X-ray crystallography to contain square-planar NiII coordinated to the terminal amino group [Ni–N, 1.932(3)A], two deprotonated amide Ns [1.884(3) and 1.831(3)A] and imidazole δN [1.908(3)A].

Rate Measurements by the Pulsed-Accelerated-Flow Method

A pulsed-accelerated-flow spectrophotometer with UV-visible capability is described that permits measurement of pseudo-first-order rate constants as large as 500 000 s(-)(1) (t(1/2) = 1.4 μs). Chemical rate processes are resolved from physical mixing rate processes by variation of flow velocities under conditions of turbulent flow. Two mixing processes are found in the mixing/observation tube. One mixing rate constant, valid for the full length of the tube, is directly proportional to the flow velocity. The other mixing behavior, proportional to the square of the flow velocity, is found only in the immediate vicinity of the 10 inlet reactant streams that collide with one another in the middle of the observation tube. Contributions from the latter mixing become more important for very fast reactions that take place close to the inlet jets. These mixing models and improved signal/noise permit the measurement of rate constants for very fast reactions. Applications of the PAF method to electron-transfer, proton-transfer, hydrolysis, and non-metal redox reactions are reported for pseudo-first-order and second-order reactions.

THE CRYSTAL STRUCTURE OF TRIETHYLENETETRAMINENICKEL(II) PERCHLORATE

Abstract The crystal structure of Ni(trien)(ClO4)2(trien = triethylenetetramine, NH2CH2CH2NHCH2CH2NHCH2CH2NH2) has eight formula units in an orthorhombic unit cell of dimensions: a = 14.67, b = 14.95, and c = 13.85 A, with a space group of Pna21-C 9 2v . The structure was solved by Patterson and Fourier methods using film data and refined by least squares to a conventional R factor of 0.136 for the 1583 observed reflections. The coordination around each nickel atom is planar with four nitrogens in a trapezoidal arrangement. The perchlorate groups do not occupy the fifth and sixth coordination positions, but link, through hydrogen bonds, the essentially identical ligand complexes that comprise the asymmetric unit. The complex is the meso isomer with the central ring in the eclipsed cis form. Although the normal gauche conformation is not present in the central ring formed by the secondary nitrogens, the gauche conformation is present in the chelates formed between the secondary and primary nitrogens.

CYCLIC VOLTAMMETRIC STUDIES OF THE REDUCTION OF COPPER(II)-PEPTIDE COMPLEXES

Abstract Cyclic voltammetry is used to study the electrochemical reduction of copper(II) triglycine complexes and the electrochemical reduction of a ternary copper(II)-diglycine-(2,9-dimethyl-1,10-phenanthroline) complex, Cu(H−1G2)dmp. The reduction of the copper(II) triglycine complexes at a hanging mercury drop electrode occurs via two-electrons to form copper amalgam. Under some conditions the dissociation of the copper(II) triglycine complexes to Cuaq 2+ is not sufficiently rapid to maintain diffusion control, and two reduction waves appear, one due to reduction of Cuaq 2+ and the other, appearing at more negative values than the first, due to reduction of a residual copper(II) triglycine complex. The reduction of the ternary complex CuII(H−1G2)dmp at a carbon paste electrode provides evidence of a transient copper(I) complex, CuI(H−1G2)dmp.