Kinetic and mechanistic study of the reaction of 2,6-dichlorophenol-indophenol and cysteine

In this work the reaction of cysteine (H(2)Q) with 2,6-dichlorophenolindophenol (D) is studied kinetically in the pH range 2.5-9.0. Taking into consideration the distribution diagrams for the species H(3)Q(+), H(2)Q, HQ(-), Q(2-) for cysteine and DH(+)(2), DH, D(-) for 2,6-dichlorophenolindophenol the reaction rate constants k(i) for all possible combinations of the reacting species were determined. The maximum reactivity appears at pH 6.88 with an overall reaction constant k = 306 1.mole(-1).sec(-1) at 22 degrees . The effect of the concentrations of the reagents and the ionic strength on the reaction rate is also given. From Arrhenius plots an activation energy E(a) = 8.1 kcal/mole was calculated. Working curves for the determination of cysteine in aqueous solutions are also presented applying the reaction rate method. Finally the paper includes important analytical information for the calculation of the errors due to interference of cysteine by the kinetic determination of ascorbic acid, using its reaction with 2,6-dichlorophenolindophenol.     

Kinetic and mechanistic study of the reduction of 2,6-dichlorophenol indophenol by dithionites

The reaction of 2,6-dichlorophenolindophenol (DCPI) and dithionites is studied kinetically by applying the stopped-flow technique. Reaction rate constants are given for the pH range 1.30–6.80. The reaction was found to follow first-order kinetics with respect to each of the reactants. For pH 3.97, 5.10 and 6.80, the second-order reaction rate constant was determined by applying four different technique. Mean values of k = 172±5, 200±2 and 276±4 l mol^?1s^?1 are given for pH 3.97, 5.10 and 6.80, respectively. A mechanism is proposed for the reaction, which suggests partial reactions of all possible species of DCPI and dithionites at any pH. An equation for the calculation of k at any pH is derived, which gives k as a function of [H⁺], the partial reaction rate constants and the dissociation constants of DCPI and H₂S₂O₄. Values of reaction rate constants of all possible partial reactions are also presented.     

A kinetic study of the reactions of vanadium, iron, and cobalt with sulfur dioxide

A kinetic study of the reactions of ground state V, Fe, and Co with SO2 is reported. V, Fe, and Co were produced by the 248 nm photodissociation of VCl4, ferrocene, and Co(C5H5)(CO)2, respectively, and were detected by laser-induced fluorescence. V + SO2 proceeds by an abstraction reaction with rate constants given by k=(2.33 +/- 0.57)x 10(-10) exp[-(1.14 +/- 0.19) kcal mol(-1)/RT] cm3 molecule(-1) s(-1) over the temperature range 296-571 K. Fe + SO2 was studied in the N2 buffer range of 10-185 Torr between 294 and 498 K. The limiting, low-pressure third-order rate constants are given by k(0)=(3.45 +/- 1.19)x 10(-30) exp[-(2.81 +/- 0.24) kcal mol(-1)/RT] cm6 molecule(-2) s(-1). Co + SO2 was studied in the CO2 buffer range of 5-40 Torr between 294 and 498 K. This reaction is independent of temperature over the indicated range and has a third-order rate constant of k0=(5.23 +/- 0.28)x 10(-31) cm6 molecule(-2) s(-1). Results of this work are compared to previous work on the Sc, Ti, Cr, Mn, and Ni + SO2 systems. The reaction efficiencies for the abstraction reactions depend on the ionization energies of the transition metal atoms and on the reaction exothermicities, and the reaction efficiencies of the association reactions are strongly dependent on the energies needed to promote an electron from a 4s2 configuration to a 4s1 configuration.     

Laser-spectroscopic laboratory studies of atmospheric aqueous phase free radical chemistry

Nitrate radical (NO(3)) reactions with benzene (R-1), toluene (R-2), p-xylene (R-3), p-cresol (R-4) and mesitylene (R-5) have been studied by laser photolysis/long path laser absorption (LP-LPLA) in aqueous solution. Rate constants of k(1)=(4.0+/-0.6). 10(8), k(2)=(1.2+/-0.3). 10(9), k(3)=(1.6+/-0.1). 10(9), k(4)= (8.4+/-2.3). 10(8) and k(5)=(1.3+/-0.3). 10(9) lmol(-1)s(-1) were obtained at T=298 K. In addition, reaction rate coefficients for SO(-)(5)+Fe(2+)-->prod. (R-6) and SO(-)(5)+Mn(2+)-->prod. (R-7) of k(6)=(4.3+/-2.4). 10(7) lmol(-1)s(-1) and k(7)=(4.6+/-1.0). 10(6) lmol(-1)s(-1) (T=298 K, I-->0) have been obtained by the application of laser photolysis/UV-VIS broadband diode array spectroscopy. A new laser photolysis/UV-long path laser absorption experiment has been applied to study the reaction of the Cl(-)(2) radical anion with dissolved sulfur(IV). For the reactions Cl(-)(2)+HSO(-)(3)-->2Cl(-)+H(+)+SO(-)(3) (R-8) and Cl(-)(2)+SO(2-)(3)-->2Cl(-)+SO(-)(3) (R-9) rate coefficients of k(8)=(1.7+/-0.2). 10(8) lmol(-1)s(-1) (T=298 K, I-->0) and of k(9)=(6.2+/-0.3). 10(7) lmol(-1)s(-1) (T=279 K, I-->0) were obtained.     

Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)

The formation constants of UO2SO4 (aq), UO2(SO4)2(2-), and UO2(SO4)3(4-) were measured in aqueous solutions from 10 to 75 degrees C by time-resolved laser-induced fluorescence spectroscopy (TRLFS). A constant enthalpy of reaction approach was satisfactorily used to fit the thermodynamic parameters of stepwise complex formation reactions in a 0.1 M Na(+) ionic medium: log 10 K 1(25 degrees C) = 2.45 +/- 0.05, Delta r H1 = 29.1 +/- 4.0 kJ x mol(-1), log10 K2(25 degrees C) = 1.03 +/- 0.04, and Delta r H2 = 16.6 +/- 4.5 kJ x mol(-1). While the enthalpy of the UO2(SO4)2(2-) formation reaction is in good agreement with calorimetric data, that for UO2SO4 (aq) is higher than other values by a few kilojoules per mole. Incomplete knowledge of the speciation may have led to an underestimation of Delta r H1 in previous calorimetric studies. In fact, one of the published calorimetric determinations of Delta r H1 is here supported by the TRLFS results only when reinterpreted with a more correct equilibrium constant value, which shifts the fitted Delta r H1 value up by 9 kJ x mol(-1). UO2(SO 4) 3 (4-) was evidenced in a 3 M Na (+) ionic medium: log10 K3(25 degrees C) = 0.76 +/- 0.20 and Delta r H3 = 11 +/- 8 kJ x mol(-1) were obtained. The fluorescence features of the sulfate complexes were observed to depend on the ionic conditions. Changes in the coordination mode (mono- and bidentate) of the sulfate ligands may explain these observations, in line with recent structural data.     

S-oxygenation of thiocarbamides I: Oxidation of phenylthiourea by chlorite in acidic media

The oxidation of 1-phenyl-2-thiourea (PTU) by chlorite was studied in aqueous acidic media. The reaction is extremely complex with reaction dynamics strongly influenced by the pH of reaction medium. In excess chlorite concentrations the reaction stoichiometry involves the complete desulfurization of PTU to yield a urea residue and sulfate: 2ClO2- + PhN(H)CSNH2 + H2O --> SO4(2-) + PhN(H)CONH2 + 2Cl- + 2H+. In excess PTU, mixtures of sulfinic and sulfonic acids are formed. The reaction was followed spectrophotometrically by observing the formation of chlorine dioxide which is formed from the reaction of the reactive intermediate HOCl and chlorite: 2ClO2- + HOCl + H+ --> 2ClO2(aq) + Cl- + H2O. The complexity of the ClO2- - PTU reaction arises from the fact that the reaction of ClO2 with PTU is slow enough to allow the accumulation of ClO2 in the presence of PTU. Hence the formation of ClO2 was observed to be oligooscillatory with transient formation of ClO2 even in conditions of excess oxidant. The reaction showed complex acid dependence with acid catalysis in pH conditions higher than pKa of HClO2 and acid retardation in pH conditions of less than 2.0. The rate of oxidation of PTU was given by -d[PTU]/dt = k1[ClO2-][PTU] + k2[HClO2][PTU] with the rate law: -d[PTU]/dt = [Cl(III)](T)[PTU]0/K(a1) + [H+] [k1K(a1) + k2[H+]]; where [Cl(III)]T is the sum of chlorite and chlorous acid and K(a1) is the acid dissociation constant for chlorous acid. The following bimolecular rate constants were evaluated; k1 = 31.5+/-2.3 M(-1) s(-1) and k2 = 114+/-7 M(-1) s(-1). The direct reaction of ClO2 with PTU was autocatalytic in low acid concentrations with a stoichiometric ratio of 8:5; 8ClO2 + 5PhN(H)CSNH2 + 9H2O --> 5SO4(2-) + 5PhN(H)CONH2 + 8Cl- + 18H+. The proposed mechanism implicates HOCl as a major intermediate whose autocatalytic production determined the observed global dynamics of the reaction. A comprehensive 29-reaction scheme is evoked to describe the complex reaction dynamics.     

Homogeneous catalytic reduction of dioxygen using transfer hydrogenation catalysts

Solutions of Cp*IrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN(SO2C6H4CH3)-) (1H(H)) with O2 generate Cp*Ir(TsDPEN-H) (1) and 1 equiv of H2O. Kinetic analysis indicates a third-order rate law (second order in [1H(H)] and first order in [O2]), resulting in an overall rate constant of 0.024 +/- 0.013 M(-2) s(-1). Isotopic labeling revealed that the rate of the reaction of 1H(H) + O2 was strongly affected by deuteration at the hydride position (k(HH2)/k(DH2) = 6.0 +/- 1.3) but insensitive to deuteration of the amine (k(HH2)/k(HD2) = 1.2 +/- 0.2); these values are more disparate than for conventional transfer hydrogenation (Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001). The temperature dependence of the reaction rate indicated DeltaH = 82.2 kJ/mol, DeltaS = 13.2 J/mol K, and a reaction barrier of 85.0 kJ/mol. A CH2Cl2 solution under 0.30 atm of H2 and 0.13 atm of O2 converted to H2O in the presence of 1 and 10 mol % of H(OEt2)2BAr(F)4 (BAr(F)4- = B(C6H3-3,5-(CF3)2)4-). The formation of water from H2 was verified by 2H NMR for the reaction of D2 + O2. Solutions of 1 slowly catalyze the oxidation of amyl alcohol to pentanal; using 1,4-benzoquinone as a cocatalyst, the conversion was faster. Complex 1 also catalyzes the reaction of O2 with RNH2BH3 (R = H, t-Bu), resulting in the formation of water and H2. The deactivation of the catalyst 1 in its reactions with O2 was traced to degradation of the Cp* ligand to a fulvene derivative. This pathway is not observed in the presence of amine-boranes, which were shown to reduce fulvenes back to Cp*. This work suggests the potential of transfer hydrogenation catalysts in reactions involving O2.     

Comparative kinetic study for rate constant determination of the reaction of ascorbic acid with 2,6-dichlorophenolindophenol

The rate constant k of the reaction of ascorbic acid with 2,6-dichlorophenolindophenol (DCPI) in oxalic acid solutions is determined, by stopped-flow techniques. Four different methods are used to evaluate the results. The values and errors are compared statistically. The average of the rate constant is 56.5 x 10(3) l. mole(-1), sec(-1) and the overall standard deviation is 0.6 x 10(3) l. mole(-1), sec(-1) or 1.0% relative. The pH-dependence of the rate constant suggests that DCPI reacts with undissociated ascorbic acid.     

Kinetics of the reaction of SH and SD with NO2

The rate constants for the reactions of NO2 with SH and SD were measured between 250 and 360 K to be 2.8 x 10(-11) exp{(270+/-40)/T(K)} and 2.6x10(-11) exp{(285+/-20)/T(K)} cm3 molecule-1 s-1, respectively. SH(SD) radicals were generated by pulsed laser photolysis of H2S(D2S) or CH3SH and detected via pulsed laser-induced fluorescence. The laser-induced fluorescence excitation spectrum of SH was found to be contaminated by the presence of the SO radical. This contamination is suggested as a possible reason for differences among some of the reported values of k1 in the literature. The title reaction influences the atmospheric lifetime of the SH radical when NO2 is greater than 100 pptv, but the revised value of k1 does not significantly alter our current understanding of SH oxidation in the atmosphere.     

Reactions of SO3 with the O/H radical pool under combustion conditions

The reactions of SO3 with H, O, and OH radicals have been investigated by ab initio calculations. For the SO3 + H reaction (1), the lowest energy pathway involves initial formation of HSO3 and rearrangement to HOSO2, followed by dissociation to OH + SO2. The reaction is fast, with k(1) = 8.4 x 10(9)T(1.22) exp(-13.9 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (700-2000 K). The SO3 + O --> SO2 + O2 reaction (2) may proceed on both the triplet and singlet surfaces, but due to a high barrier the reaction is predicted to be slow. The rate constant can be described as k(2) = 2.8 x 10(4)T(2.57) exp(-122.3 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) for T > 1000 K. The SO3 + OH reaction to form SO2 + HO2 (3) proceeds by direct abstraction but is comparatively slow, with k(3) = 4.8 x 10(4)T(2.46) exp(-114.1 kJ mol(-) 1/RT) cm(3) mol(-1) s(-1) (800-2000 K). The revised rate constants and detailed reaction mechanism are consistent with experimental data from batch reactors, flow reactors, and laminar flames on oxidation of SO2 to SO3. The SO3 + O reaction is found to be insignificant during most conditions of interest; even in lean flames, SO3 + H is the major consumption reaction for SO3.     

Kinetics and mechanisms of CF3CHFOCH3, CF3CHFOC(O)H, and FC(O)OCH3 reactions with OH radicals

The kinetics and mechanism of oxidation of CF3CHFOCH3 was studied using an 11.5-dm3 environmental reaction chamber. OH radicals were produced by UV photolysis of an O3-H2O-He mixture at an initial pressure of 200 Torr in the chamber. The rate constant of the reaction of CF3CHFOCH3 with OH radicals (k1) was determined to be (1.77 +/- 0.69) x 10(-12) exp[(-720 +/- 110)/T] cm3 molecule(-1)(s-1) by means of a relative rate method at 253-328 K. The mechanism of the reaction was investigated by FT-IR spectroscopy at 298 K. CF3CHFOC(O)H, FC(O)OCH3, and COF2 were determined to be the major products. The branching ratio (k1a/k1b) for the reactions CF3CHFOCH3 + OH --> CF3CHFOCH2* + H2O (k1a) and CF3CHFOCH3 + OH --> CF3CF*OCH3 + H2O (k1b) was estimated to be 4.2:1 at 298 K from the yields of CF3CHFOC(O)H, FC(O)OCH3, and COF2. The rate constants of the reactions of CF3CHFOC(O)H (k2) and FC(O)OCH3 (k3) with OH radicals were determined to be (9.14 +/- 2.78) x 10(-13) exp[(-1190 +/- 90)/T] and (2.10 +/- 0.65) x 10(-13) exp[(-630 +/- 90)/T] cm3 molecule(-1)(s-1), respectively, by means of a relative rate method at 253-328 K. The rate constants at 298 K were as follows: k1 = (1.56 +/- 0.06) x 10-13, k2 = (1.67 +/- 0.05) x 10-14, and k3 = (2.53 +/- 0.07) x 10-14 cm3 molecule(-1)(s-1). The tropospheric lifetimes of CF3CHFOCH3, CF3CHFOC(O)H, and FC(O)OCH3 with respect to reaction with OH radicals were estimated to be 0.29, 3.2, and 1.8 years, respectively.     

Kinetic and product study of the gas-phase reactions of OH radicals, NO(3) radicals, and O(3) with (C(2)H(5)O)(2)P(S)CH(3) and (C(2)H(5)O)(3)PS

Rate constants for the reactions of OH radicals and NO3 radicals with O,O-diethyl methylphosphonothioate [(C(2)H(5)O)(2)P(S)CH(3); DEMPT] and O,O,O-triethyl phosphorothioate [(C(2)H(5)O)(3)PS; TEPT] have been measured using relative rate methods at atmospheric pressure of air over the temperature range 296-348 K for the OH radical reactions and at 296 +/- 2 K for the NO(3) radical reactions. At 296 +/- 2 K, the rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were 20.4 +/- 0.8 and 7.92 +/- 0.27 for DEMPT and TEPT, respectively, and those for the NO(3) radical reactions (in units of 10(-15) cm(3) molecule(-1) s(-1)) were 2.01 +/- 0.20 and 1.03 +/- 0.10, respectively. Upper limits to the rate constants for the reactions of O(3) with DEMPT and TEPT of <6 x 10(-20) cm(3) molecule(-1) s(-1) were determined in each case. Rate constants for the OH radical reactions, measured relative to k(OH + alpha-pinene) = 1.21 x 10(-11) e(436/T) cm(3) molecule(-1) s(-1), resulted in the Arrhenius expressions k(OH + DEMPT) = 1.08 x 10(-11) e(871+/-25)/T cm(3) molecule(-1) s(-1) and k(OH + TEPT) = 8.21 x 10(-13) e(1353+/-49)/T cm(3) molecule(-1) s(-1) over the temperature range 296-348 K, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the reference rate constant. Diethyl methylphosphonate was identified and quantified from the OH radical and NO(3) radical reactions with DEMPT, with formation yields of 21 +/- 4%, independent of temperature, from the OH radical reaction and 62 +/- 11% from the NO(3) radical reaction at 296 +/- 2 K. Similarly, triethyl phosphate was identified and quantified from the OH radical and NO(3) radical reactions with TEPT, with formation yields of 56 +/- 9%, independent of temperature, from the OH radical reaction and 78 +/- 15% from the NO(3) radical reaction at 296 +/- 2 K.     

Kinetics and products of the gas-phase reactions of divinyl sulfoxide with OH and NO3 radicals and O3

Using relative rate methods, rate constants for the gas-phase reactions of divinyl sulfoxide [CH 2CHS(O)CHCH 2; DVSO] with NO 3 radicals and O 3 have been measured at 296 +/- 2 K, and rate constants for the reaction with OH radicals have been measured over the temperature range of 277-349 K. Rate constants obtained for the NO 3 radical and O 3 reactions at 296 +/- 2 K were (6.1 +/- 1.4) x 10 (-16) and (4.3 +/- 1.0) x 10 (-19) cm (3) molecule (-1) s (-1), respectively. For the OH radical reaction, the temperature-dependent rate expression obtained was k = 4.17 x 10 (-12)e ((858 +/- 141)/ T ) cm (3) molecule (-1) s (-1) with a 298 K rate constant of (7.43 +/- 0.71) x 10 (-11) cm (3) molecule (-1) s (-1), where, in all cases, the errors are two standard deviations and do not include the uncertainties in the rate constants for the reference compounds. Divinyl sulfone was observed as a minor product of both the OH radical and NO 3 radical reactions at 296 +/- 2 K. Using in situ Fourier transform infrared spectroscopy, CO, CO 2, SO 2, HCHO, and divinyl sulfone were observed as products of the OH radical reaction, with molar formation yields of 35 +/- 11, 2.2 +/- 0.8, 33 +/- 4, 54 +/- 6, and 5.4 +/- 0.8%, respectively, in air. For the experimental conditions employed, aerosol formation from the OH radical-initiated reaction of DVSO in the presence of NO was minor, being approximately 1.5%. The data obtained here for DVSO are compared with literature data for the corresponding reactions of dimethyl sulfoxide.     

Kinetics of the reaction of aqueous iron(vi) (FeVIO42-) with ethylenediaminetetraacetic acid

The reaction of aqueous iron(vi) (FeVIO42-, Fe(vi)) with ethylenediaminetetraacetic acid (EDTA) was studied kinetically as a function of pH (1.98-12.40) and temperature (15-45 degrees C) using a stopped flow kinetic technique. The rate law for the reaction of Fe(vi) with EDTA was found to be first-order with respect to each reactant over the entire studied pH range. The observed rate constants, k, decrease with an increase in pH, varying from 4.19 x 10(4) to 8.60 x 10(-2) M(-1) s(-1) over the pH range. The speciation of Fevi (H3FeO4+, H2FeO4, HFeO4-, and FeO42-) and EDTA (H4Y, H3Y-, H2Y2-, HY3-, and Y4-, Y = EDTA) species was used to explain the pH dependence of the k values. From the temperature effect on k at pH 5.4, 7.1, and 9.2, activation parameters, DeltaS(double dagger) and DeltaH(double dagger), were obtained for the reactions of Fe(VI) with EDTA. The values of DeltaS(double dagger) for the reactions were found to be negative, implying a highly ordered transition state in the reaction. The DeltaH(double dagger) for the reaction at pH 7.1 and 9.2 showed similar values within experimental error. Using the observed enthalpy parameters and the enthalpy of deprotonation of HFeO4- and EDTA species (HEDTA3- and H2EDTA2-), the enthalpy of deprotonation of H2FeO4 (DeltaH0H2FeO4) was determined as 5.7 +/- 3.0 kJ mol(-1). The reactivity of Fe(VI) with aminopolycarboxylates (APCs) was also studied in alkaline medium. The order of reactivity was determined as primary > secondary > tertiary, which suggests that FeVIO42- attacks at the nitrogen atom sites of APCs.     

Catalytic and direct oxidation of cysteine by octacyanomolybdate(V)

The oxidation of cysteine by [Mo(CN)(8)](3-) in deoxygenated aqueous solution at a moderate pH is strongly catalyzed by Cu(2+), to the degree that impurity levels of Cu(2+) are sufficient to dominate the reaction. Dipicolinic acid (dipic) is a very effective inhibitor of this catalysis, such that with 1 mM dipic, the direct oxidation can be studied. UV-vis spectra and electrochemistry show that [Mo(CN)(8)](4-) is the Mo-containing product. Cystine and cysteinesulfinate are the predominant cysteine oxidation products. The stoichiometric ratio (Deltan(Mo(V))/Deltan(cysteine)) of 1.4 at pH 10.8 is consistent with this product distribution. At pH 1.5, the reaction is quite slow and yields intractable kinetics. At pH 4.5, the rates are much faster and deviate only slightly from pseudo-first-order behavior. With 2 mM PBN (N-phenyl-tert-butyl nitrone) present at pH 4.5, the reaction rate is about 20% less and shows excellent pseudo-first-order behavior, but the stoichiometric ratio is not significantly changed. The rates also display a significant specific cation effect. In the presence of spin-trap PBN, the kinetics were studied over the pH range 3.48-12.28, with [Na(+)] maintained at 0.09-0.10 M. The rate law is -d[Mo(V)]/dt = k[cysteine](tot)[Mo(V)], with k = {2(k(b)K(a1)K(a2)[H(+)] + k(c)K(a1)K(a2)K(a3))}/([H(+)](3) + K(a1)[H(+)](2) + K(a1)K(a2)[H(+)] + K(a1)K(a2)K(a3)), where K(a1), K(a2), and K(a3) are the successive acid dissociation constants of HSCH(2)CH(NH(3)(+))CO(2)H. Least-squares fitting yields k(b) = (7.1 +/- 0.4) x 10(4) M(-1) s(-1) and k(c) = (2.3 +/-0.2) x 10(4) M(-1) s(-1) at mu = 0.1 M (NaCF(3)SO(3)) and 25 degrees C. A mechanism is inferred in which k(b) and k(c) correspond to electron transfer to Mo(V) from the thiolate forms of anionic and dianionic cysteine.     

Hydrogen atom transfer reactions of a ruthenium imidazole complex: hydrogen tunneling and the applicability of the Marcus cross relation

The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives Ru(III)(acac)2(py-im) (Ru(III)im) and the hydroxylamine TEMPO-H by transfer of H(*) (H(+) + e(-)) (acac = 2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k(3H) = 1400 +/- 100 M(-1) s(-1) at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (Delta G(o)). Deuterium transfer from Ru(II)(acac)2(py-imD) (Ru(II)imD) to TEMPO(*) is surprisingly much slower at k(3D) = 60 +/- 7 M(-1) s(-1), with k(3H)/k(3D) = 23 +/- 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E(a3D) - E(a3H) = 1.9 +/- 0.8 kcal mol(-1). The large k(3H)/k(3D) and DeltaE(a) values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru(II)imH and Ru(III)im, measured by (1)H NMR line broadening, occurs with k(4H) = (3.2 +/- 0.3) x 10(5) M(-1) s(-1) at 298 K and k(4H)/k(4D) = 1.5 +/- 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A(4H)/A(4D)) = -0.5 +/- 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru(II)imH(D) + TEMPO(*) that are greater than those observed: k(3H,calc)/k(3H) = 31 +/- 4 and k(3D,calc)/k(3D) = 140 +/- 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.     

Kinetics of the reactions of Cl*(2P(1/2)) and Cl(2P(3/2)) atoms with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH at 295 K

The title reactions were studied using laser flash photolysis/laser-induced-fluorescence (FP-LIF) techniques. The two spin-orbit states, Cl*(2P(1/2)) and Cl(2P(3/2)), were detected using LIF at 135.2 and 134.7 nm, respectively. Measured reaction rate constants were as follows (units of cm3 molecule(-1) s(-1)): k(Cl(2P(3/2))+CH3OH) = (5.35 +/- 0.24) x 10(-11), k(Cl(2P(3/2))+C2H5OH) = (9.50 +/- 0.85) x 10(-11), k(Cl(2P(3/2))+n-C3H7OH) = (1.71 +/- 0.11) x 10(-10), and k(Cl(2P(3/2))+i-C3H7OH) = (9.11 +/- 0.60) x 10(-11). Measured rate constants for total removal of Cl*(2P(1/2)) in collisions with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH were (1.95 +/- 0.13) x 10(-10), (2.48 +/- 0.18) x 10(-10), (3.13 +/- 0.18) x 10(-10), and (2.84 +/- 0.16) x 10(-10), respectively; quoted errors are two-standard deviations. Although spin-orbit excited Cl*(2P(1/2)) atoms have 2.52 kcal/mol more energy than Cl(2P(3/2)), the rates of chemical reaction of Cl*(2P(1/2)) with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH are only 60-90% of the corresponding Cl(2P(3/2)) atom reactions. Under ambient conditions spin-orbit excited Cl* atoms are responsible for 0.5%, 0.5%, 0.4%, and 0.7% of the observed reactivity of thermalized Cl atoms toward CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH, respectively.     

Hydrogen atom transfer from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)] to TEMPO: a negative enthalpy of activation predicted by the Marcus equation

The transfer of a hydrogen atom from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)], [FeII(H2bip)3]2+, to the stable nitroxide, TEMPO, was studied by stopped-flow UV-vis spectrophotometry. The products are the deprotonated iron(III) complex [FeIII(H2bip)2(Hbip)]2+ and the hydroxylamine, TEMPO-H. This reaction can also be referred to as proton-coupled electron transfer (PCET). The equilibrium constant for the reaction is close to 1; thus, the reaction can be driven in either direction. The rate constants for the forward and reverse reactions at 298 K are k1 = 260 +/- 30 M-1 s-1 and k-1 = 150 +/- 20 M-1 s-1. Interestingly, the rate constant for the forward reaction decreases as reaction temperature is increased, implying a negative activation enthalpy: DeltaH1 = -2.7 +/- 0.4 kcal mol-1, DeltaS1 = -57 +/- 8 cal mol-1 K-1. Marcus theory predicts this unusual temperature dependence on the basis of independently measured self-exchange rate constants and equilibrium constants: DeltaHcalcd = -3.5 +/- 0.5 kcal mol-1, DeltaScalcd = -42 +/- 10 cal mol-1 K-1. This result illustrates the value of the Marcus approach for these types of reactions. The dominant contributor to the negative activation enthalpy is the favorable enthalpy of reaction, DeltaH1 degrees = -9.4 +/- 0.6 kcal mol-1, rather than the small negative activation enthalpy for the H-atom self-exchange between the iron complexes.     

Intramolecular general base catalyzed ester hydrolysis. The hydrolysis of 2-aminobenzoate esters

Rate constants have been obtained for the hydrolysis of the trifluoroethyl, phenyl, and p-nitrophenyl esters of 2-aminobenzoic acid at 50 degrees C in H(2)O. The pseudo-first-order rate constants, k(obsd), are pH independent from pH 8 to pH 4 (the pK(a) of the amine group conjugate acid). The 2-aminobenzoate esters hydrolyze with similar rate constants in the pH-independent reactions, and these water reactions are approximately 2-fold slower in D(2)O than in H(2)O. The most likely mechanism involves intramolecular general base catalysis by the neighboring amine group. The rate enhancements in the pH-independent reaction in comparison with the pH-independent hydrolysis of the corresponding para substituted esters or the benzoate esters are 50-100-fold. In comparison with the hydroxide ion catalyzed reaction, the enhancement in k(obsd) at pH 4 with the phenyl ester is 10(5)-fold. Intramolecular general base catalyzed reactions are assessed in respect to their relative advantages and disadvantages in enzyme catalysis. A general base catalyzed reaction can be more rapid at low pH than a nucleophilic reaction that has a marked dependence on pH and the leaving group.     

Theoretical and experimental investigation on the oxidation of gallic acid by sulfate radical anions

By monitoring the decay of SO4*- after flash photolysis of aqueous solutions of S2O82- at different pH values, the kinetics of the reaction of SO4*- radicals with gallic acid and the gallate ion was investigated. The bimolecular rate constants for the reactions of the sulfate radicals with gallic acid and the gallate ion were found to be (6.3 +/- 0.7) x 10(8) and (2.9 +/- 0.2) x 10(9) M(-1) s(-1), respectively. On the basis of the oxygen-independent second-order decay kinetics and on their absorption spectra, the organic radicals formed as intermediates of these reactions were assigned to the corresponding phenoxyl radicals. DFT calculations in the gas phase and aqueous solution support formation of the phenoxyl radicals by H abstraction from the phenols to the sulfate radical anion. The observed recombination of the phenoxyl radicals of gallic acid to yield substituted biphenyls and quinones is also supported by the calculations. HPLC/MS product analysis showed formation of one of the predicted quinones.