Protonation of Chloro-, Bromo- and Iodo-pentacyanocobaltate(III) Complexes

The reactions between Fe(Phen)₃²⁺[phen = tris-(1,10) phenanthroline] and Co(CN)₅X³⁻ (X = Cl, Br or I) have been studied in aqueous acidic solutions at 25 °C and ionic strength in the range I = 0.001–0.02 mol dm⁻³ (NaCl/HCl). Plots of k₂ versus √I, applying Debye–Huckel Theory, gave the values −1.79 ± 0.18, −1.65 ± 0.18 and 1.81 ± 0.10 as the product of charges (Z_AZ_B) for the reactions of Fe(Phen)₃²⁺ with the chloro-, bromo- and iodo- complexes respectively. Z_AZ_B of ≈ −2 suggests that the charge on these Co^III complexes cannot be −3 but is −1. This suggests the possibility of protonation of these Co^III complexes. Protonation was investigated over the range [H⁺] = 0.0001 −0.06 mol dm⁻³ and the protonation constants K_a obtained are 1.22 × 10³, 7.31 × 10³ and 9.90 × 10² dm⁶ mol⁻³ for X = Cl, Br and I, respectively.     

Kinetic study of the promazine oxidation to promazine 5-oxide by trisoxalatocobaltate(III) in basic aqueous media

The kinetics of the oxidation of promazine by trisoxalatocobaltate(III) were studied in the presence of a large excess of the cobalt(III) in tris buffer solution using u.v.–vis spectroscopy ([Co^III] = (0.6 − 2) × 10⁻³ M, [ptz] = 6 × 10⁻⁵ M, pH = 6.6–7.8, I = 0.1 M (NaCl), T = 288−308 K, l = 1 cm). The reaction proceeds via two consecutive reversible steps. In the first step, the reaction leads to formation of cobalt(II) species and a stable cationic radical. In the second step, cobalt(III) is reduced to cobalt(II) ion and a promazine radical is oxidized to the promazine 5-oxide. Linear dependences of the pseudo-first-order rate constants (k ₁ and k ₂) on [Co^III] with a non-zero intercept were established for both redox processes. Rates of reactions decreased with increasing concentration of the H⁺ ion indicating that the promazine and its radical exist in equilibrium with their deprotonated forms, which are reactive reducing species. The activation parameters for reactions studied were as follows: ΔH^≠ = 44 ± 1 kJ mol⁻¹, ΔS^≠ = −100 ± 4 JK⁻¹ mol⁻¹ for the first step and ΔH^≠ = 25 ± 1 kJ mol⁻¹, ΔS^≠ = −169 ± 4 J K⁻¹ mol⁻¹ for the second step, respectively. Mechanistic consequences of all the results are discussed.     

Complexation of [2.2]paracyclophane with β- and γ-cyclodextrins studied by HPLC and NMR

The NMR spectra of [2.2]paracyclophane with β- or γ-cyclodextrin in DMF-d₇ at room temperature do not show significant complexation, while HPLC of the complexes in mixed H₂O:alcohol solvents demonstrate complexation with different stoichiometries. At 243 K in DMF solution the H3 and H5 NMR signals of γ-cyclodextrin (but not β) exhibit complexation-induced chemical shifts denoting complex formation. According to HPLC, at room temperature the [2.2]paracyclophane complex with β-cyclodextrin in 20% H₂O:EtOH exhibits 1:2 stoichiometry with K ₁ = 1×10² ± 2, K ₂ = 9.0×10⁴ ± 2×10³ (K = 9×10⁶) while that with γ-cyclodextrin in 50% H₂O:MeOH exhibits 1:1 stoichiometry with K ₁ = 4×10³ ± 150 M⁻¹. Thermodynamic parameters for both complexes have been estimated from the retention time temperature dependence. For the β-cyclodextrin complexation at 25°C ΔG ⁰ _CD is −39.7 kJ mol⁻¹ while ΔH ⁰ _CD and ΔS ⁰ _CD are −88.2 kJ mol⁻¹ and −0.16 kJ mol⁻¹ K⁻¹. For γ-cyclodextrin, the corresponding values are ΔG ⁰ _CD = −20.5 kJ mol⁻¹, ΔH ⁰ _CD = −33.5 kJ mol⁻¹ and ΔS ⁰ _CD = −0.04 kJ mol⁻¹ K⁻¹.      

Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution ([Co^III] = (2–6) × 10⁻³ m, [ptz] = 2.5 × 10⁻⁴ m, [H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and [Co(edta)]⁻ in aqueous HCl ([Co^III] = (1 − 4) × 10⁻³ m, [ptz] = 1 × 10⁻⁴ m, [H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on [Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of [H⁺]. In the case of the [Co(edta)]⁻ ion, the k _obs dependence on [H⁺] was linear and the increasing [H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for [Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for [Co(edta)]⁻.     

Outer-Sphere electron-transfer and the effect of linkage isomerism in the titanium(III) reduction of nitro-pentacyanocobaltate(III) and nitro-, and nitrito-pentaamminecobalt(III) complexes in aqueous acidic media

The reductions of [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺, by Ti^III in aqueous acidic solution have been studied spectrophotometrically. Kinetic studies were carried out using conventional techniques at an ionic strength of 1.0 mol dm⁻³ (LiCl/HCl) at 25.0 ± 0.1 °C and acid concentrations between 0.015 and 0.100 mol dm⁻³. The second-order rate constant is inverse—acid dependent and is described by the limiting rate law:- k₂ ≈ k₀ + k[H⁺]⁻¹,where k=k′K_a and K_a is the hydrolytic equilibrium constant for [Ti(H₂O)₆]³⁺. Values of k₀ obtained for [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺ are (1.31 ± 0.05) × 10⁻² dm³ mol⁻¹ s⁻¹, (4.53 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ and (1.7 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ respectively, while the corresponding k′ values from reductions by TiOH²⁺ are 10.27 ± 0.45 dm³ mol⁻¹ s⁻¹, 14.99 ± 0.70 dm³ mol⁻¹ s⁻¹ and 17.93 ± 0.78 dm³ mol⁻¹ s⁻¹ respectively. Values of K _a obtained for the three complexes lie in the range (1–2) × 10⁻³ mol dm⁻³ which suggest an outer-sphere mechanism.     

Kinetic, spectral and redox properties of the transients formed on one-electron reduction of 3, 3, 6, 6-tetramethyl-3, 4, 6, 7, 9-pentahydro-10-phenyl (1, 8) (2H, 5H) acridinedione in aqueous-organic mixed solvent system: A pulse radiolysis study

The phenyl substituted acridine-1,8-dione (AD) dye reacts with (CH₃)₂*COH radicals with a bimolecular rate constant of 0.6 × 10⁸ dm³ mol⁻¹ s⁻¹ in acidic aqueous-organic mixed solvent system. The transient optical absorption band (λ_max = 465 nm, ɛ = 6.8 × 10² dm³ mol⁻¹ cm⁻¹) is assigned to ADH* formed on protonation of the radical anion. In basic solutions, (CH₃)₂*COH radicals react with a bimolecular rate constant of 4.6 × 10⁸ dm³ mol⁻¹ s⁻¹ and the transient optical absorption band (λ_max = 490 nm, ɛ = 10.4 × 10³ dm³ mol⁻¹ cm⁻¹) is assigned to radical anion, AD*⁻, which has a pK_a value of 8.0. The reduction potential value of the AD/AD*⁻ couple is estimated to be between −0.99 and −1.15 V vs NHE by pulse radiolysis studies. The cyclic voltammetric studies showed the peak potential close to −1.2 V vs Ag/AgCl.     

Determination of dissociation energies of N-H bond in the 4-anilinodiphenylaminyl radical and O-H bond in the 2,5-dichloro-4-hydroxyphenoxyl radical from the equilibrium constants of chain reactions in quinoneimine-hydroquinone systems

The temperature dependences of the equilibrium constants of two chain reversible reactions in quinonediimine (quinonemonoimine)—2,5-dichlorohydroquinone systems in chlorobenzene were studied. The enthalpy of equilibrium of the reversible reaction of quinonediimine with 4-hydroxydiphenylamine was estimated from these data (ΔH = − 14.4±1.6 kJ mol⁻¹) and a more accurate value of the N-H bond dissociation energy in the 4-anilinodiphenylaminyl radical was determined (D _NH = 278.6±3.0 kJ mol⁻¹). A chain mechanism was proposed for the reaction between quinonediimine and 2,5-dichlorohydroquinone, and the chain length was estimated (ν = 300 units) at room temperature. Processing of published data on the rate constant of the reaction of styrylperoxy radicals with 2,5-dichlorohydroquinone in the framework of the intersecting parabolas method gave the O-H bond dissociation energy in 2,5-dichlorohydroquinone: D _OH = 362.4±0.9 kJ mol⁻¹. Taking into account these data, the O-H bond dissociation energy in the 2,5-dichlorosemiquinone radical was found: D _OH = 253.6±1.9 kJ mol⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1661–1666, October, 2006.     

Catalytic and Thermodynamic Characterization of Endoglucanase (CMCase) from <Emphasis Type="Italic">Aspergillus oryzae</Emphasis> cmc-1

Monomeric extracellular endoglucanase (25 kDa) of transgenic koji (Aspergillus oryzae cmc-1) produced under submerged growth condition (7.5 U mg⁻¹ protein) was purified to homogeneity level by ammonium sulfate precipitation and various column chromatography on fast protein liquid chromatography system. Activation energy for carboxymethylcellulose (CMC) hydrolysis was 3.32 kJ mol⁻¹ at optimum temperature (55 °C), and its temperature quotient (Q ₁₀) was 1.0. The enzyme was stable over a pH range of 4.1–5.3 and gave maximum activity at pH 4.4. V _max for CMC hydrolysis was 854 U mg⁻¹ protein and K _m was 20 mg CMC ml⁻¹. The turnover (k _cat) was 356 s⁻¹. The pK _a1 and pK _a2 of ionisable groups of active site controlling V _max were 3.9 and 6.25, respectively. Thermodynamic parameters for CMC hydrolysis were as follows: ΔH* = 0.59 kJ mol⁻¹, ΔG* = 64.57 kJ mol⁻¹ and ΔS* = −195.05 J mol⁻¹ K⁻¹, respectively. Activation energy for irreversible inactivation ‘E _a(d)’ of the endoglucanase was 378 kJ mol⁻¹, whereas enthalpy (ΔH*), Gibbs free energy (ΔG*) and entropy (ΔS*) of activation at 44 °C were 375.36 kJ mol⁻¹, 111.36 kJ mol⁻¹ and 833.06 J mol⁻¹ K⁻¹, respectively.     

Saturated Vapor Pressure of Iridium(III) Acetylacetonate

The temperature dependency of the saturated vapor pressure of Ir(acac)₃ has been measured by the method of calibrated volume (MCV), the Knudsen method, the flow transpiration method, and the membrane method. The thermodynamic parameters of phase transition of a crystal to gas were calculated using each of these methods, and the following values of ΔH _T ⁰ (kJ mol⁻¹) and ΔS _T ⁰ (J mol⁻¹K⁻¹), respectively, were obtained: MCV: 101.59, 156.70; Knudsen: 130.54, 224.40; Flow transpiration: 129.34, 212.23; Membrane: 95.45, 149.44 Coprocessing of obtaining data (MCV, flow transportation method and Knudsen method) at temperature ranges 110−200°C as also conducted:ΔH _T ⁰ =127.9±2.1 (kJ mol⁻¹ ); ΔS _T ⁰ =215.2±5.0 (J mol⁻¹ K⁻¹ ). This revised version was published online in August 2006 with corrections to the Cover Date.     

Rate constant for the reaction of bromine atoms with ethane: Kinetic and thermochemical implications

Relative-rate kinetic experiments were carried-out at T = 310 ± 3 K to determine rate constant ratios for the reactions of Br atoms with C₂H₆(1), CH₂ClBr(2) and neo-C₅H₁₂(3). Br atoms were produced by stationary photolysis of Br₂ and the consumption of the reactants was determined by gas-chromatography. k ₂/k ₁ = 1.174 ± 0.053 and k ₃/k ₁ = 0.458 ± 0.027 were determined (with 1σ precision given). The rate constant ratios were resolved to absolute k ₁ values, and k ₁(310 K) = (2.27 ± 0.30) × 10⁵ cm³ mol⁻¹ s⁻¹ was recommended. The recommended k ₁ was applied in a third law analysis providing Δ_f H ^o ₂₉₈(C₂H₅) = (122.0 ± 1.9) kJ mol⁻¹.     

Mechanistic studies on the intramolecular electron transfer in an adduct species of the oxo-centred trinuclear iron(III) cation and <Emphasis Type="SmallCaps">l</Emphasis>-ascorbic acid in aqueous solution

The kinetics of the intra-molecular electron transfer of an adduct of l-ascorbic acid and the [Fe₃^IIIO(CH₃COO)₆(H₂O)₃]⁺ cation in aqueous acetate buffer was studied spectrophotometrically, over the ranges 2.55 ≤ pH ≤ 3.74, 20.0 ≤ θ ≤ 35.0 °C, at an ionic strength of 0.50 and 1.0 mol dm⁻³ (NaClO₄). The reaction of l-ascorbic acid and the complex cation involves the rapid formation of an adduct species followed by a slower reduction in the iron centres through consecutive one-electron transfer processes. The final product of the reaction is aqueous iron(II) in acetate buffer. The proposed mechanism involves the triaqua and diaqua-hydroxo species of the complex cation, both of which form adducts with l-ascorbic acid. At 25 °C, the equilibrium constant for the adduct formation was found to be 86 ± 15 and 5.8 ± 0.2 dm³ mol⁻¹ for the triaqua and diaqua-hydroxo species, respectively. The kinetic parameters derived from the rate expression have been found to be: k ₀ = (1.12 ± 0.02) × 10⁻² s⁻¹ for the combined spontaneous decomposition and k ₁ = (4.47 ± 0.06) × 10⁻² s⁻¹ (ΔH ₁^‡ = 51.0 ± 2.3 kJ mol⁻¹, ΔS ₁^‡ = −100 ± 8 J K⁻¹ mol⁻¹), k ₂ = (4.79 ± 0.38) × 10⁻¹ s⁻¹ (ΔH ₂^‡ = 76.5 ± 0.8 kJ mol⁻¹, ΔS ₂^‡ = 6 ± 3 J K⁻¹ mol⁻¹) for the triaqua and diaqua-hydoxo species, respectively.     

Ligand Exchange Processes on Solvated Lithium Cations. II. Complexation by Cryptands in γ-Butyrolactone as Solvent

Kinetic studies on Li⁺ exchange between the cryptands C222 and C221, and γ-butyrolactone as solvent were performed as a function of ligand-to-metal ratio, temperature and pressure using ⁷Li NMR. The thermal rate and activation parameters are: C222: k ₂₉₈ = (3.3 ± 0.8)×10⁴ M⁻¹ s⁻¹, ΔH ^# = 35 ± 1 kJ mol⁻¹ and ΔS ^# = −41 ± 3 J K⁻¹ mol⁻¹; C221: k ₂₉₈ = 105 ± 32 M⁻¹ s⁻¹, ΔH ^# = 48 ± 1 kJ mol⁻¹ and ΔS ^# = −45 ± 2 J K⁻¹ mol⁻¹. Temperature and pressure dependence measurements were performed in the presence of an excess of Li⁺. The influence of pressure on the exchange rate is insignificant for both ligands, such that the value of activation volume is around zero within the experimental error limits. The activation parameters obtained in this study indicate that the exchange of Li⁺ between solvated and chelated Li⁺ ions follows an associative interchange mechanism. Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at . For Part I see: R. Puchta, M. Galle, N.J.R. van Hommes, E. Pasgreta and R. van Eldik: Inorg. Chem. 43, 8227 (2004).     

Stationary points on the energy hypersurface of the reaction O3 + H•→ [•O3H]* ⇆ O2 + •OH and thermodynamic functions of •O3H at G3MP2B3, CCSD(T)-CBS (W1U) and MR-ACPF-CBS levels of theory

The relative enthalpies, ΔH^o (0) and ΔH^o (298.15), of stationary points (four minimum and three transition structures) on the ^•O₃H potential energy surface were calculated with the aid of the G3MP2B3 as well as the CCSD(T)–CBS (W1U) procedures from which we earlier found mean absolute deviations (MAD) of 3.9 kJ mol⁻¹ and 2.3 kJ mol⁻¹, respectively, between experimental and calculated standard enthalpies of the formation of a set of 32 free radicals. For CCSD(T)-CBS (W1U) the well depth from O₃ + H^• to trans-^•O₃H, ΔH^o_well(298.15) = −339.1 kJ mol⁻¹, as well as the reaction enthalpy of the overall reaction O₃ + H^•→O₂ + ^•OH, Δ_rH^o(298.15) = −333.7 kJ mol⁻¹, and the barrier of bond dissociation of trans-^•O₃H → O₂ + ^•OH, ΔH^o(298.15) = 22.3 kJ mol⁻¹, affirm the stable short-lived intermediate ^•O₃H. In addition, for radicals cis-^•O₃H and trans-^•O₃H, the thermodynamic functions heat capacity C^o_p(T), entropy S^o (T), and thermal energy content H^o(T) − H^o(0) are tabulated in the range of 100 − 3000 K. The much debated calculated standard enthalpy of the formation of the trans-^•O₃H resulted to be Δ_fH^o(298.15) = 31.1 kJ mol ⁻¹ and 32.9 kJ mol ⁻¹, at the G3MP2B3 and CCSD(T)-CBS (W1U) levels of theory, respectively. In addition, MR-ACPF-CBS calculations were applied to consider possible multiconfiguration effects and yield Δ_fH^o(298.15) = 21.2 kJ mol ⁻¹. The discrepancy between calculated values and the experimental value of −4.2 ± 21 kJ mol⁻¹ is still unresolved. Note added in proof: Yu-Ran Luo and J. Alistair Kerr, based on the discussion in reference 12, recently presented an experimental value of Δ_fH^o(298.15) = 29.7 ± 8.4 kJ mol⁻¹ in the 85th edition of the CRC Handbook of Chemistry and Physics (in progress).     

The structure and thermochemistry of 3:4,5:6-dibenzo-2-hydroxymethylene-cyclohepta-3,5-dienenone (1) and some related compounds

Condensed and gas phase enthalpies of formation of 3:4,5:6-dibenzo-2-hydroxymethylene-cyclohepta-3,5-dienenone (1, (−199.1 ± 16.4), (−70.5 ± 20.5) kJ mol⁻¹, respectively) and 3,4,6,7-dibenzobicyclo[3.2.1]nona-3,6-dien-2-one (2, (−79.7 ± 22.9), (20.1 ± 23.1) kJ mol⁻¹) are reported. Sublimation enthalpies at T=298.15 K for these compounds were evaluated by combining the fusion enthalpies at T = 298.15 K (1, (12.5 ± 1.8); 2, (5.3 ± 1.7) kJ mol⁻¹) adjusted from DSC measurements at the melting temperature (1, (T _fus, 357.7 K, 16.9 ± 1.3 kJ mol⁻¹)); 2, (T _fus, 383.3 K, 10.9 ± 0.1) kJ mol⁻¹) with the vaporization enthalpies at T = 298.15 K (1, (116.1 ± 12.1); 2, (94.5 ± 2.2) kJ mol⁻¹) measured by correlation-gas chromatography. The vaporization enthalpies of benzoin ((98.5 ± 12.5) kJ mol⁻¹) and 7-heptadecanone ((94.5 ± 1.8) kJ mol⁻¹) at T = 298.15 K and the fusion enthalpy of phenyl salicylate (T _fus, 312.7 K, 18.4 ± 0.5) kJ mol⁻¹) were also determined for the correlations. The crystal structure of 1 was determined by X-ray crystallography. Compound 1 exists entirely in the enol form and resembles the crystal structure found for benzoylacetone.     

Kinetics of reactions of chlorine atom/arene complexes withn-heptane

Room temperature rate constants have been determined for reactions ofn-hepatane with Cl/benzene (k=6×10⁷ dm³ mol⁻¹ s⁻¹), Cl/toluene (k=1×10⁷ dm³ mol⁻¹ s⁻¹) and Cl/m-xylene (k=1.7×10⁶ dm³ mol⁻¹ s⁻¹) complexes, respectively, in carbon tetrachloride, using the laser flash photolysis of nitrogen trichloride as a chlorine atom source.     

Kinetics and mechanism of the acid-catalysed hydrolysis of the carbonatotetraimidazolecobalt(III) ion

Summary The kinetics of the acid-catalysed hydrolysis of the [(imidazole)₄Co(CO₃)]⁺ ion was found to follow the rate law -dln[complex]/dt = k ₁ K[H⁺](1 + K[H ⁺]) in the 25–45 °C range, [H⁺] 0.05–1.0 m range and I = 1.0m. The reaction sequence consists of a rapid protonation equilibrium followed by the one-end dissociation of the coordinated carbonato ligand (rate-determining step) and subsequent fast release of the monodentate carbonato ligand. The rate parameter values, k ₁ and ITK, at 25 °C are 6.48 × 10⁻³s⁻¹ and 0.31m ⁻¹, respectively, and activation parameters for k ₁ are ΔH ₁ ^≠ = 86.1 ± 1.2kJ mol⁻¹ and ΔS ₁ ^≠ = 2.1 ± 6.3 J mol⁻¹K⁻¹. The hydrolysis rate increases with increase in ionic strength. The different ways of dealing with the data fit are presented and discussed. The kinetic results are compared with those for the similar cobalt(III) complexes.     

Spectroscopic study of protonation of oligonucleotides containing adenine and cytosine

Acidobasic properties of purine and pyrimidine bases (adenine, cytosine) and relevant nucleosides (adenosine, cytidine) were studied by means of glass-electrode potentiometry and the respective dissociation constants were determined under given experimental conditions (I = 0.1 M (NaCl), t = (25.0 ± 0.1) °C): adenine (pK _HL = 9.65 ± 0.04, pK _H2L = 4.18 ± 0.04), adenosine (pK _H2L = 3.59 ± 0.05), cytosine (pK _H2L = 4.56 ± 0.01), cytidine (pK _H2L = 4.16 ± 0.02). In addition, thermodynamic parameters for bases: adenine (ΔH ⁰ = (−17 ± 4) kJ mol⁻¹, ΔS ⁰ = (23 ± 13) J K⁻¹ mol⁻¹), cytosine (ΔH ⁰ = (−22 ± 1) kJ mol⁻¹, ΔS ⁰ = (13 ± 5) J K⁻¹ mol⁻¹) were calculated. Acidobasic behavior of oligonucleotides (5′CAC-CAC-CAC3′ = (CAC)₃, 5′AAA-CCC-CCC3′ = A₃C₆, 5′CCC-AAA-CCC3′ = C₃A₃C₃) was studied under the same experimental conditions by molecular absorption spectroscopy. pH-dependent spectral datasets were analyzed by means of advanced chemometric techniques (EFA, MCR-ALS) and the presence of hemiprotonated species concerning (C⁺-C) a non-canonical pair (i-motif) in titled oligonucleotides was proposed in order to explain experimental data obtained according to literature.     

The standard enthalpy of formation of silver pivalate

The standard enthalpy of combustion of crystalline silver pivalate, (CH₃)₃CC(O)OAg (AgPiv), was determined in an isoperibolic calorimeter with a self-sealing steel bomb, Δ_c H ⁰ (AgPiv, cr)= −2786.9±5.6 kJ mol⁻¹. The value of standard enthalpy of formation was derived for crystalline state: Δ_f H ⁰(AgPiv,cr)= −466.9±5.6 kJ mol⁻¹. Using the enthalpy of sublimation, measured earlier, the enthalpy of formation of gaseous dimer was obtained: Δ_f H ⁰(Ag₂Piv₂,g)= −787±14 kJ mol⁻¹. The enthalpy of reaction (CH₃)₃CC(O)OAg(cr)=Ag(cr)+(CH₃)₃CC(O)O^.(g) was estimated, Δ_r H ⁰=202 kJ mol⁻¹.     

Radical scavenging reactions of chlorogenic acid: A pulse radiolysis study

At near neutral pH (approx. 5.5), the OH-adduct of chlorogenic acid (CGA), formed on pulse radiolysis of N₂O-saturated aqueous CGA solutions (λ _max = 400 and 450 nm) with k = 9 × 10⁹ dm³ mol⁻¹ s⁻¹, rapidly eliminates water (k = 1 × 10³ s⁻¹) to give a resonance-stabilized phenoxyl type of radical. Oxygen rapidly adds to the OH-adduct of CGA (pH 5.5) to form a peroxyl type of radical (k = 6 × 10⁷ dm³ mol⁻¹ s⁻¹). At pH 10.5, where both the hydroxyl groups of CGA are deprotonated, the rate of reaction of · OH radicals with CGA was essentially the same as at pH 5.5, although there was a marked shift in the absorption maximum to approx. 500 nm. The CGA phenoxyl radical formed with more specific one-electron oxidants, viz., Br ₂ ^·− and N ₃ ^· radicals show an absorption maximum at 385 and 500 nm, k ranging from 1–5.5 × 10⁹ dm³ mol⁻¹ s⁻¹. Reactions of other one-electron oxidants, viz., NO ₂ ^· , NO^· and CCl₃OO^· radicals, are also discussed. Repair rates of thymidine, cytidine and guanosine radicals generated pulse radiolytically at pH 9.5 by CGA are in the range of (0.7–3) × 10⁹ dm³ mol⁻¹ s⁻¹.     

Redox chemistry of o- and m -hydroxycinnamic acids: A pulse radiolysis study

Radiation chemical reactions of^•OH, O^•−, N₃ ^•and e _aq ^t- witho- and m-hydroxycinnamic acids were studied. The second-orderrateconstantsforthereaction of^•OH with ortho and meta isomers in buffer solution at pH7 are 3.9±0.2 × 10⁹ and 4.4 ± 0.3 × 10⁹ dm³ mol^-1 s^-1 respectively. At pH 3 the rate with the ortho isomer was halved (1.6 ± 0.4 × 10⁹ dm³ mol^-1 s^-1) but it was unaffected in the case of meta isomer (k = 4.2±0.6 × 10⁹dm³mol^-1 s^-1). The rate constant in the reaction of N₃ ^• with the ortho isomer is lower by an order of magnitude (k = 4.9 ± 0.4 × 10⁸ dm³ mol^-1s^-1). The rates of the reaction of e _aq ^t- with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in the^•OH witho-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue-shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of O^bb−with theo-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of the absorption peaks were centred at 370–380 nm in both the isomers. The underlying reaction mechanism is discussed.