Reactions of proton transfer and exchange with the participation of OH,SH, and CH acids and amines

A comparison of proton exchange reactions between OH, SH, and CH acids and the NH groups of trialkylammonium ions showed that regardless of the nature of the acid XH, the mechanism of exchange includes transfer of a proton in the ion pair N-H⁺ ... X^– as the slow step. At the fast steps of proton exchange XH- N⁺H, i.e., molecular exchange with breaking of a hydrogen bond X-H ... N and transfer of a proton along these bonds, differences appear in the properties of XH acids. In the sequence from OH to SH and CH acids, the hydrogen bonds X-H ... N are weakened. As a result of this, in the same sequence the kinetic acidity (k₂) decreases but the rate of molecular exchange (k_H) increases. The ratio between the values of k₂ and k_H is inverted when the strong bonds O-H ... N (k₂/k_H 1) are replaced by weak bonds C-H ... N (k₂/k_H 1). It was also established that the kinetic stability of the anions increases as the oxygen atoms are replaced by sulfur in the series RCOO^– < RCOS^– < R₂PSS^– as a result of the more effective delocalization of the negative charge on the diffuse orbitals of sulfur.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 471–475, July–August 1987.     

An NMR study of hydrogen exchange in p-hydroxybenzoic acid

The compound has been used over a temperature range in dimethylsulfoxide, with trichloroacetic acid and triethylamine as catalysts. The rate constant for uncatalyzed bimolecular exchange at 25 C is k_N=131 M^–1 sec^–1; the constant for acid-catalyzed exchange is k_A=0.9 · 10⁴ M^–1 sec^–1; and that for base-catalyzed exchange is k_B=0.5 · 10⁴ M^–1 sec^–1. The activation energy for uncatalyzed exchange is 5.75 kcal/mole. The exchange rates in dimethylformamide and acetone are higher, on account of differences in the hydrogen bond. The exchange rates of the isomers fall in the sequence ortho > para > meta, which is due to conjugation and intramolecular hydrogen bonding. The exchange mechanism is discussed. Simple relationships in dimensionless parameters are given for calculating the exchange rate from the shape of the NMR signal.     

An NMR study of proton exchange in the system p-nitrosophenol—p-quinone monooxime

The solvent used was dimethylformamide at neutral and alkaline pH. The equilibrium constants are determined by spectrophotometry. The rate of proton exchange has been measured as a function of temperature and concentration. The rate constants and activation energies have been measured; for uncatalyzed exchange k_n=(1.5±0.5) ·· 10³ M^–1 sec^–1, E=8±1 kcal/mole, while base-catalyzed exchange has k=(0.3±0.1) · 10⁶ M^–1 sec^–1 and E=6±1 kcal/mole.We are indebted to A. I. Brodskii for assistance in this work, and to V. I. Oshkaderov and L. A. Kichakova for recording the NMR spectra.     

Hydrogen isotope exchange reaction rate in tritium and methane mixed gas

Hydrogen isotope exchange reaction rate in tritium and methane mixed gas, as induced by tritium decay and beta radiation, has been experimentally measured. Initially T₂ gas was filled to 40 kPa and 20 kPa of CH₄ gas was added. The mixed gas spectrum was analyzed periodically by laser Raman spectrometry. The first order HT and H₂ formation rates and T₂ and CH₄ decay rates by hydrogen isotope exchange reaction were observed between 2.9·10^–3 h^–1 and 4.8·10^–3 h^–1. Although the estimated hydrogen isotope exchange reaction rate was 1/20–1/10 slower than the rate of H₂+T₂ mixed gases, it was nearly equivalent to the ion formation rate by tritium beta radiation. This suggested that isotopic hydrogen radicals formed via ionization would disappear in the presence of methane.     

Studies on ion exchange equilibria and kinetics VI. Prediction of ion exchange equilibria of UO 2 2+ −Na+−H+ ternary system

Experimental data are reported for the ion exchange equilibria of the binary systems UO ₂ ²⁺ –H⁺, UO ₂ ²⁺ –Na⁺ and Na⁺–H⁺, and of the ternary system UO ₂ ²⁺ –Na⁺–H⁺ on a strong acid cation exchange resin 001X7 at 25 °C. It is found that the equilibria for any pairs of ions are essentially the same in binary and ternary mixtures and that the prediction method proposed by our laboratory for SO ₂ ^2– –Cl^––NO ₃ ^– -201X7 strong base anion exchange resin system is also applicable to the ternary system studied in this paper. The predictions of the ternary system UO ₂ ²⁺ –Na⁺–H⁺ based solely on the binary data without using resin phase activity coefficients are consistent with the experimental data.     

Kinetic exchange studies of metal ion substitution in EDTA chelates Fe(III)- UO2 EDTA exchange

A kinetic study of the exchange reaction between UO₂EDTA complex and Fe(III), at a constant ionic strength of 0.1, over the concentration range of 5×10^–3–1×10^–2 M of each reactant and pH 4.5–5.5 has been carried out radiometrically. The rate of the exchange process can be expressed by the equation: R=k₁[UO₂EDTA][Fe]+k₂[EDTA][H⁺]^–1. The activation parameters calculated were H^*=25.95 kJ mol^–1 and S^*=0.67 kJ mol^–1 K^–1.     

Studies on ion exchange equilibria and kinetics: V. UO 2 2+ −Na+−H+ ternary cation exchange kinetics

The kinetics of particle-diffusion controlled ion exchange in the ternary system of cations UO ₂ ²⁺ –Na⁺–H⁺–001×7 strong acidic resin has been studied. In the [R–H⁺]/(Na⁺+UO ₂ ²⁺ ) system, the change of the amount of Na⁺ in the resin phase with time showed a high peak. In the [R–Na⁺]/(H⁺+UO ₂ ²⁺ ) system, the change of the amount of H⁺ in the resin phase with time also showed a high peak. In the [R₂–UO ₂ ²⁺ ]/(H⁺+Na⁺) system, the change of amount of H⁺ in the resin phase with time showed merely a small peak. This kinetic character of the ternary ion exchange system in the finite solution volume has been analyzed according to the Nerst-Planck equation, and on the whole, the trend of the experimental results is consistent with the resulting numerical solution of the set of Nerst-Planck equations.     

Heterogeneous isotope exchange reaction between SnCl2 and SnO2·xH2O in HCl medium

Heterogeneous isotope exchange between hydrated stannic oxide and stannous chloride in 0.1M HCl solution has been studied as a function of the stannic oxide, stannous chloride, and chloride ion concentrations, and temperature. The exchange process is a second-order reaction, which is independent of the chloride ion concentration, with a mean rate constant, k, of 2.31 dm³ mol^–1·min^–1. The activation energy of the isotope exchange process was found to equal 3.62 kcal·mol^–1. Possible use of this system, SnCl₂/SnO₂·xH₂O, as basis for a¹¹³Sn-^113mIn generator is suggested.     

Nitrogen isotope exchange between nitric oxide and nitric acid

The rate of nitrogen isotope exchange between NO and HNO₃ has been measured as a function of nitric acid concentration of 1.5–4M·1^–1. The exchange rate law is shown to beR=k[HNO₃]²[N₂O₃] and the measured activation energy isE=67.78kJ ·M^–1 (16.2 kcal·M^–1). It is concluded that N₂O₃ participates in¹⁵N/¹⁴N exchange between NO and HNO₃ at nitric acid concentrations higher than 1.5M·1^–1.     

Kinetics of proton exchange between semiquinone radicals and hydrochloric acid

The mechanism of proton exchange between semiquinone neutral radicals 3,6-di-tert-butyl-2-hydroxyphenoxyl (1), 6-tert-butyl-3-chloro-2-hydroxy-4-triphenylmethylphenoxyl, and hydrochloric acid in toluene solutions has been studied. The rate of proton exchange with hydrochloric acid is less than that with acetic acid owing to the higher thermodynamic stability of the radical cation formed upon semiquinone radical protonation by hydrochloric acid. The formation of radical cations and their dimers has been proven by spectroscopy.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No 1, pp. 84–87, January, 1993.     

Protonation of 18-crown-6 by dichloropicric acid in anhydrous and in water saturated 1,2-dichloroethane. Homoconjugation of dichloropicric acid

From conductometric and UV-VIS spectrophotometric studies of the reaction between 18-crown-6 (L) and dichloropicric acid (HA) in dry and water saturated 1,2-dichloroethane, it has been concluded that formation of a 1:1 homoconjugate HA ₂ ^– accompanies the simple protonation of L, viz, L+HALH⁺A^– and L+2HALH⁺HA ₂ ^– . The electrolytes LH⁺A^– and LH⁺HA ₂ ^– are extensively, or practically completely dissociated in both solvents under the experimental conditions. The specie LH⁺A^– appears to be a contact ion pair in DCE. The stability constant of HA ₂ ^– in the dry solvent, 5.7×10³ mol^–1-cm³, is some 10^2.4 times that in propylene carbonate reflecting the difference in H-bond accepting capacity of the two solvents. Hydration of HA, A^– and HA ₂ ^– in wet dichloroethane is negligible or slight. As expected, LH⁺ is rather strongly hydrated, the ratio of the hydration constants of LH⁺ and L being about 1×10¹.     

Inter- and intra-annular proton exchange in gaseous benzylbenzenium ions (protonated diphenylmethane)

Two distinct proton exchange reactions occur in metastable gaseous benzylbenzenium ions, generated by isobutane chemical ionization of diphenylmethane and four deuterium-labelled analogues. Whereas the proton ring-walk at the benzenium moiety is fast giving rise to a completely random intraannular proton exchange, the interannular proton exchange is surprisingly slow and competes with the elimination of benzene. A kinetic isotope effect of k_H/k_D= 5 has been determined for the interannular proton transfer, and a particularly high energy barrier of 50–75 kJ mol^?1 has been estimated. These observations are attributed to steric restrictions of the ring-to-ring proton transfer in benzylbenzenium ions and contrasted to the fast interannular proton exchange in the higher homologues.     

Ligand exchange reaction of Zn(II)-acetylacetonate complex with 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid

Ligand exchange reaction of Zn(II)-acetylacetonate complex (Zn-acac₂) with 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H₂TPPS) has been investigated spectrophotometrically and radiometrically. The exchange reaction was observed by spectral change from H₂TPPS to Zn-TPPS or activity of⁶⁵Zn(acac)₂ extracted into the chloroform phase. The 2nd order rate constants (k ₂) for the exchange reaction at 70 °C and at pH 7.8 were found to be 32.8±2.3 and 31.2±3.2 M^–1·s^–1 from the spectrometric and radiotracer experiments, respectively. For the direct complexation of Zn(II) with H₂TPPS, a similar 2nd order rate constant (k=32.4±4.7 M^–1·s^–1) was obtained as that in the ligand exchange reaction. The activation energies (E) for the exchange and the formation of Zn-TPPS were found to be 69.3±0.2 and 69.4±0.2 kJ·mol^–1, respectively, in the temperature range from 40 to 70 °C.     

Equilibrium and kinetics of proton transfer from thiocarboxylic acids to γ-collidine

NMR and IR spectroscopy have been used in studying the equilibrium in the reaction of proton transfer from thiocarboxylic acids RCOSH [R=CH₃ (a), C₆H₅ (b) or CH₂Cl (c)] to -collidine (d), and also the kinetics of CH/NH proton exchange between protonated -collidine and excess RCOSH. For compoundsa and b, partial protonation of the -collidine is observed; and for compound c, complete protonation. The heat of reaction of proton transfer with the participation of binary acidamine associates is 30 (a) and 45 (b) kJ/mole. The rate of proton exchange decreases and the activation energy E increases with increasing acidity of the RCOSH [E=44 (b) and 88 (c) kJ/mole] and with increasing basicity of the amine (E_d < E_TEA), which, in accordance with the orders of reaction that were found for the exchanged components, is due to a mechanism in which the slow stage is proton transfer in the ion pair NH⁺...^–SOCR. The thiocarboxylate ion of c is unstable; and after splitting out Cl^–, it forms the compounds Cl(CH₂COS)₂ ^– and (CH₂COS)₂.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 187–194, March–April, 1985.     

Modeling water exchange on monomeric and dimeric Mn centers

Water exchange on Mn centers in proteins has been modeled with density functional theory using the B3LYP functional. The reaction barrier for dissociative water exchange on [Mn^IV(H₂O)₂(OH)₄] is only 9.6 kcal mol^–1, corresponding to a rate of 6×10⁵ s^–1. It has also been investigated how modifications of the model complex change the exchange rate. Three cases of water exchange on Mn dimers have been modeled. The reaction barrier for dissociative exchange of a terminal water ligand on [(H₂O)₂(OH)₂Mn^IV(-O)₂Mn^IV(H₂O)₂(OH)₂] is 8.6 kcal mol^–1, while the bridging oxo group exchange with a ring-opening mechanism has a barrier of 19.2 kcal mol^–1. These results are intended for interpretations of measurements of water exchange for the oxygen evolving complex of photosystem II. Finally, a tautomerization mechanism for exchange of a terminal oxyl radical has been modeled for the synthetic O₂ catalyst [(terpy)(H₂O)Mn^IV(-O)₂Mn^IV(O)(terpy)]³⁺ (terpy=2,2:6,2-terpyridine). The calculated reaction barrier is 14.7 kcal mol^–1.Contribution to the Björn Roos Honorary Issue     

Isotope exchange between U/III/ and U/IV/ in the HCl-TBP extraction system

The measurement of the isotope-exchange reaction between U/IV/ in the organic phase and U/III/ in the aqueous phase in the extraction systems: 7-8M HCl—5–40% TBP /aromatic diluent or CCL₄/ were made. The high rate of exchange with the rate constant >10²M^–1min^–1 was observed.     

1H N.m.r. Chemical shifts and hydrogen exchange rates oftrans-[Co(en)2XY]n+ complexes in liquid ammonia

Summary ¹H n.m.r. spectra of coordinated ethylenediamine HN< hydrogens for 23trans-[Co(en)₂XY]ⁿ⁺ complexes and¹H-²H exchange rates for some selected compounds are reported in the solvent liquid ammonia. Assignments are presented for asymmetric complexes. These are based on the chemical shift order of the symmetric complexes. The assignments are confirmed by independent experimental findings including the effect of specific ion-association and the observed exchange rates. The exchange rates follow the relation k_obsd=k⁰+k¹ [N²H₄ClO₄]^–1, illustrative of general base-catalysis; in this case rate-determining proton abstraction by N²H₃ and N²H ₂ ^– , respectively. Chemical shifts and exchange rates in liquid ammonia follow the published order of the aqueous parameters.     

Characteristics of oxygen exchange in phosphoric acid

The oxygen exchange between phosphoric acid and water at various temperatures and concentrations has been investigated in the presence and absence of K₂HPO₄. The minimum rate of exchange is observed for a solution concentration of 2–2.5 mole/l. It has been shown that in concentrated solutions of phosphoric acid the formation of H₄PO ₄ ⁺ is possible, and the true rate constant of the acid-catalytic conversion and the basicity constant of H₃PO₄ has been calculated. It has been found that in a solution where the mobility of the oxygen atom towards the anion increases, the value of the limiting current on the anodic dissolution of copper diminishes.     

Base catalyzed enolization and hydrogen exchange of trifluoroacetone. A comparison to acetone

The kinetics of the pyridine catalyzed hydrogen exchange of 1,1,1-trifluoroacetone in 50% D₂O-dioxane have been measured using ¹H-NMR. Rates of hydrogen exchange of acetone were also measured under comparable conditions and the rate of deuterium uptake by trifluoroacetone was found to exceed that of acetone by a factor of 1700 at 25°C. However trifluoroacetone is known to be extensively hydrated under these conditions. The hydrogen exchange of trifluoroacetone is interpreted as most probably proceeding through proton abstraction by pyridine from the free ketone to form the enolate followed by deuteration on carbon, with the rate of proton abstraction from trifluoroacetone exceeding that of acetone by a factor of 10⁵ to 10⁶. Other possibilities are also considered.     

MnO2/SiO2–SO3H nanocomposite as hydrogen peroxide scavenger for durability improvement in proton exchange membranes

Membrane durability was a key problem to the development of proton exchange membrane fuel cells (PEMFCs). A novel nanocomposite MnO₂/SiO₂–SO₃H was prepared to mitigate the hydrogen peroxide attack to the membranes at fuel cell condition. The nanocomposites were synthesized by the wet chemical method and three-step functionalization. The crystal structure was characterized by X-ray powder diffraction (XRD), the crystallite size and the distribution of the nanocomposites were investigated by TEM. SEM-EDX was used to analyze the elemental distribution on the surface of the nanocomposite. And the surface functional groups (–SO₃H) were evaluated by FT-IR. The amount of sulfonic acid groups introduced onto the silica surface was determined by titration method. The radical scavenging ability was estimated by UV–VIS spectroscopy using dimethyl sulfoxide (DMSO) as the trapping agent. The membrane durability was investigated via ex situ Fenton test and in situ open circuit voltage (OCV) accelerated test. In these tests, the fluoride emission rate (FER) reduced by nearly one order of magnitude with the dispersion of MnO₂/SiO₂–SO₃H nanocomposites into Nafion membrane, suggesting that MnO₂/SiO₂–SO₃H nanocomposites had a promising application to mitigate the degradation of the proton exchange membrane.