Kinetics of deuterium isotope exchange between t-butylthiol and diphenylphosphine

The rate constant k (dm³ mol^–3 s^–1)=(3.7±0.4)×10²exp[–38(kJ mol^–1)/RT] for the bimolecular reaction of H-iosotope exchange between t–C₄H₉SD and (C₆H₅)₂PH in the temperature region 283–343 K was established by means of¹HNMR. The possible mechanisms of exchange are briefly discussed.

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–C₄H₉SD (C₆H₅)₂PH 283–343 ¹H (³·^–1·^–1)=(3,7±0,4)·10² exp [–38(·^–1)/RT]. .

     

Kinetics and isotope fractionation in deuterium exchange between N1-methyl-N2-phenylthiourea and S-deuterated t-butylthiol in aprotic solvents

The kinetics and isotope fractionation of deuterium in the exchange reaction between N¹-methyl-N²-phenylthiourea and S-deuterated t-butylthiol in selected aprotic solvents: methylene chloride, tetrahydrofuran, acetone, dimethyl sulphoxide and acetonitrile have been determined. The rate of the deuterium exchange in N¹H site is higher than that in N²H site and both depend visibly on the solvent. The higher the polarity of the solvent the lower the rate constant of the exchange reaction. The equilibrium isotope effect is relatively high and depends on the solvent, however, no distinct correlation could be found between the fractionation factor and electron donor or acceptor properties of the solvent.     

Homogeneous isotope exchange reaction between hydrogen and deuterium

The isotope exchange reaction between hydrogen and deuterium was studied in incident and reflected shock waves over the temperature range 1200–1800 K by vacuum ultraviolet absorption spectroscopy. The observed exchange reaction ensued after long induction periods at rates that proved to account for the amounts of exchange previously seen in single-pulse shock-tube and reflected-wave mass-spectrometric investigations. From the absence of detectable reaction during the induction period, lower bounds of 70 and 45 kcal were placed on the barriers to molecular exchange in four- and six-center transition structures, respectively.     

Mechanism of isotope exchange reaction between methane and deuterium

The rate of CH₄ disappearance in a shock-heated CH₄:D₂:Ar = 9.70:9.86:80.44 mixture was monitored by coincidence absorption of the 2948 cm^?1 He-Ne laser line over the shockfront temperature range of 1900–2300K. Comparison with CH₄ pyrolysis results by means of computer simulations suggested that atom and free radical chains are responsible for the homogeneous D/H exchange reaction on CH₄. Additional simulations for the experimental conditions of previous single-pulse shock tube experiments led to the recognition of a high sensitivity of the exchange rate to trace amounts of hydrocarbon impurity and to the dissociation rate of CH₄.     

Kinetics of tritium isotope exchange between liquid pyrrole and gaseous hydrogen

The kinetics of tritium isotope exchange between liquid pyrrole and gaseous hydrogen has been studied over the temperature range of 290–303 K. The reaction was carried out in the presence of platinum black but in spite of that, it appeared to be relatively slow. The kinetics of the exchange reaction studied could be described by the simple McKay equation. The results obtained suggest that diffusion is the rate-determining step. A mechanism of exchange is proposed.     

Kinetics of deuterium exchange of furan and methylfurans in dimethylsulfoxide

Rates of exchange between deuterium in 2D-, 3D-, 2D-3CH₃-, and 2D-5CH₃ -furan and a solution of potassium tert-butoxide in dimethylsulfoxide are measured. Protophilic deuterium exchange at position 2 in the furan molecule is 600 times faster than that at position 3, while a methyl group at position 3 or 5 retards it 10-fold. Catalytic activities of potassium and lithium butoxides are compared.We thank A. E. Berzin for supplying 3-chloromercurifuran, and P. P. Alikhanov and L. Ya. Mashkova for checking the purities of the compounds by GLC.     

High temperature solid state catalytic isotope exchange with deuterium and tritium

A method for synthesizing tritium- or deuterium-labeled amino acids, peptides and biogenic amines through high temperature solid state catalytic isotope exchange (HSCIE) is proposed. The dependence of the degree of isotope exchange in HSCIE on the structure of the compound, the reactivity of hydrogen at different carbon atoms and the conditions of the process has been examined. If HSCIE is performed in the temperature range of 373 to 413K, the selectivity of isotopic label incorporation comes to 70% or higher. When the tritium label is introduced into peptides, they retain the configuration of asymmetric atoms, even upon the substitution of tritium for hydrogen at the -carbon atoms of the amino acid residues. HSCIE at 453–513K leads to an even distribution of the isotopic label over the organic compound molecule. The results of³H NMR spectroscopy highlighting the distribution of the tritium label in the organic compound molecules are presented. The configuration of asymmetric atoms in amino acids is preserved to a high extent upon 80–90% substitution of isotopes for hydrogen atoms.Presented at the 12th Radiochemical Conference, Marianské Lazné May 7–11, 1990.     

Kinetics of chlorine isotope exchange reaction between sodium chloride-36 and triphenyltin chloride in mixed solvents

Fluorine is an important trace element for life and human well-being. Food, in general, provides about 40 percent of the fluorine intake in the human body. In order to measure fluorine levels in human diet samples, Instrumental Neutron Activation Analysis (INAA) and Proton Induced Gamma-ray Emission (PIGE) analysis were used. Reactions¹⁹F(n,)²⁰F and¹⁹F(n, p)¹⁹O were employed for determination of the fluorine concentration using a reactor neutron spectrum and epithermal neutrons. Corrections were made for the sodium matrix interference caused by the²³Na(n, )²⁰F threshold reaction in the case of reactor neutron cyclic activation analysis and for the oxygen interference via¹⁸O(n, )¹⁹O reaction when using the epithermal cyclic NAA method. The fluorine content of the diet samples was also determined by PIGE analysis making use of the resonance reaction¹⁹F(p, )¹⁶O at 872 keV. Cyclic Neutron Activation Analysis (CNAA) when combined with mass fractionation was found to be the most suitable for determination of low concentration of fluorine, through the¹⁹F(n, )²⁰F reaction with a detection limit of 2.2 ppm in Bowen's Kale elemental standard.     

Kinetics of chlorine isotope exchange reaction between sodium chloride-36 and triphenyltin chloride in mixed solvents

Isotope exchange reaction between NaCl-36 and triphenyltin chloride in dioxane-water (8020% w/w) and ethanol-water (9010% w/w) mixed solvents has been studied at 25, 35 and 50 °C. The exchange reaction was found to proceed via a bimolecular S_N2, limiting mechanism with reaction rates depending on the solvent used. Inhibition of the exchange in ethanol-water is probably due to solvation of chloride ion through hydrogen bond formation. The rate laws for the exchange reactions are: R_e=3.24×10⁹ e^–65550/RT [Rh₃SnCl] [NaCl] in dioxanewater and R_e=6.61×10⁸ e^–69600/RT [Ph₃SnCl] [NaCl] in ethanol-water, where is the degree of dissociation of NaCl and R_e is the rate of exchange in mol l^–1 s^–1. The activation parameters H^*, S^* and G^* are reported.     

Laser isotope separation of deuterium

Highly selective isotope separation of deuterium has been demonstrated in laser-irradiated formaldehyde mixtures of H₂CO and HDCO. Single step deuterium enrichment factors of 14 were achieved by photodissociation into HD and CO, using HeCd laser light at 325.03 nm. Initial deuterium concentration ranged from 0.015 mole percent (natural abundance) to 5 mole percent.     

Kinetic isotope effect in hydrogen isotope exchange between water and phosphines

The kinetic parameters of the tritium exchange between^*H₂O(^*D₂O) and (CH₃)₂PH in the gas phase and between^*H₂O(^*D₂O) and (C₆H₅)₂PH in liquid were measured. Both reactions appeared to be heterogeneous ones. The^*HH/^*DD kinetic isotope effects were estimated and compared with the results of exchange between methanol and phosphines. The differences in isotope effects are explained by the change of the symmetry of the four center cyclic transition complex resulting from the various type of solvation of this complex.     

Kinetics of heterogeneous isotope exchange in the systems containing porous particles

The porous particle is represented as a system of slabs of different thickness in order to evaluate the kinetic curve of isotope exchange. The physical reasons of such a representation are discussed. The model curves are compared for different distributions of slab thickness.     

Kinetics of hydrogen isotope exchange in molecules with strong intramolecular hydrogen bonds

Rate constants and activation energies of hydrogen exchange in solution between methanol and molecules with intramolecular H-bonds have been measured. It has been established that the rate-determining step is the dissociation of this bond.

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H-. , .

     

An NMR relaxation study of hydrogen exchange and its deuterium isotope effects in aqueous carboxylic acid solutions

Exchange-rate measurements of water protons and deuterons in aqueous solutions of acetic, malonic and glutaric acid by the NMR T₂ Carr-Purcell method, employing oxygen-17-enriched water, are reported. In contrast to previous results on acetic-acid solutions, intermolecular proton exchange via solvent between COOH and COO^? groups is found to contribute considerably to the exchange rate for all three acids investigated. The rate constants necessary for the calculation of the proton residence time in the COOH groups have been determined. Observed deuterium isotope effects on the rate constants for hydrogen-ion and acetic-acid-catalyzed exchange are discussed in terms of fractionation-factor theory.     

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.