Photooxidation of water by molecular oxygen: isotope exchange

The phototransfer of isotopic nuclei from water molecules to oxygen as evidence for the direct photooxidation of water by molecular oxygen was detected experimentally. The mechanism and its ecochemical consequences and significance are presented.     

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.     

Hydrogen isotope exchange between pentafluoroethane and water: kinetics and equilibrium

It is shown experimentally that pentafluoroethane undergoes rapid protium-deuterium exchange with water in the presence of hydroxide ion. Addition of dimethylsulfoxide enhances the rate at least by a factor of 100. The first measured fractionation factor data are presented for the temperature range of 50–120°C. These values are compared with the theoretical estimations calculated by using isotopic reduced partition function ratios based on molecular vibrational frequencies. Although catalytic exchange is slow at ambient temperature, the reaction rate becomes measureable above around 60°C because of large activation energy (92 kJ/mole). Comparisons are made with similar data available for various halomethane and haloethane systems. © 1997 John Wiley & Sons, Inc.     

Photo-oxidation of aldehydes with molecular oxygen in the presence of catalytic bromine or hydrobromic Acid

Aldehydes were found to be oxidized with molecular oxygen to the corresponding carboxylic acid in the presence of catalytic hydrobromic acid or bromine under photo-irradiation.     

Speciative determination of phosphorus in environmental water by the isotope exchange method

An isotope exchange method for the speciative determination of phosphorus (PO ₄ ^3– , PO ₃ ^3– and total P) in natural water samples is proposed by using the exchange system of moly bdophosphate in the aqueous phase and tetraphenylarsonium molybdophosophate in the organic phase. In this exchange system, only PO ₄ ^3– exchanges and is determined. When the sample water is treated with Br₂ water in advance, the amount of PO ₄ ^3– +PO ₃ ^3– can be obtained. When the sample is treated with H₂SO₄ and HNO₃, then the amount of total P can be determined.     

Mechanism of isotope exchange of dihydrogen with liquid water catalyzed by hydroxide ions

In the analysis of the reaction between dissolving hydrogen and aqueous alkali hydroxide solutions the interaction of hydrogen with the molecular ion (H₃O₂)^– should be studied. This interaction is analyzed by the interacting bonds method. The activation energy (E_a) is estimated.

---

, (H₃O₂)^–. . (E_a).

     

Nonadiabatic relaxation dynamics of water anion cluster and its isotope effects by ring‐polymer molecular dynamics simulation

We have applied a recently developed hybrid quantum ring‐polymer molecular dynamics method to the nonadiabatic p → s relaxation dynamics in water anion clusters to understand the isotope effects observed in previous experiments. The average relaxation times for (H₂O)₅₀^? and (D₂O)₅₀^? were calculated at 120 and 207 fs, respectively, and are comparable to the experimental results. Therefore, we conclude that nuclear quantum effects play an essential role in understanding the observed isotope effects for water anion cluster nonadiabatic dynamics. The nonadiabatic relaxation mechanisms are also discussed in detail. © 2014 Wiley Periodicals, Inc.     

Radioiodination of amitriptyline by isotope exchange

The catalysed isotope exchange method was studied with the aim to elaborate the optimal procedure for labeling of amitriptyline with iodine¹²⁵I. Besides the catalysts published, cuprous salts and ammonium sulphate, a series of transition metal complexes as catalysts were tested at different reaction conditions. The best result (91% radiochemical yield) was achieved with trans-chlorocarbonylbis-(triphenylphosphine) iridium complex (Vaska catalyst) and sonification.     

Reliable determination of oxygen and hydrogen isotope ratios in atmospheric water vapour adsorbed on 3A molecular sieve

The isotope ratio of atmospheric water vapour is determined by wide-ranging feedback effects from the isotope ratio of water in biological water pools, soil surface horizons, open water bodies and precipitation. Accurate determination of atmospheric water vapour isotope ratios is important for a broad range of research areas from leaf-scale to global-scale isotope studies. In spite of the importance of stable isotopic measurements of atmospheric water vapour, there is a paucity of published data available, largely because of the requirement for liquid nitrogen or dry ice for quantitative trapping of water vapour. We report results from a non-cryogenic method for quantitatively trapping atmospheric water vapour using 3A molecular sieve, although water is removed from the column using standard cryogenic methods. The molecular sieve column was conditioned with water of a known isotope ratio to 'set' the background signature of the molecular sieve. Two separate prototypes were developed, one for large collection volumes (3 mL) and one for small collection volumes (90 microL). Atmospheric water vapour was adsorbed to the column by pulling air through the column for several days to reach the desired final volume. Water was recovered from the column by baking at 250 degrees C in a dry helium or nitrogen air stream and cryogenically trapped. For the large-volume apparatus, the recovered water differed from water that was simultaneously trapped by liquid nitrogen (the experimental control) by 2.6 per thousand with a standard deviation (SD) of 1.5 per thousand for delta(2)H and by 0.3 per thousand with a SD of 0.2 per thousand for delta(18)O. Water-vapour recovery was not satisfactory for the small volume apparatus.     

Isotopic exchange between molecular oxygen and MO·SiO2

Russian Chemical Bulletin -     

Deuterium labeling of glyprolines by isotope exchange

Deuterium-labeled 5-oxo-Pro-Arg-Pro, 5-oxo-Pro-His-Pro-Gly-Pro-NH₂, and Phe-Pro-LeuPro-Ala were synthesized. The effect of the method of peptide deposition on the catalyst, the catalyst nature, temperature, and structure of peptides on the inclusion of deuterium with the formation of the appropriate number of isotopomers was demonstrated. The 5-oxo-Pro-Arg-Pro, 5-oxo-Pro-His-Pro-Gly-Pro-NH₂, and Phe-Pro-Leu-Pro-Ala peptides containing, on average, 4.6, 7.7, and 5.4 deuterium atoms per peptide molecule, respectively, were obtained.     

The determination of bromine and iodine in environmental water samples by thermal neutron activation and isotope exchange

A routine-method for the determination of bromine and iodine in environmental water by neutron activation is presented. The elements are isolated by isotope exchange between the irradiated sample and a solution of Br₂ or I₂ CCl₄. The method is not sensitive to the chemical species in which the halogen is present. The lower limit of the determination is 1.0 μg Br·1⁻¹ and 0.1 μgI·1⁻¹.     

The kinetics of isotope exchange reactions: Use of initial rates to measure isotope effects on carbon acid ionization

Multistep hydrogen isotope exchange reactions, such as the íonization of a carbon acid via a carbanion intermediate in a protic solvent, when conducted using an isotopic tracer to monitor the exchange, have the unusual feature that their rate-determining steps always refer to the transfer of the tracer isotope and never to the isotope present in macroscopic amounts. This property of these reactions is discussed and rationalized using a free energy versus reaction coordinate diagram. It is further shown that this property does not invalidate a commonly used method of measuring kinetic isotope effects on carbon acid ionization in which rates of incorporation of tritium tracers into RH and RD substrates are compared, despite the fact that tritium transfer is rate determining in both exchanges, but it is valid only if initial rate measurements are used. When the comparison is made in a protio solvent, e.g., H₂O, the portion of the initial reaction which may be used depends strongly on the magnitude of the isotope effect. It ranges from less than 1% tritium incorporation for large isotope effects to 10% or more for isotope effects near unity. On the other hand, when a deuterated solvent, e.g., D₂O, is used, the range of validity of the method for large isotope effects is extended dramatically.     

Radiochemical separation of thorium by isotope ion exchange

The aim of the present work is to obtain the separation of²³³Th from the radioisotopes formed in the irradiation of Mn, U, Ba, Cs, Co and the lanthanide elements with thermal neutrons, because they may interfere in the neutron activation analysis of Th, when the activity of²³³Th is used. The experiments were performed with the resin Bio-Rad AG 50W X-4 and X-8 (100–200 mesh) in the thorium form. The separation of²³³Th from the interfering radioisotopes is based on the retention of²³³Th by the resin (isotope exchange) and the elution of the interfering radioisotopes with a dilute solution of Th in 0.5M HCl. Batch experiments were made in order to determine the equilibrium time for the isotopic ion exchange of thorium and also the distribution coefficients of the interfering elements between the solution and the resin. Column experiments were carried out with the purpose of establishing the conditions that allow the maximum isotope exchange of²³³Th and the minimum retention of the interfering radioisotopes in the resin. With this purpose, a statistical interpretation of a four variable experimental design is presented.From a thesis submitted by C. Sepúlveda Munita to the Instituto de Pesquisas Energéticas e Nucleares (CNEN/SP) University of São Paulo, in partial fulfillment of a Doctor of Science's Degree. Work supported by the Brazilian Atomic Energy Commission.