Clear Isotope Effect Research Articles

Anomalous role of carbon and nitrogen isotopes in (DMe-DCNQI) 2Cu system

We studied 13C, 13N isotope effect in electronic state of the (DMe-DCNQI) 2Cu system. Isotope labeling on a terminal cyanoimino moiety exhibited clear isotope effect. The 15N effect was weaker than the 13C effect.

Isotope effects of the mechanical relaxation of H and D in dilute PdY and PdGd alloys

The internal friction spectra of dilute PdY and PdGd alloys charged with H or D show three relaxation maxima. The maxima attributed to the stress induced ordering of H or D pairs (Zener maximum) and to the reorientation of H–Y or H–Gd pairs (D–Y or D–Gd pairs, respectively) (reorientation maximum) show clear isotope effects concerning their temperature position as well as their activation parameters. In case of the reorientation maximum (activation enthalpy Ha,HM2=0.18–0.26 eV, Ha,DM2=0.20–0.26 eV, depending on H or D as well as Y or Gd concentration) which occurs at temperatures slightly above 100 K the pre-exponential factor of the jump frequency is of the same order of magnitude as the Debye frequency of the host lattice (νDeb=5.7·1012 s−1) for D, whereas it is one order of magnitude smaller for H. In case of the Zener maximum (Ha,HM1=0.11 eV, Ha,DM1=0.15 eV) which occurs at temperatures of about 80 K the pre-exponential factors for D and H are lower by two and four orders of magnitude, respectively, as compared to the Debye frequency. These results are interpreted in terms of thermally activated tunnelling jump processes which occur either adiabatically for higher temperatures or non-adiabatically for lower temperatures.

Two components of boundary potentials at the lipid membrane surface: electrokinetic and complementary methods studies

Bilayer lipid membranes (BLM) are commonly used as models for cell membranes to study their interactions with inorganic ions and molecules of biological importance. In this work the principal electrostatic effects at the BLM surface are demonstrated by two methods: by the inner membrane field compensation (IFC) which is applied to planar BLM and sensitive to changes in the total boundary potential; and by electrokinetic measurements in liposome suspensions, sensitive to diffuse (surface) component of this potential. The difference in these two potentials allows us to conclude on changes in the dipole component of the boundary potential caused by structural changes at the membrane–water interface. No difference in the experimental data of both methods was observed for Be 2+ and other divalent cation adsorption to unchanged phosphatidyl choline (PC) membranes. These data are in a good agreement with the Gouy–Champan–Stern (GCS) theory of diffuse double layer. This theory gives the value of binding constants for Be 2+ about 400 M −1 and 10 4 M −1 for DPPC liposomes in the liquid and solid states of the lipids, respectively. Clear isotope effects for normal and heavy water solutions of Be 2+ were observed both by the electrostatic measurements and by differential scanning calorimetry. In contrast to PC, the electrostatic potentials induced by Be 2+ and Gd 3+ adsorption to membranes from phosphatidyiserine (PS) show the difference between the data of mentioned methods — total boundary potential changes are much higher in comparison to the surface potential. Dipole potential changes (about 150 mV) caused by changes in PS head group orientation may be more important in this specific case.

Strong rotational effects in the adsorption dynamics of H 2/Pd(111): evidence for dynamical steering

We have used seeded beam techniques to determine the influence of the rotational energy of hydrogen and deuterium molecules on the adsorption dynamics. A strong decrease of sticking with increasing rotational energy has been observed. In the low translational energy regime where the sticking coefficient decreases with beam energy the strongest effects are encountered. This result points to the importance of dynamical steering rather than to adsorption via a classical molecular precursor. Because rotational energy is quantized, clear isotope effects between hydrogen and deuterium can be observed.

パラジウム触媒によるＣ‐Ｈ結合活性化を経るアルカンのカルボキシル化反応

Palladium (II) -catalyzed carboxylation of alkanes including propane, ethane, and methane with carbon monoxide has been achieved in the presence of potassium peroxodisulfate and trifluoroacetic acid. Regioselectivity by this method is secondary>primary>tertiary due to steric and electronic factors. Stereochemistry is determined thermodynamically. An equimolar amount of copper additive improves the catalyst activity dramatically and thus gives higher yield than Pd and Cu alone except for the reaction of methane. In contrast, acetic acid synthesis from methane is best promoted by the copper catalyst. The reaction with Pd and Pd-Cu catalysts has been proved to be electrophilic C-H activation, whereas radical process is at work in the carboxylation with Cu catalyst. In the former systems, clear isotope effect has been observed. Spectroscopic investigation of the Pd-Cu mixed catalyst suggests that copper forms a 1 : 1 complex with palladium and enhances the electrophilicity of Pd.

Palladium and Copper-Catalyzed Carboxylation of Alkanes with Carbon Monoxide. Remarkable Effect of the Mixed Metal Salts

Abstract The mixed catalyst Pd(OAc)2–Cu(OAc)2 promotes the carboxylation of cyclohexane and propane with CO in higher yield than either a Pd(II) or Cu(II) catalyst alone. The mixed catalyst has the highest activity when the ratio of Cu(II)/Pd(II) is more than unity. The carboxylations of p-xylene with Pd(II)–Cu(II) and Pd(II) afford 2,5-dimethylbenzoic acid, but no carboxylic acid is detected in the reaction with Cu(II), giving rise to p-xylene dimer as the major product instead. Clear isotope effect (3.0—3.2) is observed in the reactions of cyclohexane with Pd(II)–Cu(II) and Pd(II), unlike the reaction with Cu(II) (1.0).

Protonic conduction in calcium, strontium and barium zirconates

Electrical conduction in sintered oxides based on calcium, strontium or barium zirconate at high temperature was investigated electrochemically. When Zr in MZrO 3 (M=Ca, Sr or Ba) was partially replaced by trivalent element such as In, Sc, Y, etc., the resultant solid solutions exhibited protonic conduction under hydrogen-containing atmosphere at elevated temperatures. In general, the protonic conductivities of BaZrO 3- and SrZrO 3-based ceramics were higher than those of CaZrO 3-based one although the values were low compared to those of SrCeO 3 or BaCeO 3-based ceramics. Clear isotope effects in IR spectrum and conductivity were observed between protonated and deuteronated SrZr 0.95Y 0.05O 3−α.

Isotope effects on structural phase transitions in several sulfates

N and Li isotope effects on structural phase transitions in several sulfates, (${\mathrm{NH}}_{4}$${)}_{2}$${\mathrm{SO}}_{4}$, ${\mathrm{NH}}_{4}$${\mathrm{LiSO}}_{4}$, ${\mathrm{RbLiSO}}_{4}$, and ${\mathrm{CsLiSO}}_{4}$, are reported (underlined elements are those for which isotopes were substituted). No clear isotope effect has been observed in any of the sulfates studied. This result is similar to that obtained for ${\mathrm{NaNO}}_{2}$, but in contrast to those obtained in other materials such as ${\mathrm{BaTiO}}_{3}$.

Electron mobility in dense gases of deuterated methanes as functions of electric field strength, temperature, and density

Values of electron mobility μ were measured in gaseous deuterated methanes CHxD4−x(x=0–4) as a function of density normalized electric field strength at 0.2≤n/1026 molecule m−3≤61=nc (subscript c denotes critical fluid), and 98≤T/K≤433. The mobility at low E/n had a constant value μ0 but at fields above a threshold (E/n)th, μ increased. In the saturated vapor the ratio of the electron drift velocity at (E/n)th to the speed of sound in the vapor was determined mainly by T, while nμ0 was mainly determined by n; there was no clear isotope effect. The effect of density on nμ0 is discussed in terms of the deformation potential and dielectric screening models. The former fits better.

Elastic scattering of 30 MeV positive and negative pions from nickel isotopes.

Elastic cross sections for the scattering of positive and negative pions from /sup 58,60,64/Ni were measured at 30 MeV incident pion energy. A clear isotope effect was observed in the ..pi../sup -/ cross sections while no effect was observed in the ..pi../sup +/ cross sections. The cross sections were compared to predictions made by various optical model calculations. In general, both ..pi../sup +/ and ..pi../sup -/ cross sections were poorly described and the ..pi../sup -/ isotope effects were not well reproduced. Anomalous effects first observed in pionic atoms may also appear in the present data.

Dynamic impurities in ultrapure semiconductors

Theoretical and experimental results are presented which shed light on a series of unusual shallow level impurities in ultrapure germanium. These impurities, both donors and acceptors, (1) show hydrogenic spectra with signatures identical to all other shallow impurities; (2) exhibit unusual dependence in their spectra to applied uniaxial stresses; (3) display a clear isotope effect (hydrogen to deuterium) in the electronic spectrum when the crystal growing conditions are changed -- a clear violation of the Born-Oppenheimer approximation. They are dynamic impurities consisting of a two-atom center in a fully tetrahedral environment, in which coherent nuclear motion is coupled to the electronic degrees of freedom. Experimental data, together with group-theoretical analysis, permit a complete description of these systems.

Diffusion in liquid mixtures of isotopes

Molecular dynamic calculations of the velocity autocorrelation functions and self-diffusion coefficients in two-component liquid mixtures consisting of the argon isotope of mass 40 and, in order to obtain a clear isotope effect, fictitious argon isotopes of one of the masses 32, 80 and 200 have been made. A perturbation theory, in principle being an expansion in powers of the mass difference of the two isotopes in the mixtures, has been developed and used for the analysis of the results for the velocity autocorrelation functions, permitting also the calculation of isotope effects for the mass differences occurring in mixtures of real isotopes. The connection between the mutual and self-diffusion coefficients and the relation of the results to binary collision and hydrodynamic theory has been briefly considered.

Separative Fluodiffusion

Abstract Earlier work on fluodiffusion is discussed. General phenomenological equations for fluodiffusion are given. It is shown that in a Poiseuille flow of a molecular mixture, separational fluodiffusion is expected to be much bigger in the direction of the flow than perpendicular to it. Experiments with liquid lithium are reported. They show a clear isotope effect, but it remains a question whether this effect is due to fluodiffusion or to an interaction of the lithium with the iron in which it was contained