Effect Of Isotopic Substitution Research Articles

Resistivity, specific heat, and magnetocaloric effect of La0.8Ag0.1MnO3: Effect of isotopic substitution of 16O → 18O

Results of the experimental studies of resistivity, specific heat, and magnetocaloric effect in lanthanum deficient manganite of La0.8Ag0.1MnO3 are presented. The influence of isotopic substitution of oxygen 16О → 18О on the resistivity, specific heat, and magnetocaloric effect in La0.8Ag0.1MnO3 is estimated. The magnetocaloric effect has been studied by two different methods, which agree well enough. The entropy change as a result of the magnetocaloric effect varies between 2 and 4.1 J/kg K in the magnetic field of 26 kOe in the study samples.

Non-canonical mass laws in equilibrium isotopic fractionations: Evidence from the vapor pressure isotope effect of SF6

We report experimental observations of the vapor pressure isotope effect, including 33S/32S and 34S/32S ratios, for SF6 ice between 137 and 173K. The temporal evolution of observed fractionations, mass-balance of reactants and products, and reversal of the fractionation at one temperature (155K) are consistent with a subset of our experiments having reached or closely approached thermodynamic equilibrium. That equilibrium involves a reversed vapor pressure isotope effect; i.e., vapor is between 2‰ and 3‰ higher in 34S/32S than co-existing ice, with the difference increasing with decreasing temperature. At the explored temperatures, the apparent equilibrium fractionation of 33S/32S ratios is 0.551±0.010 times that for 34S/32S ratios—higher than the canonical ratio expected for mass dependent thermodynamic fractionations (∼0.515). Two experiments examining exchange between adsorbed and vapor SF6 suggest the sorbate–vapor fractionation at 180–188K is similar to that for ice–vapor at ∼150K. In contrast, the liquid–vapor fractionation at 228–300K is negligibly small (∼0.1‰ for 34S/32S; the mass law is ill defined due to the low amplitude of fractionation). We hypothesize that the observed vapor pressure isotope for SF6 ice and sorbate is controlled by commonly understood effects of isotopic substitution on vibrational energies of molecules, but leads to both an exotic mass law and reversed fractionation due to the competition between isotope effects on intramolecular vibrations, which promote heavy isotope enrichment in vapor, and isotope effects on intermolecular (lattice) vibrations, which promote heavy isotope enrichment in ice. This explanation implies that a variety of naturally important compounds having diverse modes of vibration (i.e., varying greatly in frequency and particularly, reduced mass) could potentially exhibit similarly non-canonical mass laws for S and O isotope fractionations. We examined this hypothesis using a density function model of SF6 vapor and lattice dynamic model of SF6(ice). These models support the direction of the measured vapor pressure isotope effect, but do not quantitatively agree with the magnitude of the fractionation and poorly match the phonon spectrum of SF6 ice. A strict test of our hypothesis must await a more sophisticated model of the isotopic dependence of the phonon spectrum of SF6 ice.

Car–Parrinello simulation of the vibrational spectra of strong hydrogen bonds with isotopic substitution effects: Application to oxalic acid dihydrate

The nature of strong intermolecular hydrogen bonding in oxalic acid dihydrate in the crystal phase was examined by infrared spectroscopy and Car–Parrinello molecular dynamics simulation. We studied region of infrared spectra associated with the O—H modes. The spectra were calculated using harmonic approximation with crystal field and time course of the dipole moment as obtained from Car–Parrinello simulation with quantization of the O—H motion, and isotopic substitution. We obtained good agreement of the molecular dynamic simulation with experiment. To our best knowledge, this is one of the first Car–Parrinello calculations of infrared spectra including anharmonicity effects and crystal field interactions.

Isotope exponent in disordered underdoped and overdoped La214

The effect of oxygen isotope substitution on Tc has been analyzed for heavily underdoped and overdoped La2-xSrxCu1-yZnyO4 compounds with different Zn contents in the CuO2 plane. The effect of Zn on the isotope exponent, α, was more pronounced in the case of the underdoped (x = 0.09) compounds compared to the overdoped (x = 0.22) ones. The variation of α with in-plane disorder can be described quite well within a model based solely on impurity induced Cooper pair-breaking in the case of the underdoped compounds. This model, on the other hand, fails to describe the behavior of α(y) for the overdoped samples, even though Zn still breaks pairs quite effectively at this hole (Sr) content. We discuss the implications of these finding in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as number of charge carriers in the CuO2 plane, p (≡ x), is varied.

Magnetocaloric properties of La0.7Ca0.3Mn16O3 and La0.7Ca0.3Mn18O3 manganites and their “sandwich”

The effect of 16О → 18О isotope substitution on specific heat and magnetocaloric effect of polycrystalline La0.7Ca0.3MnO3 manganite is studied. Mainly the effect of isotope substitution for the specific heat and magnetocaloric effect is only the reduction of temperatures of anomalies. ΔTad values at magnetic field change ΔH = 18 kOe are equal to ΔTad = 2.41 K and 2.60 K for La0.7Ca0.3Mn16O3 (LCMO16) and La0.7Ca0.3Mn18O3 (LCMO18), respectively. The sandwich of the LCMO16 and LCMO18 samples was produced for direct measurement of ΔTad. The use of sandwich from materials with near similar magnetocaloric properties increases the relative cooling power by about 20%.

Effect of isotopic substitution in the electronic absorption spectrum of acetone: VUV photoabsorption studies of acetone-d6

The VUV photoabsorption spectrum of acetone-d6 is recorded in the energy region 6–10eV using synchrotron radiation at a resolving power of∼1000. Extensive Rydberg series are observed converging to its first ionization limit (9.708eV). Quantum defect analysis and vibronic analysis of higher Rydberg series members have been reported for the first time. A comparative study of the Rydberg series and accompanying vibronic structure observed in acetone-h6 and acetone-d6 has clarified and consolidated many of the vibronic assignments. The frequency of the ν19′ mode (CO in-plane bend) of the 3py state is obtained as 340±5cm−1 on the basis of observation of an extensive progression of this mode.

1H relaxation dispersion in solutions of nitroxide radicals: Effects of hyperfine interactions with 14N and 15N nuclei

(1)H relaxation dispersion of decalin and glycerol solutions of nitroxide radicals, 4-oxo-TEMPO-d(16)-(15)N and 4-oxo-TEMPO-d(16)-(14)N was measured in the frequency range of 10 kHz-20 MHz (for (1)H) using STELAR Field Cycling spectrometer. The purpose of the studies is to reveal how the spin dynamics of the free electron of the nitroxide radical affects the proton spin relaxation of the solvent molecules, depending on dynamical properties of the solvent. Combining the results for both solvents, the range of translational diffusion coefficients, 10(-9)-10(-11) m(2)/s, was covered (these values refer to the relative diffusion of the solvent and solute molecules). The data were analyzed in terms of relaxation formulas including the isotropic part of the electron spin - nitrogen spin hyperfine coupling (for the case of (14)N and (15)N) and therefore valid for an arbitrary magnetic field. The influence of the hyperfine coupling on (1)H relaxation of solvent molecules depending on frequency and time-scale of the translational dynamics was discussed in detail. Special attention was given to the effect of isotope substitution ((14)N/(15)N). In parallel, the influence of rotational dynamics on the inter-molecular (radical - solvent) electron spin - proton spin dipole-dipole coupling (which is the relaxation mechanism of solvent protons) was investigated. The rotational dynamics is of importance as the interacting spins are not placed in the molecular centers. It was demonstrated that the role of the isotropic hyperfine coupling increases for slower dynamics, but it is of importance already in the fast motion range (10(-9)m(2)/s). The isotope effects is small, however clearly visible; the (1)H relaxation rate for the case of (15)N is larger (in the range of lower frequencies) than for (14)N. It was shown that when the diffusion coefficient decreases below 5 × 10(-11) m(2)/s electron spin relaxation becomes of importance and its role becomes progressively more significant when the dynamics slows done. As far as the influence of the rotational dynamics is concerned, it was show that this process is of importance not only in the range of higher frequencies (like for diamagnetic solutions) but also at low and intermediate frequencies.

Chain-length and mode-delocalization dependent amide-I anharmonicity in peptide oligomers

The diagonal anharmonicities of the amide-I mode in the alanine oligomers are examined in the normal-mode basis by ab initio calculations. The selected oligomers range from dimer to heptamer, in either the α-helical or β-sheet conformations. It is found that the anharmonicity varies from mode to mode within the same oligomer. For a given amide-I mode, the anharmonicity is closely related to the delocalization extent of the mode: the less it delocalizes, the larger the anharmonicity it has. Thus, the single-mode potential energy distribution (PED(max)) can be used as an indicator of the magnitude of the anharmonicity. It is found that as the peptide chain length increases, the averaged diagonal anharmonicity generally decreases; however, the sum of the averaged diagonal and off-diagonal anharmonicities within a peptide roughly remains a constant for all the oligomers examined, indicating the excitonic characteristics of the amide-I modes. Excitonic coupling tends to decrease the diagonal anharmonicities in a coupled system with multiple chromophores, which explains the observed behavior of the anharmonicities. The excitonic nature of the amide-I band in peptide oligomers is thus verified by the anharmonic computations. Isotopic substitution effect on the anharmonicities and mode localizations of the amide-I modes in peptides is also discussed.

Effect of Partial Oxygen Isotope Substitution on the Phase Diagram of (Pr<sub>1-y</sub>Eu<sub>y</sub>)<sub>0.7</sub>Ca<sub>0.3</sub>CoO<sub>3</sub> Cobaltites

Effect of the partial oxygen isotope substitution on (Pr1-yEuy)0.7Ca0.3CoO3 cobaltites was studied in detail for the crossover region (0.12<у<0.26) of the phase diagram. We prepared a series of the cobaltite samples with the different degrees of enrichment by the heavy oxygen isotope 18O, namely, containing 90%, 67%, 43%, 17%, and 0% of 18O. The Eu doping of the samples was chosen to be near and on the both sides of the crossover doping level ycr. Based on measurements of the ac magnetic susceptibility χ(Т) and electrical resistivity ρ(Т), we were able to analyze the evolution the sample properties with the change Eu and 18O content.

Quantum dynamics of H + LiH reaction and its isotopic variants

Time-dependent quantum wave packet dynamics study is carried out to investigate the initial state selected channel specific reactivity of H + LiH collisional system on a new and more accurate ab initio potential energy surface developed by Wernli et al. [J. Phys. Chem. A 113, 1121 (2009)]. The H + LiH reaction proceeds through LiH depletion and H-exchange paths. While the former path is highly exoergic (by ∼2.258 eV), the latter path is thermoneutral. State selected and energy resolved integral reaction cross sections and thermal rate constants are reported and compared with the literature data. The reactivity of the LiH depletion channel is found to be greater than the H-exchange channel. Rotational excitation of the reagent LiH molecule causes a decrease of reactivity of both the channels. On the other hand, the vibrational excitation of the reagent LiH decreases the reactivity of the LiH depletion channel and increases the reactivity of the H-exchange channel. The effect of isotopic substitution (H by D) on the reaction dynamics is also examined.

Wave-packet propagation study of the early-time non-adiabatic dissociation dynamics of NH3Cl: Diabatic picture, effects of isotope substitution and varying the initial vibration levels

Reduced two-dimensional (2D) diabatic potential energy surfaces (PESs) of the two lowest electronic states of NH3Cl were constructed by using the adiabatic 2D PESs, and the early-time dissociation dynamics of the charge-transferred excited electronic state of NH3Cl (Ronen et al. in Phys Rev Lett 93:048301, 2004) were investigated. In the diabatic representation, it was shown that the dissociation path from H2NH+–Cl− to H2N + HCl includes two different processes: the one-step dynamics of diabatic proton transfer followed by electron adjustment, and the two-step dynamics consisting of firstly electron transfer from the other direction mediated (stimulated) by proton movement to form the ground electronic state of H2NH–Cl and then secondly adiabatic H atom tunneling to H2N–HCl. In order to find a means of controlling the branching ratio of the two paths toward the H2N + HCl and H2NH + Cl limits, the effects of varying the initial vibration levels of the precursor anion NH3Cl− and those of isotope substitution (H2NDCl) were also studied. Only the H2N + HCl limit was observed regardless of the initial vibration level and isotope substitution. The overall features of the dynamics are almost unchanged by deuterium substitution (H2NDCl); only the timescale is increased, as expected.

Vector correlations in the F + HO → HF + O reaction and its isotopic variant

The F + H(D)O → HF(DF) + O reactions have been studied using quasi-classical trajectory (QCT) calculation method, based on the three different potential energy surfaces (PESs) of Gomez-Carrasco et al. (J Chem Phys 2004, 121:4605; J Chem Phys 2005, 123:114310; Chem Phys Lett 2007, 435:188). Facilitated with the analysis of the QCT results, the pictures for product scattering and product polarizations have been presented to investigate the vector correlations in the two reactions, with effects of isotope substitution and electronic state as well as collision energy being revealed at a chemical stereodynamical level.

Quantum tautomerization in porphycene and its isotopomers: Path-integral molecular dynamics simulations

Path-integral molecular dynamics simulations have been performed for porphycene and its isotopic variants in order to understand the effect of isotopic substitution of inner protons on the double proton transfer mechanism. We have used an on-the-fly direct dynamics technique at the semiempirical PM6 level combined with specific reaction parameterization. Our quantum simulations show that double proton transfer of the unsubstituted porphycene at T=300K mainly occurs via a so-called concerted mechanism through the D2h second-order saddle point. In addition, we found that both isotopic substitution and temperature significantly affect the double proton transfer mechanism. For example, the contribution of the stepwise mechanism increases with a temperature increase. We have also carried out hypothetical simulations with the porphycene configurations being completely planar. It has been found that out-of-plane vibrational motions significantly decrease the contribution of the concerted proton transfer mechanism.

Use of isotopes for studying reaction mechanisms

The effects of isotopic substitution on equilibrium constants and reaction rates in processes which do not directly involve the isotopic atom are described. In particular, the mechanistic details which can be obtained by quantifying the secondary kinetic isotope effect and steric isotope effect are illustrated.

Isotope-induced desymmetrization can mimic isotopic perturbation of equilibria. On the symmetry of bromonium ions and hydrogen bonds.

The desymmetrizing effect of isotopic substitution on the geometry of otherwise symmetrical structures can be surprisingly large when there is anharmonic coupling between a desymmetrizing mode and modes whose zero-point energies are strongly isotope-dependent. This phenomenon is found to account for experimental observations that had been previously interpreted as implying asymmetry in bromonium ions and in hydrogen phthalate ions in aprotic solvents. The unlabeled structures are concluded to have the higher symmetry found in computed structures. Great care must be taken in applying isotopic perturbation as a test of asymmetry.

Effects of disorder and isotopic substitution in the specific heat and Raman scattering in LuB12

Precision measurements of the specific heat and spectral intensity I(ω) of Raman scattering for LuNB12 single crystal samples with various boron isotopes (N = 10, 11, nat) have been performed at low and intermediate temperatures. A boson peak in the low-frequency part of the I(ω) spectrum has been observed for the first time for lutetium dodecaboride at liquid nitrogen temperatures. It has been shown that low-temperature anomalies in the specific heat, along with the features of Raman spectra, can be interpreted in terms of the transition to a cageglass state at T* = 50−70 K, which appears when Lu3+ ions are displaced from the centrosymmetric position in cavities of a rigid covalent boron sublattice towards the randomly located boron vacancies. The concentrations of various two-level systems that correspond to two types of vibrational clusters with correlation lengths of 12–15 and 18–22 A, respectively, have been estimated. The vibrational density of states of LuB12 has been calculated from Raman spectra in the model of soft atomic potentials. An approach has been proposed to explain the dielectrization of the properties of the YbB12 compound at T < T*, as well as the features of the formation of magnetic structures in RB12 antiferromagnets (R = Tb, Dy, Ho, Er, Tm) and the suppression of superconductivity in LuB12.

Full-dimensional time-dependent wave packet dynamics of H2 + D2 reaction

Collision induced dissociation (CID), four center reaction (4C), and single exchange reaction (SE) in H(2) (v(1) = high) + D(2) (v(2) = low) were studied by means of time-dependent wave packet approach within a full-dimensional model. Initial state-selected total reaction probabilities for the three competitive processes have been computed on two realistic global potential energy surfaces of Aguado-Suárez-Paniagua and Boothroyd-Martin-Keogh-Peterson (BMKP) with the total angular momentum J = 0. The role of both vibrationally excited and rotationally excited reagents was examined by varying the initial vibrational and rotational states. The vibrational excitation of the hot diatom gives an enhancement effect on the CID process, while the vibrational excitation of the cold diatom gives an inhibition effect. The rotational excitation of both reagents has a significant effect on the reaction process. The 4C and SE probabilities are at least one order of magnitude smaller than the CID probabilities over the energy range considered. Isotope substitution effects were also studied by substituting the collider D(2) by H(2) and HD on the BMKP potential energy surfaces. The CID process is most efficient for the H(2) + D(2) combination and least efficient for the H(2) + H(2) combination and is different for the 4C and SE processes.

Resonant two-photon ionization spectroscopy of the 35Cl and 37Cl isotopomers of cis and trans 3-chloro-4-fluoroanisole

The effects of conformation and isotopic substitution on the properties of 3-chloro-4-fluoroanisole (3C4FA) were studied by mass-analyzed resonant two-photon ionization (R2PI) technique and theoretical calculations. In the one color R2PI spectra, the band origins of the S 1 ← S 0 electronic transitions (0 0 bands) of cis 35Cl–3C4FA and cis 37Cl–3C4FA were found to be equivalent at 34,703 ± 3 cm −1, while the 0 0 bands of trans 35Cl–3C4FA and trans 37Cl–3C4FA were found to be equivalent at 34,747 ± 3 cm −1. Assignments of the observed vibrational bands of R2PI spectra were made mainly based on the 10-electron, 8-orbital CASSCF/6-31g calculations and on conformity with the available data of the similar aromatic molecules in the literature. With the two color R2PI technique, the adiabatic ionization energies (IEs) of cis 35Cl–3C4FA and cis 37Cl–3C4FA were determined to be equivalently 67,349 ± 15 cm −1, while the IEs of trans 35Cl–3C4FA and trans 37Cl–3C4FA were determined to be equivalently 67,595 ± 15 cm −1. The conformational effect on the transition energies, ionization energies and vibrational frequencies of 3C4FA is greater than the isotopic effect. The hetero-dihalogen-substitution effect on the transition energies is also discussed.

Quasi-classical trajectory study of the O(1D) + HF reaction dynamics on 11A′ potential energy surface

The quasi-classical trajectory (QCT) calculations for the title reaction were carried out using the recently developed, accurate potential energy surface (PES) of the [Formula: see text] singlet state of the OHF system The integral cross section and the product rotational alignment factor [Formula: see text] were calculated as a function of collision energy. In addition, I discovered the effect of isotopic substitution on stereodynamics for the title reaction, and the influence of the rotation excitation of the reagent on stereodynamics is also presented. Both the scalar and vector properties of the reaction O(1D) + HF → OH + F(2P) are studied in this paper. It was found that the reaction is mainly controlled by an indirect reaction mechanism, and that the deep noncollinear insertion HOF well has a great impact on the dynamics of the reaction. The conclusions drawn in this paper will draw from references to similar reactions, and provide a theoretical foundation for related experiments.

Effect of reagent vibrational excitation and isotope substitution on the stereo-dynamics of the Ba + HF → BaF + H reaction

Based on an extended London—Eyring—Polanyi—Sato (LEPS) potential energy surface (PES), the Ba + HF reaction has been studied by the quasi-classical trajectory (QCT) method. The reaction integral cross section as a function of collision energy for the Ba + HF → BaF + H reaction is presented and the influence of isotope substitution on the differential cross sections (DCSs) and alignments of the product's rotational angular momentum have also been studied. The results suggest that the integral cross sections increase with increasing collision energy, and the vibrational excitation of the reagent has great influence on the DCS. In addition, the product's rotational polarization is very strong as a result of heavy-heavy-light (HHL) mass combination, and the distinct effect of isotope substitution on the stereodynamics is also revealed.