Methyl reorientation in solid 3-ethylchrysene and 3-isopropylchrysene

We have measured the proton spin-lattice relaxation rate as a function of temperature in polycrystalline 3-ethylchrysene at nuclear magnetic resonance Larmor frequencies of 53.0 and 22.5 MHz and in polycrystalline 3-isopropylchrysene at 53.0, 22.5 and 8.50 MHz. The syntheses of these new compounds are presented. The relatively large chrysene backbone creates an ideal and unique environment for the alkyl groups such that methyl group rotation is the only motion on the nuclear magnetic resonance Larmor frequency timescale over a large temperature range. The relaxation rate data are interpreted in terms of the simplest possible dynamical model; that of random hopping for the methyl group(s), all of which are equivalent in the solid state. The barriers of 11–12 kJ mol⁻¹ are typical for methyl groups in ‘isolated' ethyl and isopropyl groups.     

Molecular dynamics in stiff ionene below glass transition

Temperature dependences of proton and fluorine second moments and spin-lattice relaxation time T₁ below glass transition were measured in glassy “I-Do,Pip-Me-BF₄” ionene. The existence of motions of methyl groups and segments linking the cationic centers, namely piperidinium rings and trimethylene groups, for the polymeric part of ionene were established. Isotropic rotation of the counter-ion was evidenced and its limited diffusion suggested. To interpret the proton and fluorine relaxation data, a Davidson-Cole distribution of correlation times was assumed.     

NMR study of monomethylammonium cation in (CH3NH3)5Bi2Cl11 ferroelectric polycrystal

Spin-lattice relaxation times T₁ and T_1ρ are determined for protons in three polycrystals (CH₃NH₃)₅Bi₂Cl₁₁, (CD₃NH₃)₅Bi₂Cl₁₁ and (CH₃ND₃)₅Bi₂Cl₁₁. The temperature dependencies of the relaxation times obtained for (CH₃NH₃)₅Bi₂Cl₁₁ and (CD₃NH₃)₅Bi₂Cl₁₁ are interpreted as a result of correlated motions of the three-proton groups of the monomethylammonium cation. The minimum of the T_1ρ relaxation time is explained as a result of the oscillations of the symmetry axis of the whole cation.     

Molecular dynamics in solid riboflavin as studied by 1H NMR

Spin–lattice relaxation times T₁ and T_1d as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B₂) in the temperature range 55–350 K. The broad and flat T₁ minimum observed at low temperatures is attributed to the motion of two nonequivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. An additional mechanism of relaxation in the high temperature region is provided by the motion of a proton in one of the hydroxyl groups. The Davidson–Cole distribution of correlation times for this motion is assumed.     

Investigation of cerebral ischemia using magnetization transfer contrast (MTC) MR imaging

The effects of cerebral ischemia in rat brain were monitored as a function of time using proton MR imaging. Spinspin relaxation time (T₂), proton density, and magnetization transfer contrast (MTC) were measured by MR imaging at various time intervals during a 1-week period following the induction of ischemic damage. Ischemic injury was characterized by a maximization of both T₂ value and MTC appearance at 24 hr postischemic injury. These changes were accompanied by a gradual increase in MR observable water density over the first few days of ischemia. A reduction in the magnetization exchange rate between “free” and “bound” water protons as measured by MTC imaging is at least partially responsible for the elevation in T₂ values observed during ischemia, and may accompany breakdown of cellular structure.     

Classical and quantum molecular dynamics of cation in (CH3NH3)3 Sb2Br9 polycrystal as studied by 1H NMR.

¹H spin-lattice relaxation times and second moments were determined for polycrystalline (CH₃NH₃)₃Sb₂Br₉ sample in a wide range of temperature (5–200 K) at 24.6 and 55.2 MHz. ²H NMR spectra of (CD₃NH₃)₃Sb₂Br₉ were recorded between 5 K and room temperature. The relaxation time is interpreted as a result of motion of two different non-equivalent types of monomethylammonium cations occurring at the 2:1 proportion in a unit cell. Below 30 K, the relaxation processes via tunneling are suggested to dominate. Above 30 K, only classical behaviour of methylammonium cations is detected. Two monomethylammonium cations relax with the classical correlated C₃ reorientation and the rotational tunnelling mechanism, while the third cation exhibits only the classical correlated reorientation. The dynamic parameters of these motions have been determined.     

Thermal expansion of solid CH4 and CD4

The linear expansion coefficients of solid light methane CH₄ and deuteromethane CD₄ have been denned in the temperature range 10°–24°K and 6°–58°K, respectively. The question about the nature of rotational motion of molecules in CH₄ and CD₄ crystals and about the existence of low temperature phase transition in solid methane is discussed.     

Local and global dynamics in the glass-forming di-isobutyl phthalate as studied by H NMR

Spin-lattice relaxation times T₁ and T_1ρ as well as ¹H NMR spectra have been employed to study the dynamics of the glass-forming di-isobutyl phthalate in the temperature range extending from 100 K, through the glass transition temperature T_g, up to 340 K. Below T_g NMR relaxation is governed by local dynamics and may be attributed to rotation of methyl groups at low temperatures and to motion of isobutyl groups in the intermediate temperature interval. Above T_g the main relaxation mechanism is provided by overall molecular motion. The observed relaxation behavior is explained by motional models assuming asymmetrical distributions of correlation times. The motional parameters obtained from Davidson-Cole distribution, which yields the best fit of the data at all temperatures are given.     

Molecular associations in binary mixtures of pyridine and chlorobenzene in benzene solution using microwave absorption data

The dielectric relaxation time (τ) of binary mixtures of different molar concentrations of pyridine (C₅H₅N) and chlorobenzene (C₆H₅Cl) in benzene solution at different temperatures (25, 30, 35 and 40 °C) has been calculated by using standard microwave techniques and Gopala Krishna's single frequency (9.875 GHz) concentration variation method. The energy parameters (ΔH_ε, ΔF_ε, and ΔS_ε) for the dielectric relaxation process of the binary mixture containing 0.5 mol fraction of pyridine have been calculated at the respective temperatures. Comparisons have been made with the corresponding energy parameters for the viscous flow (ΔH_η, ΔF_η, and ΔS_η). From the observations it is found that the dielectric relaxation process can be treated as the rate process. Based upon above studies, solute–solvent type of molecular associations arising from the interaction of chlorobenzene and benzene and pyridine and benzene molecules has been proposed. No solute–solute type of molecular association has been observed.     

Pressure and solvent induced low-temperature synthesis of monodisperse superparamagnetic nanocrystals: The case of Fe3O4 in alkanols

The synthesis of nanoparticles via the decomposition of organometallic compounds has been studied extensively, but the effect of pressure has received little attention. Here we firstly focus on the pressure effect, and utilize it as well as the solvent and temperature effects to develop a “green” cheap low-temperature (140–200 °C) strategy via the decomposition of iron oleate in alcohol (or other alkanols) using oleic acid as the protecting reagent. The obtained monodisperse Fe₃O₄ nanocrystals (NCs) exhibit satisfying superparamagnetism. The mechanism of the process has been investigated and it was shown both high pressure and alkanol as the solvent can facilitate the formation of magnetite NCs. Furthermore, the solvent used in the preparation can control the shape, size and dispersion of NCs. In addition, the effects of reactant concentration, reaction time, temperature, and the mol ratio of oleic acid on the Fe₃O₄ NCs have also been discussed.     

Thermal transport properties in a He---He mixture near and below the superfluid transition

Measurements of the thermal conductivity and of X/T are reported for a ³He---⁴He mixture with a ³He mole fraction X = 0.622. At T_λ, X/T passes through a sharp peak. A comparison with the theory of Khalatnikov is presented. The relaxation times τ(T) to reach steady state conditions show qualitatively the same behaviors as X/T.     

Proton and deutron NMR study of PTFE ionomer membranes

Proton and deutron NMR have been conducted to investigate the ionic motion in perfluorinated ionomer membranes from Dow Chemical (XUS) and DuPont (Nafion^R). Two proton relaxation peaks were found in the XUS specimen absorbed with H₂O. The major (narrow) peak presented a spin-lattice relaxation time (T₁) of 107 ms while the minor (broader) one gave much longer T₁. While the former was attributed to the water molecules involved in restricted motion, the latter was expected to be associated with the protons located in the vicinity of the sulfonate groups. Similar to the previous results from the others, only a single peak was detected in Nafion^R in ¹H spectra, indicating that the protons in the different environments were engaging rapid exchange within NMR time scale. In contrast to the inverse proportion dependence of the linewidth on the water sorption in Nafion^R, the major line of the XUS membrane exhibited insensitive linewidth dependence on the variation of H₂O concentration. The difference was attributed to the existence of narrow breaths of the pores in XUS sample, such that free water contribution to the enhancement of proton mobility was limited. The ²H spectra of Nafion^R were found to possess a doublet, due to nuclear quadrupolar interaction. Dow (XUS) membrane treated in at 100% relative humidity (RH) D₂O presented a single peak with the linewidth insensitive to the amount of heavy water absorbed. An additional rise emerged on the “shoulder” of this single peak when treated at 33% RH. It is concluded that XUS membrane does not provide strong hydrogen bonding to eliminate the rapid motion average over the nuclear quadrupole interaction.     

Cl nuclear quadrupole resonance studies of molecular motions of ClO3 and water diffusion in Ca(ClO3)2·2H2O

A ³⁵Cl nuclear quadrupole resonance (³⁵Cl-NQR) investigation of polycrystalline Ca(ClO₃)₂·2H₂O is described. The ³⁵Cl-NQR frequencies (ν_Q) for two resonance lines (ν_Q1 and ν_Q2), the spin lattice relaxation time (T_1Q) for ν_Q2 only and the line width δν_Q2 were measured in the temperature range 292–345 K, except for the frequency measured up to 455 K. The observed decrease in the resonance frequencies with increasing temperature permitted the determination of the frequencies of librations of the ClO₃⁻ ion about two axes perpendicular to the three-fold axis of the ion mainly responsible for this effect. The temperature dependence of the relaxation time T_1Q proved the occurrence of water diffusion and hindered rotation of ClO₃⁻ ions. The activation energies of these two molecular motions were determined, and their effect on the electric field gradient at the site of a chlorine nucleus was discussed. Temperature measurements of the line width δν_Q2 confirmed the conclusions drawn from the analysis of T_1Q(T).     

Localized in vivo proton spectroscopy of the bone marrow in patients with leukemia

Volume selective magnetic resonance (MR) proton spectroscopy was used to investigate the haemopoietic (iliac bone) and fatty bone marrow (tibia) in patients with leukemia and polycythaemia vera. Selective measurements of the relaxation times T₁ and T₂ for the “water” and “fat” resonances in the bone marrow spectra were performed. Nine patients with acute leukemia and three patients with chronic leukemia were examined at diagnosis. Three patients with acute leukemia in remission were also examined. Five of the leukemic patients had follow-up examinations performed in relation to chemotherapeutic treatment. Nine patients with polycythaemia vera and 21 normal control subjects were examined with identical methods for comparison. All patients had bone marrow biopsies performed prior to every MR examination. Significant differences could be detected in the spectral patterns from iliac bone marrow in patients with leukemia at diagnosis compared to the healthy normal controls. The “relative water content” was increased in the iliac bone marrow spectra of the leukemic patients compared to the normal subjects, which indicates an increase in the amount of haemopoietic tissue and a corresponding decrease in marrow fat content. The T₁ relaxation times of the “water” resonance in the spectra from the iliac bone marrow of the leukemic patients were significantly prolonged at diagnosis, compared to the normal controls and the patients with polycythaemia vera. After chemotherapeutic induction of remission, the spectra from the iliac bone marrow in the patients with leukemia resembled normal spectra. Four leukemic patients had abnormal spectra from the tibial bone marrow and one patients showed early changes in tibial marrow during chemotherapeutic treatment, before any major changes could be detected in the iliac bone marrow.     

Frequency, field, and temperature dependence of the AC penetration depth of a GdBa2Cu3O7−δ film in the mixed state

We report on a systematic mutual induction measurement of the complex AC penetration depth λ of a sputtered high-quality GdBa₂Cu₃O_7−δ film in the mixed state by a very small coil system arranged near the sample surface. The complex penetration depth λ(B, T, ω) for DC inductions B 0.65 T (perpendicular to the film), for temperatures 36 K T 81 K, and for frequencies 1 kHz ω/2π 500 kHz was determined from the measured signal by a novel inversion scheme. The results are consistent with theoretical predictions based upon single vortex pinning. The Labusch parameter , the flux creep relaxation time τ, as well as the effective activation energy U are simulateneously determined.     

Microscopic theory of atomic and electronic stretched exponential relaxation in high temperature superconductors

Measured dimensionless room-temperature conductivity relaxation stretching fractions β in YBCO are in excellent agreement with theoretical predictions, which in 1995 identified two magic fractions, β=3/5 and β=3/7. Thus, relaxation studies provide an absolute measure of “ideality” in these complex materials, independent not only of composition x but even of crystal structure. The relaxation stretching fractions β associated with T_c itself, reported in 2000, are also explained by the magic fraction β=3/5 predicted by microscopic theory. One can infer that the interactions responsible for high-temperature superconductivity are short range, non-magnetic, and primarily associated with resonant trapping centers in semiconductive layers.     

Methane activation over Ag-exchanged ZSM-5 zeolites: A theoretical study

Methane activation catalyzed over Ag-exchanged ZSM-5 zeolites was investigated by using the density functional theory (DFT) with a cluster model. Two different pathways were taken into account in this work: the “alkyl” and the “carbenium” pathways. The activation barriers obtained are 34.09 and 66.63 kcal/mol for the “alkyl” and the “carbenium” pathway, respectively. The calculated results show that the activation barrier of the “alkyl” pathway is smaller than that of “carbenium” pathway. Consequently, the “alkyl” pathway is the preferential reaction pathway. A new mechanism of methane conversion in the presence of ethene was proposed. In the catalytic cycle, the initial step of methane activation proceeds with the “alkyl” pathway and the Ag⁺ cation acts as an acceptor of the methyl group, then ethene reacts with the Ag⁺CH₃⁻ group to form propene. In addition, it is found that the Ag⁺ cations play an important role in the methane activation, compared with the reaction of methane activation over H-ZSM-5.     

Solvent fluctuations and viscosity-dependent rates of solution reactions in a regime indescribable by the transition state theory

An organic molecule isomerizes in viscous solvents when appropriate cavities are formed around it in the course of slow diffusive thermal fluctuations of solvent molecules. The isomerization occurs when fast twisting (vibrational) fluctuations around a bond get to have large amplitudes in such cavities. This situation can be described by the two-reaction-coordinate model of Sumi and Marcus originally proposed for electron transfer reactions. In fact, the rate constant derived from this model fits nicely to that observed for thermal Z→E isomerization of substituted azobenzenes and N-benzylideneanilines. The rate constant is influenced by slow speeds of diffusive motions of solvent molecules, whose relaxation time τ is usually proportional to the solvent viscosity η. It has a form of k = 1/(k_TST⁻¹+k_f⁻¹), where k_TST, independent of τ, represents the rate constant expected from the transition state theory (TST), while k_f ∝ τ⁻ with 0 < ≤ 1 represents the part controlled by solvent fluctuations. An analytic expression of for the isomerization reactions is given in terms of physical parameters underlying the reaction mechanism with cavity formation.

This rate-constant formula is a general one applicable widely also to other solution reactions, covering from the TST-validated regime for a small τ to the TST-invalidated one for a large τ. In the former, k approaches k_TST since k_f k_TST, while in the latter, k approaches k_f since k_f k_TST, becoming decreasing with a decrease in the typical speed (∝ τ⁻¹) of solvent fluctuations. The dependence of k ≈ k_f ∝ η⁻ in the non-TST regime has often been observed also in biological reactions such as enzymatic ones. In this case, it is not appropriate to say that reactions are controlled by slow speeds of solvent fluctuations, but we should rather say that enzymes utilize this situation, which has been called conformational gating, in the course of solvent-fluctuation-driven conformational fluctuations of proteins. It has important meanings in protein functions.     

Outer curvature and conformal geometry of an imbedding

The differential geometry of an imbedded (e.g. string or membrane world sheet) surface in a higher-dimensional background is shown to be conveniently describable (except in the null limit case) in terms of what are designated as its first, second, and third fundamental tensors, which will have the respective symmetry properties η_μν = η_(μν) as a trivial algebraic identity, K_μν^ρ = K_(μν)^ρ as the “generalised Weingarten identity”, which is the (Frobenius type) integrability condition for the imbedding, and Ξ_λμν^ρ = Ξ_(λμν)^ρ as a “generalised Codazzi equation”, which depends on the background geometry being flat or of constant curvature, needing replacement by a more complicated expression for a generic value of the background curvature B_κλ^μ_ν. The “generalised Gauss equation” expressing the dependence on this background curvature of the internal curvature tensor R_κλ^μ_ν of the imbedded surface is converted into terms of the first and second fundamental tensors, and it is thereby demonstrated that the vanishing of the (conformally invariant) “conformation tensor”, i.e. the trace free part C_μν^ρ of the second fundamental tensor K_μv^ρ, is a sufficient condition for conformal flatness of the imbedded surface (and thus in particular for the vanishing of its (Weyl type) conformal curvature tensor C_κλ^μ_ν) provided the background is itself conformally flat. In a trio of which the first two members are the generalised Gauss and Codazzi equations, the “third” member is shown to give an expression in terms of C_μν^ρ for the (trace free, conformally invariant) “outer curvature” tensor Ω_κλ^μ_ν whose vanishing is the condition for feasibility of the natural generalisation of the Walker frame transportation ansatz. The vanishing of C_μν^ρ is shown to be sufficient in a conformally flat background for the vanishing also of Ω_κλ^μ_ν.