A ProtonT1,T2, and NOE Study of Relative Motion of the Indole Ring of Tryptophans in Gramicidin Analogs Incorporated into SDS Micelles

The proton spin–lattice relaxation time, spin–spin relaxation time, and NOE of the indole ring NH proton of tryptophan residues have been determined for seven analogs of gramicidin A incorporated into SDS micelles. The data obtained indicate that the motion of the indole rings systematically decreases, proceeding from the aqueous interface to the interior of the micelle.     

Molecular Diffusion and DNP Enhancement in Aqueous Char Suspensions

The heterogeneous¹H dynamic nuclear polarization (DNP) effect is studied at low magnetic fields for a system consisting of several newly synthesized carbon chars suspended in water. By using Fourier Transform pulsed-field-gradient spin–echo NMR spectroscopy, several different self-diffusion coefficients have been observed in aqueous char suspensions, corresponding to regions of differing water mobility in the porous structure. Proton spin–lattice relaxation data generally confirm the results of molecular diffusion measurements. Through utilization of the Torrey model, the influence of “cage effects” on DNP enhancement in porous media is discussed. Results suggest that short-range nuclear–electronic interactions in pores have a dominant effect on DNP enhancement in char suspensions.     

H NMR study of N(CH3)4H(ClF2CCOO)2

The proton spin–lattice relaxation times and ¹H NMR second moments were measured over a wide range of temperature. The results were compared with those of the ¹⁹F NMR relaxation that we obtained earlier. For both nuclear species, the evolution of the longitudinal magnetizations with time is observed to be strongly bi-exponential and were in good quantitative agreement with the cross-relaxation theory.     

Lattice-assisted proton hopping in oxides at low temperatures

Stimulated diffusion of protons in oxides such as ABO₃ crystals and rutile TiO₂ is discussed in the context of quantum Brownian motion. A self-consistent lattice-assisted proton hopping (LAPH) model is developed by going from white noise (characteristic of the standard stochastic theory of superionic conduction) to colored noise in the Markovian limit. This model differs from the commonly used ion jump models in that the hydrogen diffusion rate prefactor is identified as a quantity proportional to the frequency of phonon assistance. Application of the quantum fluctuation–dissipation theorem suggests that the dynamic activation energy for diffusion is a function of a bath-mode frequency. The LAPH model can predict enhanced rates of barrier jumping at room temperature compared to thermally activated proton diffusion. This indicates that low-temperature solid oxide devices are potential candidates for use in hydrogen energy research. The LAPH model offers a valid explanation for the mechanism of high protonic mobility recently observed for TiO₂ in a picosecond transient pump-probe experiment. This unexpected dominant lattice relaxation channel must be considered as a new classical-like (but low-temperature) proton transfer mechanism. For vibration-assisted protonic jumps to occur at low temperature, the phonon assistance must be classified as a low-frequency vibration specific to each lattice.     

Molecular Dynamics in a New Solid Glucofuranose-Based Low-Molecular-Weight Organogelator as Studied by 1H NMR

The proton spin–lattice relaxation time T ₁ and the nuclear magnetic resonance second moment were used to study the molecular dynamics of 1,2-O-(1-ethylpropylidene)-α-D-glucofuranose, a new low-molecular-weight organogelator, in the temperature range of 85–308 K. The observed T ₁ minima were attributed to the motion of methyl groups. The experimental data were interpreted in terms of Haupt's theory assuming the tunneling-assisted relaxation process. Authors' address: Jadwiga Tritt-Goc, Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, Poznań 60-179, Poland     

NMR with Hyperpolarised Protons in Metals

Proton pulse NMR, established as a versatile method in Solid State Physics, Chemistry, Biology and Medical Science, requires on the order of 10¹⁸ nuclei to detect an electromagnetic signal in a free induction decay (FID). The main cause for this small sensitivity is the low polarisation in the order of a few ppm due to the Boltzmann distribution in the magnetic field. Thus, NMR experiments on hydrogen are limited to metals with extremely high hydrogen solubility like Pd near room temperature. Using a polarised proton beam, a NMR signal is possible with as few as 10¹³ implanted nuclei. For the first time spin–spin and spin–lattice relaxation times were measured in Au and W with this technique at the Bonn cyclotron.     

Spin–triplet current in half metal/conical helimagnet/superconductor heterojunctions

The BTK theory is extended to investigate spin–triplet current and differential conductance spectrum in the half metal/conical helimagnet (Holmium)/s-wave superconductor heterojunctions. We show that the effective spin–split and spin–flip scatterings of the Holmium layer control the conversion efficiency between the spin–singlet and equal-spin triplet pair correlations, leading to a tunneling current oscillation with the thickness of the Holmium layer. This can provide qualitative explanations on the current oscillation in Ho/Co/Ho-based Josephson junction experiment. The differential conductance spectrum confirms spin–flip Andreev reflection induced long-ranged equal-spin triplet pair correlations.     

Measurement of Relaxation Rates of N and H Backbone Protons in Proteins with Tailored Initial Conditions

Several methods are presented for the selective determination of spin–lattice and spin–spin relaxation rates of backbone protons in labeled proteins. The relaxation rates of amide protons in ¹⁵N labeled proteins can be measured by using two-way selective cross-polarization (SCP). The measurement of H^α relaxation rates can be achieved by combining this method with homonuclear Hartmann–Hahn transfer using doubly selective irradiation. Various schemes for selective or nonselective inversion of the longitudinal proton magnetization lead to different initial recovery rates. The methods have been applied to lysine K6 in ¹⁵N-labeled human ubiquitin and to leucine L5 in ¹⁵N- and ¹³C-labeled octapeptide YG*G*F*LRRI (GFL) in which the marked residues are ¹⁵N- and ¹³C-labeled.     

Relaxation and resonance ultrasound attenuation by Jahn-Teller centers in a GaAs:Cu crystal

The interaction of ultrasound with Cu_Ga4As in a GaAs:Cu crystal has been experimentally studied. The temperature dependences of the attenuation of all normal ultrasonic modes propagating in the ??110?? direction both in doped copper and in nominally pure gallium arsenide crystals have been measured. In the GaAs:Cu crystal, the attenuation peak has been revealed for a transverse wave polarized along the ??110?? axis whose elastic shifts correspond to the symmetry of the tetragonal mode of the Jahn-Teller effect. The temperature dependence of the attenuation of this wave indicates that two types of attenuation??relaxation and resonance??occur. The constructed temperature dependence of the relaxation time indicates that tunneling through the potential barrier between the minima of the adiabatic potential energy is the main relaxation mechanism at temperatures below 10 K. Tunneling splitting estimated from experimental data is in good agreement with the theoretical estimate.     

Dynamics of α-Tocopherol Acetate: Proton Relaxation Studies Supported by Molecular Dynamics Simulations

Dynamical properties of α-tocopherol acetate (commonly known as vitamin E) have been investigated in a broad temperature range (below and above the glass transition) by means of proton spin–lattice relaxation. Two distinct relaxation processes have been detected in the studied temperature range. One of them, present in the solid phase, has been attributed to reorientation of methyl groups. In order to identify the motional process leading to the proton relaxation above the glass transition temperature (T _g), molecular dynamics (MD) simulations have been performed, which provided time correlation functions for several internuclear vectors in the molecule. The high-temperature relaxation process is primarily due to dynamics of the aromatic rings of the tocopherol molecule; however, a considerable contribution of diffusion of the aliphatic chain cannot be excluded. Comparing the nuclear magnetic resonance (NMR) results with MD and relaxation data of dielectric spectroscopy (DS) available in the literature (K. Kamiński, S. Maślanka, J. Zioło, M. Paluch, K.J. McGrath, C.M. Roland, Phys Rev E 75:011903-7, 2007; E. Szwajczak, J. Świergiel, R. Stagraczyński, J. Jadżyn, Phys Chem Liq 47:460–466, 2009), the motional process observed in NMR relaxation studies above T _g has been identified with the δ process observed in DS.     

Laser-Heated High-Temperature NMR: A Time-Resolution Study

The time resolution achievable in in situ high-temperature nuclear magnetic resonance experiments is investigated using laser heating of refractory materials. Three case studies using ²⁷Al in alumina nanoparticles, ²⁹Si in silicon carbide and ²³Na in a glass-forming mixture of sodium carbonate and quartz have been conducted to distinguish the cases of (a) a fast-relaxing, high natural abundance nucleus, (b) a probe nucleus with low abundance and low spin–lattice relaxation rate, and (c) a complex and changing system of industrial relevance. The most suitable nucleus for in situ high-temperature studies is one with high abundance but slow relaxation because the differential relaxation time between hot and cold parts of the sample effectively removes the signal from the cold material. There is no "in situ penalty" from the diminishing Boltzmann polarization at high temperature since this effect is balanced by a corresponding increase of the spin–lattice relaxation rate. Authors' address: Rudolf Winter, Materials Physics, University of Wales Aberystwyth, Penglais, Aberystwyth SY23 3BZ, United Kingdom     

Blood, Sweat, and Tears. Toward a Rehabilitation of the INADEQUATE Experiment

The double-quantum-filtered carbon–carbon correlation experiment (INADEQUATE) can be accelerated significantly through a reduction in the spin–lattice relaxation times by dissolving oxygen gas in the solution. The effect is enhanced by lowering the temperature and by pressurizing the sample tube with oxygen. This offers a fourfold reduction in the relaxation times of the carbon-13 resonances in the 125-MHz spectrum of methyl salicylate. The addition of perfluorotertiarybutanol (related to the artificial blood substitutes) increases the amount of oxygen that can be dissolved, so that without oxygen pressurization, similar reductions in the relaxation times can be achieved. The nuclear Overhauser enhancements are only slightly reduced by addition of oxygen. Polarization transfer from the directly attached protons (INEPT) further increases the sensitivity if at least one of the two coupled carbon sites is protonated, principally because theprotonspin–lattice relaxation times of oxygenated samples are shortened by the relaxation agent. These modest improvements in sensitivity are in general complementary to existing enhancement schemes.     

Determination of the Rashba and Dresselhaus coupling constants using the conductance of a ballistic nanowire

We study the conductance steps of a ballistic nanowire in the presence of a harmonic potential, an in-plane magnetic field, and spin–orbit interactions induced by Rashba and Dresselhaus effects. Calculations of the conductance, at low temperature, using the Landauer–Büttiker formalism, reveal different patterns of steps that are strongly dependent on the magnetic field. Such dependence provides a powerful tool for determining the strengths of the spin–orbit interaction independently, especially in nanowires with low carrier density.     

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T₁∼T₂ in the range of 0.50–1.1 μs at ambient temperature. Both spin–lattice relaxation T₁ and spin–spin relaxation T₂ are longer for ¹⁵N- than for ¹⁴N-nitroxides. The dominant contributions to T₁ are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T₁ on nitrogen nuclear spin state m_I was observed for both ¹⁴N and ¹⁵N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.     

Proton Spin–Lattice Relaxation Study of the Setting Reaction in Conventional and Resin-Modified Glass-Ionomer Dental Cements

The setting processes in KetacCem, AquaCem and Fuji I glass-ionomer dental cements (GIC) as well as in the resin-modified glass-ionomer dental cement (RM-GIC) Fuji Plus have been studied by proton spin–lattice, T ₁, and spin–spin, T ₂, relaxation. The setting time dependence of the degree of hydration was studied as well. In contrast to zinc oxide dental cements, the changes in T ₁ and T ₂ are determined by the interactions of the water with the internal surface. Proton nuclear magnetic resonance relaxation is thus suitable to follow the setting of GIC and RM-GIC and gives valuable information on the GIC hardening dynamics. Authors' address: Tomaz Apih, J. Stefan Institute, Jamova 39, Ljubljana 1000, Slovenia     

Use of Additional Fast-Relaxing Paramagnetic Species for Improvement of RIDME Performance

A method of improving relaxation-induced dipolar modulation enhancement (RIDME) performance by an artificial increase of electron spin–lattice relaxation rate is introduced and tested on a model nitroxide biradical. A substantial increase of its electron spin–lattice relaxation rate is achieved by the addition of a fast-relaxing paramagnetic holmium complex to the sample solution. The electron spin–spin relaxation rate of the nitroxide does not change dramatically upon this addition. The suggested method allows obtaining a deep dipolar modulation in the RIDME experiment, which is free from distortions caused by spectral diffusion processes.     

High mobility in a two dimensional electron system with a thinned barrier

A variety of novel experiments involving mesa structures of a two dimensional electron system (2DES) require the fabrication of a metal electrode on top of the mesa. We describe a fabrication process in which the top barrier layer is thinned to achieve low interface resistance, and the effect of the diminished barrier layer on the transport characteristics of carriers in the 2DES is studied. The sample is an InAs inserted heterostructure with strong intrinsic spin–orbit interaction α. Shubnikov–de Haas, resistivity and Hall experiments are used to characterize the carrier density, mobility and spin–orbit interaction of the carriers and to compare characteristics with a sample in which the structure is not altered. Our results show that the integrity of the heterostructure and the characteristics of the carriers can be maintained when the thickness of the top barrier is as little as 5 ± 2 nm.     

1H NMR study of lithium D-lactate.

Polycrystalline D-lactic acid lithium salt [(R)-2-hydroxypropanoic acid lithium salt, lithium D-lactate] has been investigated by pulsed proton magnetic resonance methods between 77 and 300 K at 25 MHz. The main relaxation mechanism is methyl rotation; the motion is characterized by an activation energy Ea = 14.5 +/- 0.5 kJ/mol and time factor tau 0 = (1.5 +/- 0.5) x 10(-13) s. The activation energy is higher than the potential barrier obtained by ESR and ENDOR techniques for methyl rotation in the lactate radical. The methyl rotation is also responsible for a reduction of the dipolar second moment. Below 100 K the reduction of the dipolar second moment is ascribed to quantum-mechanical tunneling; an excitation energy of 6.1 +/- 1 kJ/mol is derived from a contribution to the spin-lattice relaxation times from the tunneling.     

Methyl and t-butyl reorientation in an organic molecular solid

We have determined the molecular and crystal structure of 4,5-dibromo-2,7-di-t-butyl-9,9-dimethylxanthene and measured the ¹H spin–lattice relaxation rate from 87 to 270 K at NMR frequencies of ω/2π=8.50, 22.5, and 53.0 MHz. All molecules in the crystal see the same intra and intermolecular environment and the repeating unit is half a molecule. We have extended models developed for ¹H spin–lattice relaxation resulting from the reorientation of a t-butyl group and its constituent methyl groups to include these rotors and the 9-methyl groups. The relaxation rate data is well-fitted assuming that the t-butyl groups and all three of their constituent methyl groups, as well as the 9-methyl groups all reorient with an NMR activation energy of 15.8±1.6 kJ mol⁻¹ corresponding to a barrier of 17.4±3.2 kJ mol⁻¹. Only intramethyl and intra-t-butyl intermethyl spin–spin interactions need be considered. A unique random-motion Debye (or BPP) spectral density will not fit the data for any reasonable choice of parameters. A distribution of activation energies is required.     

The influence of the O–O isotope substitution on spin dynamics of (La0.25Pr0.75)0.7Ca0.3MnO3: La NMR data

The ¹³⁹La NMR spectra and spin–spin relaxation times have been measured for the ¹⁶O and ¹⁸O isotope-substituted manganite (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ in the external magnetic field of 5 T. The NMR signal wipe-out has been observed in the ¹⁸O-enriched sample in the charge-ordered state. This phenomenon is connected with a sharp increase in the spin–spin relaxation rate. The great isotope-effect observed provides a clear evidence of an essential role of oxygen motion in controlling the long-range magnetic order in manganites.