Improvement of duty-cycle heating compensation in NMR spin relaxation experiments

To reliably measure NMR relaxation properties of macromolecules is a prerequisite for precise experiments that identify subtle variations in relaxation rates, as required for the determination of rotational diffusion anisotropy, CSA tensor determination, advanced motional modeling or entropy difference estimations. An underlying problem with current NMR relaxation measurement protocols is maintaining constant sample temperature throughout the execution of the relaxation series especially when rapid data acquisition is required. Here, it is proposed to use a combination of a heating compensation and a proton saturation sequence at the beginning of the NMR relaxation pulse scheme. This simple extension allows reproducible, robust and rapid acquisition of NMR spin relaxation data sets. The method is verified with (15)N spin relaxation measurements for human ubiquitin.     

FOI-PERFECT程序对电磁驱动高能量密度系统的三维弛豫磁流体力学模拟

提出一个完整的弛豫磁流体力学模型用于电磁驱动高能量密度系统的数值模拟, 它由弛豫电磁波动、弛豫热输运、P1/3近似辐射输运以及流体力学构成。电磁部分在真空区退化为电磁传播, 在等离子体物质区退化为磁扩散近似, 并且相速和群速是有上界的。这意味着弛豫磁流体力学能退化到传统的电阻性磁流体力学, 并且能用显式方法数值求解, 便于大规模高效并行化。基于此弛豫磁流体力学模型开发了三维辐射磁流体力学程序FOI-PERFECT, 指出了所采用的关键数值技术, 并给出了一些应用例子。     

Solution to the Glarum model with a finite relaxation rate

The one-dimensional version of the Glarum model of target relaxation by diffusing defects is generalized by considering a finite relaxation rate upon an encounter defect-target. This eliminates the difficulties associated with the instantaneous relaxation of the targets initially occupied by a defect. The diffusion and relaxation processes are described in terms of the master equation for a continuous-time random walk. An exact solution valid for all times and relaxation rates is obtained by the use of the van Kampen-Oppenheim method. The transition from exponential relaxation at short times to fractional exponential behaviour at long times is described in detail.     

Loss tangent (internal friction) for high relaxation strengths and a lognormal distribution of relaxation and retardation times

Summary The loss tangent (internal friction) is evaluated numerically for a lognormal distribution of retardation or relaxation times and high relaxation strengths. It is shown that the loss tangent peak becomes more asymmetrical and the peak width, at half-maximum, decreases as the relaxation strength increases, dropping down to values corresponding to a Debye peak at very large relaxation strengths. Furthermore, the asymmetry of the peak can be used to establish if the lognormal distribution is in the retardation or in the relaxation spectrum. Finally, a procedure which can be used to obtain the relaxation strength, the geometrical parameter of the distribution and the average retardation or relaxation time, from the experimental loss tangentvs. frequency curves, is described. This procedure is applied to actual experimental data reported in the literature for polystyrene. The authors of this paper have agreed to not receive the proofs for correction.     

Rotating-Frame Proton Cross Relaxation in the Presence of Paramagnetic Metal Ions

Rotating-frame cross relaxation for a pair of protons rotating in a spherical molecule with external relaxation is examined theoretically. The results of this study allow us to model intensities in 1D ROE and 2D ROESY spectra of protons in the presence of a paramagnetic metal ion. External relaxation moves the threshold correlation time for spin diffusion to longer times. In contrast to the effect of external relaxation on longitudinal cross relaxation (NOESY), the range of observable transverse cross relaxation (ROESY) expands with increasing external relaxation. At the same time, external relaxation compresses the overall time scale for cross-peak evolution. The initial slopes of cross-peak evolution are unaffected by external relaxation, but are sensitive to the rotational correlation time of the proton pair. Very short mixing times are necessary for accurate estimation of the initial Slopes.     

Complex local dynamics in DNA on the picosecond and nanosecond time scales

Time-resolved Stokes shift measurements of the local structural relaxation of three DNA oligonucleotides are presented. Logarithmic relaxation is seen for over three decades in time (40 ps-40 ns), indicating a complex relaxation among a large number of conformational substates. The observed relaxation is the same in all the sequences. Sequence dependence of the localized dynamics of DNA does not appear within this time range. We infer that 30%-50% of the relaxation is faster than 40 ps, has a nonlogarithmic decay and has a sequence dependent amplitude.     

Influence of surface relaxation on the surface vibrational modes of some ionic crystals

In this paper we evaluate the effects of surface relaxation on the surface vibrational modes of the (001) surfaces of NaCl, NaI and MgO, using bulk shell models which have been modified to allow for distance dependence of the short-range interactions. It is found that these dynamical shell models do not lead to convergent results for the static surface relaxation. In spite of this inconsistency, it is found that surface relaxation gives rise to a new structure of relaxationinduced surface modes, which is common to the three crystals and independent of the details of the relaxation. By using assumed (convergent) relaxation configurations for the MgO (001) surface it is shown that relaxation can account for the high-frequency peak in the surface-excess density of vibrational states of MgO, as found in inelastic neutron scattering experiments on MgO microcrystals. In general, surface relaxation has a profound effect on surface vibrational phenomena, and better shell models will have to be developed to deal with the static and dynamical surface phenomena in a consistent fashion.     

14N核四极共振自旋系统自旋-晶格弛豫时间的测量

¹⁴N核四极共振自旋系统的自旋-晶格弛豫是一种双指数弛豫。本文介绍了¹⁴N核四极共振自旋系统的自旋-晶格弛豫时间的3种测量方法,利用可变多面体方法对实验数据进行拟合,获得了¹⁴N核四极共振自旋系统的自旋-晶格弛豫时间T_1s和T₁₁,对有关文献中关于核四极共振弛豫时间的测量的3个观点提出了质疑。     

Non-newtonian flow for concentrated polymer systems

The modified Graessley theory with the three-dimensional Maxwell model can well explain some of the nonlinear viscoelastic behavior of concentrated polymer systems at least qualitatively with the assumption of a box-type relaxation spectrum for the equilibrium state.

The relaxation spectrum of concentrated polymer systems in shear flow is obtained by means of Graessley's theory. It is assumed that the characteristic time for the entanglement formation is the same order as that for its breakage and that the spectral density of the relaxation spectrum in the flow system is proportional to the number of entanglements between two molecules. The spectral density decreases approximately proportionally to 1/γ for relaxation times larger than 1/γ The non-Newtonian viscosity and other viscoelastic properties, such as the so-called stress overshoot and the stress relaxation, are calculated by using the obtained relaxation spectrum. Our theory explains very well the experimental results in many cases. Good agreement with experimental results is found if we assume that the so-called box-type relaxation spectrum in the equilibrium state has a finite gradient of the order of ?0.5 in the edge region of larger relaxation time on log-log plots.     

Linewidth-resolved 15N HSQC, a simple 3D method to measure 15N relaxation times from T1 and T2 linewidths

A three-dimensional approach for measuring 15N relaxation times is described. Instead of selecting particular values for the relaxation period, in the proposed method the relaxation period is incremented periodically in order to create a 3D spectrum. This additional frequency domain of the transformed spectrum contains the relaxation time information in the T1 and T2 linewidths, and thus the longitudinal and transverse 15N relaxation times can be measured without determination of 2D cross peak volumes/intensities and subsequent curve fitting procedures.     

Relaxation of hyperpolarized 129Xe in a deflating polymer bag

In magnetic resonance imaging with hyperpolarized (HP) noble gases, data is often acquired during prolonged gas delivery from a storage reservoir. However, little is known about the extent to which relaxation within the reservoir will limit the useful acquisition time. For quantitative characterization, 129Xe relaxation was studied in a bag made of polyvinyl fluoride (Tedlar). Particular emphasis was on wall relaxation, as this mechanism is expected to dominate. The HP 129Xe magnetization dynamics in the deflating bag were accurately described by a model assuming dissolution of Xe in the polymer matrix and dipolar relaxation with neighboring nuclear spins. In particular, the wall relaxation rate changed linearly with the surface-to-volume ratio and exhibited a relaxivity of κ=0.392±0.008 cm/h, which is in reasonable agreement with κ=0.331±0.051 cm/h measured in a static Tedlar bag. Estimates for the bulk gas-phase 129Xe relaxation yielded T1bulk=2.55±0.22 h, which is dominated by intrinsic Xe-Xe relaxation, with small additional contributions from magnetic field inhomogeneities and oxygen-induced relaxation. Calculations based on these findings indicate that relaxation may limit HP 129Xe experiments when slow gas delivery rates are employed as, for example, in mouse imaging or vascular infusion experiments.     

季碳的偶极-偶极弛豫分离——睾丸甾酮丙酸酯的~(13)C NMR弛豫时间的研究

本文提出了分离偶极—偶极驰豫的新方法 参数分离法。对于稳固的刚性分子,可以通过仲碳和叔碳弛豫时间的测量算出扩散系数,这不仅可以用于研究分子的动态特性。还可以用于计算季碳的偶极—偶极弛豫。文中报道了睾九甾酮丙酸酯驰豫的研究结果,表明参数分离法可对季碳的驰豫机制进行有效的研究。     

Density matrix formalism for anelastic relaxation

A density matrix formalism is developed for anelastic (mechanical) relaxation in crystalline materials with point defects characterized by elastic dipoles. The time-dependent approach to equilibrium of the strain response under the action of a constant applied stress is deduced. The formalism parallels the one used in nuclear magnetic relaxation. The anelastic relaxation time is determined as a function of the parameters occurring in the defect hopping term in the Hamiltonian. This term is responsible for the dissipation of the anelastic ‘potential’ energy into the host lattice. In a lengthy concluding section, the following aspects are discussed point by point: the advantages of the formalism presented, its scope and special cases; the physical implications of the expression obtained for the relaxation time; the similarities and differences between magnetic relaxation and anelastic relaxation, etc.     

Theorie der Multipolrelaxation

The multipole resonance and relaxation is discussed starting with a general consideration of the quantum mechanical dynamics of spinsystems with arbitrary spinI. The irreversible entropy production as a function of the multipolarisations is calculated from the density operator, expanded in terms of a complete set of orthonormal multipole operators. By use of the methods of thermodynamics of irreversible processes one obtains relaxation equations, which are a generalization of the Bloch equations of pure magnetic dipole relaxation. They connect the time derivatives of the multipolarisations with affinities, which are defined by the expansion of the logarithmus of the density operator; for small multipolarisations the affinities are equal to the multipolarisations themselves. Relations for the relaxation matrix are discussed, especially a derivation is given for reciprocal relations between the relaxation coefficients. In the special case of spinI=1 the equations of quadrupole relaxation are studied for axial symmetry.     

Bulk model of rapid thermal oxidation of silicon

A model of rapid thermal oxidation of silicon in dry oxygen based on the reaction of volume oxidation is constructed. It is assumed that the coefficient of oxygen diffusion for silicon dioxide decreases because of internal compressive stress, which is at a maximum near the SiO₂-Si interface; as the distance from the interface increases, this stress decreases according to the time-dependent exponential law because of viscoelastic relaxation from the value of the diffusion coefficient for strained oxide to that for fused quartz. The characteristic relaxation time of the coefficient of oxygen diffusion in silicon dioxide correlates with the relaxation time of internal stress in silicon dioxide films on silicon and with the relaxation time of the refraction index. Because the refraction index is related to the density of silicon dioxide, we arrive at the conclusion that the relaxation of the diffusion coefficient is related not only to the relaxation of internal mechanical stress, but also to the relaxation of the density of silicon dioxide.     

石英中Al3+-空穴取向变化弛豫过程中的红外发散响应

基于α石英的晶体结构，将红外发散响应模型和双势阱模型应用到含Ａｌ杂质的α石英中Ａｌ³⁺－空穴的取向变化弛豫过程，研究其低温介电损耗特性，结果表明Ｔ＜６．５Ｋ时，介电损耗的主要贡献来自于单声子助隧道弛豫过程；Ｔ＞１０Ｋ时，主要贡献来自于热跃迁弛豫过程；而在中间温区，介电损耗是两种过程的迭加，同一弛豫体不同的弛豫过程对应于不同的红外发散响应，还讨论了同一弛豫体引起的超声弛豫损耗。 关键词：     

On the equivalence of the parallel channel and the correlated cluster relaxation models

The question of the origins of nonexponential relaxation is addressed in terms of the probabilistic approach to relaxation. The interconnection between two differently rooted probabilistic models, i.e., between the parallel channel and the correlated cluster models, is presented. We show that clearly different probabilistic origins yield in both approaches a well-defined class of universally valid two-power-law responses with the stretched-exponential and exponential decay laws as special cases. The equivalence of both models indicates that variations in the local environment of the relaxing configurational units (parallel channel relaxation) can provide a basis for self-similar relaxation dynamics without the need for hierarchically constrained dynamics (correlated clusters relaxation).     

Self-induced avalanches in relaxing media

A theory of boson avalanches (self-induced waves) has been constructed with allowance for longitudinal relaxation. It is shown that inclusion of longitudinal relaxation requires a completely different approach to the theory of superradiance than that taken previously. For the first time an equation is derived for an avalanche with allowance for longitudinal and transverse relaxation. Numerical solution of this equation makes It possible to establish all the distinctive features of a coherent self-induced wave under the conditions of the action of relaxation mechanisms. An inversion effect is found to occur in the action of longitudinal and transverse relaxation on the delay parameter of a self-induced wave, i.e., the superradiance delay time. Analytic expressions are found of the ensemble-averaged delay time of an avalanche and the relative fluctuations of the delay time as a function of irreversible phase relaxation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 81–86, September, 1986.     

Electron spin relaxation of radicals in γ-irradiated malonic acid and methyl malonic acid

Radicals generated by γ-irradiation of malonic acid and methyl malonic acid were studied as a function of temperature by inversion recovery, echo-detected saturation recovery and electron-electron double resonance (ELDOR) at X-band, and by continuous-wave saturation recovery at X-band and S-band. ELDOR reductions for malonic acid radical in polycrystalline and single-crystal samples indicate that nuclear spin relaxation is faster than both electron spin relaxation and cross relaxation between 93 and 233 K. Deuteration of the carboxylate protons caused the maximum ELDOR reduction to shift from about 110 to 150 K, consistent with the assignment of the rapid nuclear spin relaxation to hydrogen-bonded proton dynamics. ELDOR enhancements for both radicals formed in methyl malonic acid indicate that cross relaxation is faster than both electron spin relaxation and nuclear spin relaxation between 77 and 220 K. Enhanced cross relaxation and electron spin relaxation are attributed to the rotation of methyl groups at a rate comparable to the electron Larmor frequency. The temperature dependence of the enhancement of 1/T _1e was analyzed to determine the activation energies for methyl rotation. The same radical is formed in irradiated methyl malonic acid and L-alanine, but the barrier to rotation of the α-methyl is 500 K in the methyl malonic acid host and 1500 K in the L-alanine host, which indicates a large impact of the lattice on the barrier to rotation.