共查询到20条相似文献,搜索用时 15 毫秒
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(1)H nuclear spin-lattice relaxation has been investigated in sodium acetate trihydrate and sorbic acid using field-cycling NMR in the solid state. The relaxation is dominated by the reorientation of the methyl groups. Resonant features arising from coherent tunnelling are observed in both the magnetic field dependence of the spin lattice relaxation rate, T(1)(-1)(B(z)) and in the inverse temperature dependence, T(1)(-1)(1/T). The two systems have different barrier heights and tunnelling frequencies, providing different perspectives on the tunnel resonance phenomena. The magnetic field dependence enables different spectral density components to be separately investigated and in the carboxylic acid, sorbic acid, concerted proton transfer in the hydrogen bonds is also identified at low field and low temperature. The methyl hindering barriers and the correlation times characterising the reorientational dynamics has been accurately determined in both materials. 相似文献
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Günter Voll 《Zeitschrift für Physik B Condensed Matter》1993,90(4):455-466
It is our goal to obtain a reliable prediction of the rotational tunnelling spectrum to be expected for a long chain of coupled one-dimensional quantum rotors. The problem is intractable by the simple methods used so far for up to three coupled methyl groups. Therefore, an efficient, nevertheless sufficiently precise method for solving the stationary Schrödinger equation of interacting methyl groups is developed first; it proves to be valid for a broad range of not too weak potential strengths. Then, three scenarios are investigated: they differ with respect to the relative strength of the single-rotor potential and the interaction potential. For each scenario, we illustrate the dependence of the energy level scheme on the number of coupled groups. For strong coupling and weak single-particle potential, the characteristic features of the energy level scheme of interacting methyls are most clearly observable: For as few as four coupled methyl groups we predict tunnelling spectra which are hardly distinguishable from single-methyl spectra. However, the collective behaviour is still important for the value of the tunnelling splitting. Therefore, the interpretation of such a spectrum in terms of single-methyl tunnelling is obvious but misleading with respect to the potential seen by a methyl group in the crystal. 相似文献
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A. Heidemann I. Anderson B. Jeffryes B. Alefeld 《Zeitschrift für Physik B Condensed Matter》1982,49(2):123-128
The elastic, quasielastic and inelastic structure factors for neutron scattering on CH3 groups tunnelling in a sinusoidal hindering potential with threefold symmetry are calculated as a function of the momentum transferQ. A comparison is made with data on methyl groups in nitromethane obtained with high resolution inelastic neutron scattering experiments at high momentum transfer. 相似文献
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The proton spin-lattice relaxation times, T1, of methyl groups in (CH3CO)2O, CH3COCl, CH3 COBr and (CH3)2S2 have been measured below melting points at 52 MHz. The observed T1 minima display the presence of tunnelling rotation. From the fit of the experimental results the ground, the first excited state tunnelling frequencies and the energy difference between the ground and the first excited states of the compounds have been estimated. 相似文献
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《Physica B+C》1978,93(3):344-348
The lock-in detected proton NMR-signals of methyl groups in solids is considered at arbitrary temperatures of the spin system. In the high temperature approximation the results reduce to those of Goldman and others, extended with coupled spin-lattice relaxation equations. At low temperatures of the tunnelling system some quite unusual features are to be expected, due to spin symmetry conversion causing time- as well as temperature-dependent lineshape and deviation of the magnetization from the Curie law. In particular, the lock-in detected signals in the absorption and dispersion modes are discussed. 相似文献
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B E Hammer 《Magnetic resonance imaging》1989,7(2):235-240
Proton decoupled 13C images were obtained at 2.1 Tesla. 13C[1H] images showed an increase in sensitivity over nondecoupled 13C images because of the nuclear Overhauser effect and elimination of multiple lines from scalar 13C-1H spin-spin couplings. The improvement in S/N for 13C[1H] images was smaller than expected because of a significant decrease in decoupling efficiency when 13C spin echoes were acquired in a readout gradient. Images of 13C compounds that had a wide range of chemical shifts showed separated and/or overlapping images, which is consistent with chemical shift imaging artifacts seen in 1H images. This work examines the technical constraints of acquiring and the difficulties of interpreting 13C[1H] images. 相似文献
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黄酮类化合物-O-甙的13C NMR谱 总被引:1,自引:0,他引:1
概述了黄酮类化合物-O-甙(包括双糖甙及寡糖甙)的13C NMR谱鉴定中涉及的溶剂选择,糖的数目与鉴定,糖的端基信号与甙化位移,甙元的鉴定,双糖及寡糖甙中各单糖的连接次序,以及糖链上的酰基取代等. 相似文献
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N. Makhiyanov 《Journal of Applied Spectroscopy》1994,61(1-2):509-513
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Gestodene是避孕药物18甲基炔诺酮的衍生物,动物实验表明它的孕激素活性为18甲基炔诺酮的3~5倍.本文采用各种2D NMR技术,包括:1H-1H COSY,1H-13C COSY和HMBC等,归属了它的1H和13C的谱线,并得到了有关质子间的偶合常数. 相似文献
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报道了11种新的3-卤代喹啉的13C NMR谱.应用13C NMR等谱确定了这11种新化合物的分子结构,并对全部谱峰进行了归属,初步探讨了分子结构对13C NMR化学位移的影响. 相似文献
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M. Kanowski H. Werner R. Schlögl H. -M. Vieth K. Lüders 《Applied magnetic resonance》1995,8(1):173-180
Results of13C MAS NMR measurements of the Rb x C60 system (x=2.75, 3, 4, 6) and the A6C60 compounds (A=K, Rb, Cs) are presented. Special attention was paid to sample preparation in order to suppress effects of impurities and lattice defects due to imperfect C60 starting material. The13C MAS NMR measurements of the Rb x C60 system demonstrate the usefulness of this method to reveal valuable information about its phase diagram. The existence of underdoped Rb3C60 is proved. Well resolved lines in all investigated A6C60 compounds confirm the orientational order of the C60 ions. An assignment of the signals to the three magnetically inequivalent carbon atom positions in the crystal structure is proposed. 相似文献
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《Journal of Magnetic Resonance (1969)》1984,56(1):1-9
The application of the JCH-modulated 13C spin echo is extended to chloroalkanes. One-bond coupling constants of protonated carbons are increased significantly by chlorine substitution. Nulling of resonance signals from CHx and chlorine substituted carbons is observed at selected echo delays. Assignments in 13C spectra of mixtures of chloroalkanes are derived from subspectra recorded at suitably chosen echo delays. 相似文献
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The 1H and 13C nmr assignments for all hydrogen and carbon atoms were made for β and α arteethers (5 and 6) based on chemical shift theory and 2D-nmr techniques (COSY and HETCOR). 相似文献