Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Spin Relaxation Processes in a Two‐Proton System Undergoing Anisotropic Reorientation

Abstract: The spin‐lattice relaxation time T1 and the spin‐spin relaxation time T2 for two identical spins I=½ have been calculated for anisotropic reorientation in which the spin pair reorients randomly about an axis which, in turn, tumbles randomly. The results are applicable to liquids and solids provided that the correlation time for tumbling of the axis is small compared to T2. Although the two types of motion are independent, their contributions to relaxation are not. For nonviscous liquids, T1=T2. The results are generalized to multispin systems.

Transition ion electron paramagnetic resonance

Abstract: Part 1 Introduction to electron paragmanetic resonance: EPR due to mainly two-level systems some transition-ion EPR spectra crystal fields and ligand fields. Part 2 Crystal and ligand fields and the spin Hamiltonian: review of atomic theory superposition model (SPM) molecular orbital theories the Jahn-Teller effect. Part 3 Spin Hamiltonian parameters: line broadening with fine structure resolution or ambiguities. Part 4 Experimental methods in CW-EPR: EPR spectrometers principles of design choice of microwave frequency low temperature EPR high temperatures computers in EPR. Part 5 Linewidths and computer simulation in CW-EPR: power and power-like EPR spectra quantitative spin determination statistical models of line broadening. Part 5 Representative examples of transition-metal ion EPR: planar microcycles oxygen co-ordination semiconductors zeolites and other catalysts irradiated complexes. Part 7 Coupled paramagnetic systems: dipolar coupled dimer complexes metal-nitroxy and metal-radical interactions. Part 8 Paramagnetic relaxation: classical rate theory measurement of relaxation times. Part 9 Double resonance: introduciton to the theory of ENDOR. Part 10 Electron spin echo modulation: electron spin echo ESE experimental model calculations for ESEEM. Part 11 Biological applciations of transition-ion EPR: mitochondrial electron transport chain molybdenum-containing enzymes paramagnetic metal-substituted enzymes. Part 12 Zero-field EPR: introductory theory of zero-field resonance examples of ZFR. (part contents)

Time Saving in Measurement of NMR and EPR Relaxation Times

Abstract: By producing a train of absorption or dispersion signals (continuous‐wave magnetic resonance) or free induction decays (pulsed magnetic resonance) it is possible to save time in spin‐lattice relaxation measurements due to the fact that it is not necessary to wait for equilibrium magnetization before initiating the train. The relaxation time may be calculated from the train according to a simple rapidly converging iteration.

Parahydrogen and synthesis allow dramatically enhanced nuclear alignment

Abstract: The PASADENA effect is a method for transient high-sensitivity proton spin-labelling by molecular addition of dihydrogen. When the parahydrogen mole fraction differs from the high-temperature limit of 1/4, this population difference constitutes a form of spin order which can be converted to magnetization observable by NMR. Large NMR signals are observed, if subsequent to the hydrogen addition, the two protons experience magnetic inequivalence and spin-spin coupling and if observation is made before spin-lattice relaxation restores the equilibrium spin order. The analogous effect for D2 is also possible. The kinetic mechanisms of the homogeneous hydrogenation catalysts which permit the realization of the PASADENA effect have been the target of the experimental applications. The enhancement of the NMR transitions has facilitated the determination of true molecular rate constants. Ordinarily, the activity of a catalyst is assessed by dividing the observed rate by the total catalyst concentration. However, the question as to whether most of the catalytic rate is due to a tiny fraction of active species or a large fraction with a relatively low molecular rate is not clearly addressed by such an analysis. This ambiguity is entirely avoided in the PASADENA studies, since only active catalyst molecules can contribute to the enhanced signals from which all kinetic inferences are made. The sensitivity enhancement has also led to the identification of a novel intermediate in the mechanism for the Rh(DIPHOS)+ catalyzed hydrogenation of styrene. The rate of conversion of this species into product and starting material has been studied using two-dimensional NMR. The dramatically improved sensitivity should make it possible to observe key catalytic intermediates which do not build up in sufficient quantity to allow detection by conventional NMR arising from Curie-Law magnetization. The study of surface sites which bind pairwise with H2 is also a potentially fruitful area for future experimental work. The ambient temperature NMR spectroscopy of surfaces is not often feasible due to sensitivity limitations. Simulations have been performed using typical shift and coupling parameters in an effort to characterize the enhanced lineshapes which can be expected. The inverse of the PASADENA effect has also been proposed, whereby the spin order of a molecule containing hydrogen is probed by measuring the branching ratio to ortho and para dihydrogen. This RAYMOND phenomenon (radiowave application yields modulated ortho number desorbed) has the potential for measuring precursor NMR with extraordinary sensitivity, since it finesses the need for detection of radiowaves.