Transverse nuclear relaxation time

The disadvantage of the fast echo decay in two-pulse ESEEM can be circumvented with the three-pulse ESEEM experiment shown in Figure 5b. In this pulse sequence the first two nil pulses create nuclear coherence that develops during the evolution time T and decays with the transverse nuclear relaxation time 72n which is usually much longer than the corresponding relaxation time 7m of the electrons. The third nJl pulse transfers the nuclear coherence back to observable electron coherence. The modulation of the stimulated echo is given by... 

LiCl and LiBr in mixed aqueous-methanol solutions Chemical shifts and transverse nuclear relaxation times correlation with viscosity data 94... 

Relaxation is an inherent property of all nuclear spins. There are two predominant types of relaxation processes in NMR of liquids. These relaxation processes are denoted by the longitudinal (Ti) and transverse (T2) relaxation time constants. When a sample is excited from its thermal equihbrium with an RF pulse, its tendency is to relax back to its Boltzmann distribution. The amount of time to re-equilibrate is typically on the order of seconds to minutes. T, and T2 relaxation processes operate simultaneously. The recovery of magnetization to the equilibrium state along the z-axis is longitudinal or the 7 relaxation time. The loss of coherence of the ensemble of excited spins (uniform distribution) in the x-, y-plane following the completion of a pulse is transverse or T2... 

C-NMR relaxation parameters may also be employed to access the rates of overall and internal motions for saccharide molecules.230 Longitudinal and transversal heteronuclear relaxation times as well as hetero-nuclear NOEs depend on the molecular motion of the molecule, including overall and internal motions. Thus, careful analysis of these parameters can be employed to demonstrate the presence of conformational heter-eogeneity and/or dynamics, as well as the restriction to motion and the timescale of the existing fluctuations. Different examples of application of this methodology to trisaccharides,231 232 tetrasaccharides,233 pentasaccharides,234 and polysaccharides235,236 have been described. 

In Eqs. (4)-(7) S is the electron spin quantum number, jh the proton nuclear magnetogyric ratio, g and p the electronic g factor and Bohr magneton, respectively. r//is the distance between the metal ion and the protons of the coordinated water molecules, (Oh and cos the proton and electron Larmor frequencies, respectively, and Xr is the reorientational correlation time. The longitudinal and transverse electron spin relaxation times, Tig and T2g, are frequency dependent according to Eqs. (6) and (7), and characterized by the correlation time of the modulation of the zero-field splitting (x ) and the mean-square zero-field-splitting energy (A. The limits and the approximations inherent to the equations above are discussed in detail in the previous two chapters. 

There is, in addition, another generally different time required to specify the radiofrequency behavior of the nuclear spin system. This time is called the transverse relaxation time and is the time constant for the exponential decay of the transverse (x and y) components of nuclear magnetization M- and M ... 

Nuclear Magnetic Resonance (NMR) Spectroscopy. Longitudinal and transverse relaxation times (Ti and T2) of 1H and 23Na in the water-polyelectrolytes systems were measured using a Nicolet FT-NMR, model NT-200WB. T2 was measured by the Meiboom-Gill variant of the Carr-Purcell method (5). However, in the case of very rapid relaxation, the free induction decay (FID) method was applied. The sample temperature was changed from 30 to —70°C with the assistance of the 1180 system. The accuracy of the temperature control was 0.5°C. 

Nuclear magnetic resonance (NMR) has been used to study segmental motions in block copolymer solutions. The mobility of protons in polymer chains in dilute solutions has been probed using high-resolution H NMR. Association of chains into micelles leads to a reduction in mobility in the core, which leads to a broadening of the respective NMR lines that has been studied for a number of systems, as described by Tuzar and Kratochvil (1993). The sol-gel transition in concentrated solutions has been located via ]H transverse relaxation time experiments, as outlined in Chapter 4. 

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