Transverse spin relaxation

Accurate measurements of the frequency-resolved transverse spin relaxation T2) of Rb NMR on single crystals of D-RADP-x (x = 0.20, 0.25, 0.30, 0.35) have been performed in a Bq field of 7 Tesla as a function of temperature. The probe head was placed in a He gas-flow cryostat with a temperature stability of 0.1 K. To obtain the spin echo of the Rb - 1/2 -o-+ 1/2 central transition we have used the standard (90 - fi - 180y -ti echo - (2) pulse sequence with an appropriate phase-cycling scheme to ehminate quadrature detection errors and unwanted coherences due to pulse imperfections. To avoid sparking in the He gas, the RF-field Bi had to be reduced to a level where the 7T/2-pulse length T90 equalled 3.5 ps at room temperature. 

Measurements of i N iH -NOEs, and longitudinal and transverse spin relaxation times can provide supplementary information on the globular PrP domain in the form of data on the internal mobility along the polypeptide chain (Peng and Wagner, 1992). Furthermore, realtime... 

In liquid crystals the proton resonance line width and the related effective transverse spin relaxation time T2 depend on the angle 9 between the magnetic field and the director n, which is described approximately... 

In this case, no meaningful information is available from the featureless NMR signals. On the contrary, better spectral resolution has been achieved by CP-MAS and DD-MAS NMR spectra of [3- C]Ala- or [l- C]Val-labeled bR from in which problems arising from dense spin networks of directly bonded pairs that cause such accelerated transverse spin relaxation rate are... 

Surface-Induced Order Detected by DNMR The situation is different with the transverse spin relaxation rate Tr... 

Another way to detect slow motions is by deuteron transverse spin relaxation experiments using a modified Carr-Purcell-Meiboom-Gill spin echo pulse sequence [42-44]. Information about the dynamics of liquid crystals can be retrieved from the frequency dispersion (see Fig. 2) and the anisotropy of relaxation rates. 

The ESEEM spectrum is substantially better resolved for stimulated echoes than for the two-pulse echo, because it does not contain combination frequencies (sums and differences of the basic resonance frequencies), and the lines are not broadened by fast transverse spin relaxation. Thus stimulated ESEEM is the more preferable for studying electron-nuclear interactions. 

Spin-spin relaxation is the steady decay of transverse magnetisation (phase coherence of nuclear spins) produced by the NMR excitation where there is perfect homogeneity of the magnetic field. It is evident in the shape of the FID (/fee induction decay), as the exponential decay to zero of the transverse magnetisation produced in the pulsed NMR experiment. The Fourier transformation of the FID signal (time domain) gives the FT NMR spectrum (frequency domain, Fig. 1.7). 

FID Free induction decay, decay of the induction (transverse magnetisation) back to equilibrium (transverse magnetisation zero) due to spin-spin relaxation, following excitation of a nuclear spin by a radio frequency pulse, in a way which is free from the influence of the radiofrequency field this signal (time-domain) is Fourier-transformed to the FT NMR spectrum (frequency domain)... 

During the Xi and x2 periods, the spin system experiences spin relaxation along the direction of and transverse to the magnetic field, respectively. 

There is a second relaxation process, called spin-spin (or transverse) relaxation, at a rate controlled by the spin-spin relaxation time T2. It governs the evolution of the xy magnetisation toward its equilibrium value, which is zero. In the fluid state with fast motion and extreme narrowing 7) and T2 are equal in the solid state with slow motion and full line broadening T2 becomes much shorter than 7). The so-called 180° pulse which inverts the spin population present immediately prior to the pulse is important for the accurate determination of T and the true T2 value. The spin-spin relaxation time calculated from the experimental line widths is called T2 the ideal NMR line shape is Lorentzian and its FWHH is controlled by T2. Unlike chemical shifts and spin-spin coupling constants, relaxation times are not directly related to molecular structure, but depend on molecular mobility. 

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... 

One of the possibilities is to replace the evolution of the single-quantum states by the multiple-quantum ones during extended evolution periods. As theoretically predicted by Griffey and Redfield [34] and experimentally demonstrated by Grzesiek and Bax [35], keeping a spin pair in the state of multiple-quantum coherence (MQ) eliminates most of the dipolar contribution to the spin-spin relaxation. With the X-H (X is a heteronucleus) spin pairs, this is partially offset by a higher rate of proton cross-relaxation in the transverse plane with the remote H spins. Since the MQ coherences consist of transverse... 

The measured spin relaxation parameters (longitudinal and transverse relaxation rates, Ri and P2> and heteronuclear steady-state NOE) are directly related to power spectral densities (SD). These spectral densities, J(w), are related via Fourier transformation with the corresponding correlation functions of reorientional motion. In the case of the backbone amide 15N nucleus, where the major sources of relaxation are dipolar interaction with directly bonded H and 15N CSA, the standard equations read [21] ... 

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