Tj relaxation

Figure 4-6 illustrates the relaxational eontribution to the motion. Figure 4-6A shows moment vectors for a spin system in the absenee of the rf field (Hi = 0) the magnetization eomponents are = Mq, = 0, My = 0, beeause in the xy plane the magnetization eomponents caneel. In the presenee of the rf field at the resonanee frequency the spin system absorbs energy, increasing the angle between Ho and M and perturbing the thermal equilibrium so that and My components are induced and M < Mo (Fig. 4-6B). With the passage of time (comparable to the relaxation times Tj and Tj), relaxation back to the equilibrium configuration takes place, so M. increases toward Mo, whereas nd My decrease toward zero as a consequence of the gradual loss of coherence of the moment vectors.

The variation in the Tj relaxation times of the individual carbons can be minimized by keeping the time between excitation and detection constant (Radeglia and Porzel, 1984). 

Under similar conditions (-80°C, CD2C12) H2 also reacts with 3 to give bis dihydrogen complex [IrH2(H2)2(PPh3)2]+ this is detected by H-NMR spectroscopy, including Tj relaxation measurements. This loses H2 at 0°C when the H2 is removed, to form the dinuclear hydride of Eq. (5). 

Differential relaxation of in-phase and anti-phase operators involving a spin C [10], which are due to additional Tj relaxation effects active only for the anti-phase components and which depend on the geometry of the spin system, can lead to systematic errors of the coupling constant derived from cross-peak multiplets observed in an E. COSY-type experiment [11]. Since these errors depend for a given differential relaxation rate Ap on the frequency difference of the coherences with C in the a or yS state, according to Eq. (1) a remedy to the problem is to maximize the relevant J such that the condition J 3> Ap/2n is fulfilled ... 

NMR spectroscopy provides spin-lattice (ri) and spin-spin (Tj) relaxation times. Making appropriate assumptions with regard to the magnetic interactions responsible for the relaxation process, these relaxation times can be related to molecular motions. Since nuclear spin relaxation results from all processes which cause a fluctuation in the magnetic field at the nucleus, the correlation function will generally correspond to more than one kind of motion involving all possible interactions. The equations for the relaxation times are generally of the form... 

Fig. 6. Proton Tj relaxation data for crosslinked polybutadiene samples with average number of repeat units per network chain 44 (A) and 14 (O), compared with the computed results from the modified BPP equation (assuming Gaussian distribution and the model based bn spin diffusion to locations of rapid spin-lattice relaxation) (reprinted from Ref.541 with permission)...

The time it takes for the temperature of a sample to stabilise in an NMR probe can also be determined in real-time XH NMR Tj relaxation experiments, because at vulcanisation temperatures Tj is only slightly affected by a moderate crosslink density [22, 180, 181]. The T2 data obtained in the aforementioned experiments were used for determining an increase in the density of chemical crosslinks upon vulcanisation time (Figure 10.18) [179]. The method can also be used in kinetic studies of the vulcanisation of filled and oil-extended rubbers. 

Figure 23 The response of a hole burned into the amide I band of scyllatoxin by a narrowband pump pulse (width 10 cm-1, the spectrum of the pump pulse shown as thin solid line) as a function of the delay time between pump and probe pulse (thicker solid line 0 ps thick dashed line 2.4). Vibrational Tj relaxation was compensated for in this plot by scaling the 2.4 ps spectrum in order to facilitate a direct comparison with the time zero signal.

Fig. 26. Correlation of Tj relaxation time ( H NMR) in seconds with the logarithm of the corresponding number, a of residual dichloromethane molecules per phenyl group of (Sty)98(DVB)2...

Paramagnetic spin labels offer the opportunity to screen for binding sites remote from a known binding site. ° The introduction of these labels increases the Tj-relaxation rate of binding components and therefore results in an observable... 

Fig. 2. The temperature dependence of a rate of the Tj relaxation for chain units inside, (T (O), and outside, (Tyy ( ), the adsorption layer for a solution blend containing (A) 60 vol% of hydrophilic Aerosil (300 g" ) (336 phr) [8], and...

Tje = multi-frequency EPR measurement of line widths geg = multi-frequency EPR measurement of geffectivel NMR = measurement of transverse relaxation of O H NMRD = measurement of proton Tj relaxation dispersion with relaxometer. 

An extended discussion of the behavior of redox system in non-Debye solvents has been given recently [169]. These problems were further discussed by Baranski et al. [188] in their work on the oxidation of ferrocene at a Pt microelectrode in several alcohols in the temperature range 190- 295 K. One should remember that the structure of such non-Debye solvents, which is related to the large-amplitude Tj relaxations, may be changed considerably [3] under the influence of ions, and also at the charged electrode surface. 

When the RF pulse is switched off, M realigns with the Z axis at a rate (1/Tj) slower than the rate of signal loss (l/Tj). In order to do so, M must transfer the energy gained to the surrounding molecules. This process is referred to as relaxation (spin-lattice relaxation). As Tj relaxation occurs, M returns to equilibrium (M,). 

Broad resonance lines, accompanied by Tj relaxation times much longer than the T2 relaxation times, are indications that the fluorine nucleus is in a restricted-mobility environment [14, 20, 21, 71]. The specific MR characteristics of a compound in its in vivo state must be evaluated and the acquisition and analysis adjusted to account for the values of these characteristics prior to performing human 19F spectroscopy studies. Effective T2 values (T2 ) that are shorter than 2 ms have spectra that are difficult to measure using whole-body MR systems. In general, acquisition of spectra that contain short T2 -value species require the use of short (<0.5 ms) excitation RF pulses and careful attention... 

Accounting for different T2 and T relaxation times of the different resonances corrects for signal loss due to incomplete Tj relaxation of the fluorine spins. This step is required if the Tj of the compound and reference differ and if the TR is less than 5 multiples of the longest Tj value. In addition, signal from short-T2 species require correction for signal loss owing to the time between the midpoint of the excitation pulse and the start of the first useful detection time point. 

The use of the low-field NMR spin-lattice relaxation technique has recently been successfully demonstrated [75-80] as a pore structure tool for porous materials saturated with a solvent (usually water). The basic principle is that the portion of pore fluid near a pore wall undergoes spin-lattice (Tj) and spin-spin (Tj) relaxation in a magnetic field at a faster rate than the bulk fluid. This, coupled with the fast diffusional exchange of fluid between regions within... 

Observable NMR spectral parameters such as chemical shift, spin-spin coupling, and peak intensities in simple ID spectra allow one to obtain polymer composition, tacticity, sequence distribution, and mechanism of polymerization. However, the poor mobility of polymers often leads to high viscosity solutions and rapid Tj relaxation, resulting in the poor spectral resolution. The viscosity can be decreased by dilution and high measurement temperature. Fortunately, many polymers possess segmental mobility, which facilitates the observation of high-resolution spectra with the line widths in the range of 1-10 Hz. 

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