Spin-lattice relaxation influence

In a second attempt to obtain more insight into the binding location of the dienophile and now also the diene, we have made use of the influence of paramagnetic ions on the spin-lattice relaxation rates of species in their proximity. Qose to these ions the spin-lattice relaxation rate is dramatically enhanced. This effect is highly distance-dependent as is expressed by Equation 5.7, describing the spin-lattice... 

Strong evidence of the dominant Influence of molecular conformation on the properties of coals Is Implicit In the several data sets which show an extremum In the measured property when plotted against carbon rank. Examples are the extrema which occur In the solid state properties of mass density (22,23) and proton spin-lattice relaxation rate (24) as well as In solvent swelling and extractablllty ( ). 

The results that have been obtained indicate that the major influence of the crystalline regions on segmental motions, and hence to the structure of the non-crystalline regions, is in the linewidth and T2. The different morphologies are reflected in different values of T2- The segmental motions in long chain molecules which exert major influence on the spin-lattice relaxation times and the nuclear Overhauser enhancements are not in general the same motions which determine the resonant linewidth. 

The spin-lattice relaxation process is usually exponential. Theoretically, the effect of spin-diffusion, characterized by the coefficient D (order of 1(T12 cm2 s 1), has an influence on T, relaxation times when ix > L2/D, where Lis the diffusion path length. NMR studies of model systems f6r rubber networks, based on a styrene-butadiene-styrene block copolymer (SBSy, in which styrene blocks act as a crosslink for polybutadiene rubber segments of known and uniform length, indicate that spin diffusion operating between PS and PB phases causes a lowering of Tg for the PS component in SBS (as compared to the pure PS) and hindering of the motion of the PB component (as compared to the pure PB)51). 

Any change in the medium, i.e. the solvent, the concentration, the pH value, and in the temperature will affect the mobility of the molecules and hence also the spin-lattice relaxation. However, few systematic studies have so far been performed on the concentration dependence or on the precise influence of the macroscopic viscosity, the main reason, in the case of 13C, lying in the need for highly protracted measurements in concentration studies. Moreover, little is known about the pH dependence of 13C relaxation [188], Nevertheless, the concentration dependence of 13C relaxation is apparent in the case of saccharose (Table 3.20) [166], and intramolecular hydrogen bonds can be detected by measuring the concentration dependence of Tx [189]. 

Fig. 4.15. Inversion-recovery experiment for carbon-13 7, determination or2,2 -bipyridine (400 mg in 1 mL hexadeuteriobenzene, 30 °C 15.08 MHz 16 scans for a single experiment [73 i]). The principal axis of (the fastest) molecular rotation passes C-2 (2 ) and C-5 (5 ). This rotation is too fast for optimum dipolar relaxation of C-3, C-4, and C-6 but does not influence the C-5-H bond which is affected by rotation about other axes. These rotations are slower and more effectively contribute to dipolar spin-lattice relaxation of C-5 (5 ) according to Section 3.3.3.3. To conclude, C-5 (5 ) relaxes faster (3 s) than all other CH carbons (6 s) and can be clearly distinguished from C-3 (3 ) with similar shift.

Carbon-13 spin-lattice relaxation times TL (Section 3.3) are relatively insensitive to the chain length of polymers [531]. The influence of local segmental motions predominates, as shown for low-density polyethylenes in which Tx values are one to two seconds for the main chain but up to seven seconds for peripheral side-chain carbon nuclei at 120 C [532] due to segmental mobility (Section 3.3.3.4). To conclude, quantitative evaluation of polymer carbon-13 spectra as necessary for side-chain determination requires the knowledge of spin-lattice relaxation times. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

From Eq. (16.8), it is noted that the line width (AH) is inversely proportional to the relaxation time. It should be emphasized that under normal circumstances (system not saturated), it is T2 and not 7, which determines the intrinsic line width. Just like spin-lattice relaxation time (7)), T2 is also influenced by temperature, because decreasing the temperature results in larger T2 and, consequently, smaller line-width values. This means, in several situations, that EPR spectra obtained at lower temperatures are better resolved, for example, at N2 (77 K) or He (4K) liquid temperatures. 

INFLUENCE OF HYDROGEN ORDERING ON THE PROTON SPIN-LATTICE RELAXATION TIME IN LANTHANUM SUPERSTOICHIOMETRIC DIHYDRIDES LaH2+c... 

Ratishvili I.G., Namoradze N.Z. Influence of hydrogen ordering on the spin-lattice relaxation time in metal hydrides I. The high-temperature limit". Bull. Georg. Acad. Sciences. (2004), 169 (2), 279-281. II. The low-temperature limit", ibid, (2004), 169 (3), 481- 483. 

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