Spin alignment

In summary, the absolute entropies we calculate and tabulate are, in fact, not so absolute, since they do not include isotopic entropies of mixing nor nuclear spin alignment entropies. The entropies we tabulate are sometimes called practical absolute entropies. They can be used to correctly calculate AS for a chemical process, but they are not true" absolute entropies. 

Pulsed deuteron NMR is described, which has recently been developed to become a powerftd tool for studying molectdar order and dynamics in solid polymers. In drawn fibres the complete orientational distribution function for the polymer chains can be determined from the analysis of deuteron NMR line shapes. By analyzing the line shapes of 2H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. In addition, motional heterogeneities can be detected and the resulting distribution of correlation times can directly be determined. 

In polymers one will often particularly be interested in very slow dynamic processes. The solid echo technique just described is still limited by the transverse relaxation time T being of the order of a few ps at most. The ultimate limitation in every NMR experiment however, is not T but the longitudinal relaxation time T, which for 2H in solid polymers typically is much longer, being in the range 10 ms to 10 s. The spin alignment technique (20) circumvents transverse relaxation and is limited by Tx only, thus ultraslow motions become accessible of experiment. 

Fig. 9. Observed and calculated spin alignment spectra of solid HMT showing the influence of the tetrahedral jump motion for long mixing times t2 > rc. Note the strong influence of the evolution time r, on the line shape...

Fig. 10. Correlation times for the tetrahedral jump motion in solid HMT obtained from 2H line shape analysis and 2H spin alignment...

Fig. 12a and b. Calculated 2H spin alignment spectra for diffusive motion, a unrestricted rotational diffusion for different mixing times x2 b diffusion restricted to angular regions as indicated for long mixing times t2... 

Fig. 19. Experimental spin alignment decay curves of chain deuterated PS-d3 at temperatures above and below the glass transition for various evolution times t,. Note the different timescales of t2 at the different temperatures. The straight lines indicate the decays of the plateau values on the timescale of the spin-lattice relaxation time T,. Sample characterization Mw = 141000, Mw/Mn = 1.13, atactic...

Fig. 20. Experimental spin alignment spectra of chain deuterated PS-d3 above and below Tg corresponding to the decay curves shown in Fig. 19...

Note added in proof The calculated spin alignment spectra for diffusive motion plotted in Fig. 12 are incorrect, in particular the oscillations in the central part, due to a sign-error in the computer program. 

Specimen preparation 122 Spin alignment 33—35, 41 —43 —lattice relaxation 41... 

The compound is ferromagnetic below F( because the magnetization is higher. Above the Curie temperature, the compound is a simple paramagnet with randomly oriented spins, but below that temperature, the spins align and the magnetization increases. 

Figure 1.5 (a) Vector representation displaying a greater number of spins aligned... 

High-spin organic molecules and spin alignment in organic molecular assemblies. 26, 179 Homoaromaticity, 29, 273... 

Spectroscopic observations of alkylcarbonium ions in strong acid solutions, 4, 305 Spectroscopy, l3C NMR, in macromolecular systems of biochemical interest, 13,279 Spin alignment, in organic molecular assemblies, high-spin organic molecules and, 26,179 Spin trapping, 17, 1... 

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