NMR second moment

It is curious that the chair- boat problem, which is most associated with small, liquid-state molecules, arises in the context of solid-state research (B3, II). Although the paucity of useful experiments militates against a definitive solution here E3), the frequency independence of the NMR second moment (E2), the absence of an observable free-induc-tion decay (Tj <25 fis) in the pulsed NMR spectrum (El), and the smoothness of the absorption mode itself (SI), all argue against the... 

Slichter, (1959) found that a narrowing of the proton resonance line width occurred 10—20° C above Tg in the case of polyisobutylene and natural rubber but closely at Tg in the case of atactic polypropylene. The two room temperature transitions in polytetrafluoroethylene, which are so clearly visible in the specific heat-temperature curve, Fig. 12, were found by Slichter (1958a) in a fluorine nucleus magnetic resonance study to cause a drop in the second moment with rise of temperature, but the whole effect occurred over the wide temperature range of 225 to 320° K. The slight bulge" in the specific heat-temperature curve of polymethyl methacrylate from 130 to 180° K, Fig. 13, might possibly be correlated with the drop in the NMR second moment between 150° and 200° K found by Powles (1956). 

An appropriate analysis of the dynamics of the orientational disorder of the cations can be performed by means of proton NMR second-moment studies [53-55]. Such studies have established in the case of TEA(TCNQ)2 that the cation disorder is dynamic above 270 K while it is static below 205 K, with some transitional regime in between [53,54]. The wide transition of TEA(TCNQ)2 at about 210 K may then be considered to be a continuous structural transition from a state of dynamic disorder at high T to a state... 

The rotational motion of the C5H5 rings about their coordination axis in crystals of several metallocenes was suggested many years ago by Anderson [35] on the basis of the temperature dependence of the NMR second moment. Subsequently, several studies [36-43] have been devoted to the determination of the reorientational barrier of Cp and other ji-bonded cyclic ligands in organometallic systems. Actually the measurements of the temperature dependence of H T was preferred as it allows the exploitation of a wider temperature range with respect to the second moment [44]. 

A parameter that has been widely used in the study of the structure and thermal properties of solid organic materials (13-16) and particularly coals (14-16) is the NMR second moment. To compute such a parameter from our solid echo data requires its Fourier transformation from the time domain to obtain a complex frequency domain NMR spectrum, i.e. g (v) u (v) + iv (v). The quadrature components of this spectrum represent linear combinations of the pure absorptive (u(v)) and dispersive (v(v)) modes of the true spectrum, i.e,... 

Biomedical Applications. Spin-lattice relaxation times Ti and Tid as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B-2) in the temperature range 55-350 K. The broad and flat T1 minimum observed at low temperatures is attributed to the motion of two non-equivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt s theory, which takes into account the tunneling assisted relaxation. 

Nuclear magnetic resonance data on cyclohexane are reproduced together with heat capacity information in Fig. 3.2. The transition parameters are listed in Table 3.1. Below 150 K the experimental proton NMR second moment of 26.0 + 0.5 G corresponds to that calculated for a crystal of rigid molecules of Djj dymmetry in the chair conformation. The decrease in secoixi moment from 155 to 180 K is caused by jump-reorientation about the triad axis with a 46 kJ/mol activation energy. The experimental second moment somewhat below T of 6.4 G corresponds to the calculated value of 6.1 l.OG for such motion. At the transition the ond moment drops to 1.4 G which is in line with additional reorientation about aU other axes (1.3 to 1.1 G calculated for different assumptions). Above 240 K,... 

Fig. 4.10. Thermal analysis and proton NMR second moment curves of trans-l,4-polybutadiene. The heat capacity for the solid (glass and low temperature crystal form I) have been computed by fitting at low temperature to an approximate skeletal vibration spectrum. The gradual decrease of the second moment of the proton resonance spectrum below T comes mainly from increasing mobility in the amorphous phase, Ref. The melting curve is of an approximately 90% crystalline sample. Ref.. Compare also to Fig. 1.3...

Arndt R (1965) NMR second moment of solid cyclododecane, J. Chem. Phys, 43 1885... 

We can see that the NMR second moment depends on quantities such as cos j i and cos i. It is therefore easy to appreciate, in the spirit of the discussion of Raman spectroscopy above, that the second moments will define orientation functions cos 0 as well as cos d. 

In the past, this has been studied semi-quantitatively by means of NMR second moments and NMR quadrupole echoes (5,6,7). Here we show that at high temperatures ( -iyOK) and NMR lineshapes can be obtained with sharp, well-defined features, so that quadrupole coupling or nuclear shielding (chemical shift) tensor components can be measured directly to yield information on guest molecule orientation in the clathrate hydrate cages. 

The viscoelastic behaviour of the majority amorphous phase is important in terms of the mechanical properties of the material. Dynamic mechanical (S) and dielectric studies (8) reveal three relaxations, labelled a, p and 7 in order of decreasing temperature. Variations in the strength of these relaxations with systematic changes in the mole fraction of each component have led to the association of the 7 relaxation with the HBA component and the p relaxation with the HNA component, with the a relaxation displaying features typical of a glass transition process. Support for these assignments has been obtained from analysis of the proton NMR second moments (9,10),... 

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