Lineshape and motion

Additional evidence for the applicability of the above data interpretation is obtained by using Equation 3 to predict values of l/(Tlp) min. These are 2.8 X 104 sec-1 and 6.4 X 103 sec-1 for H = 4.7 G and 20.6 G, respectively, and are consistent with the T p and T2 data. The motional properties reported here differ significantly from previous measurements the activation energy obtained from a temperature above 120°C is two to three times the values obtained in Ref. 13, and motional narrowing of the lineshape occurs at a significantly higher temperature than that in Ref. 12. 

It is a pleasure for me to introduce Volume 43 of Annual Reports on NMR Spectroscopy and to express my thanks to the reporters for their pellucid and refulgent accounts of NMR activities in a number of areas of molecular science. The volume commences with a report on Solid-State NMR Studies of Molecular Motion by M. J. Duer this is followed by a contribution from J. Higinbotham and I. Marshall on NMR Lineshapes and Lineshape Fitting Procedures the final chapter is on Recent Progress in Solid-State NMR of Low-7 Nuclei by M. E. Smith. My gratitude to the production staff at Academic Press (London) for their meticulous efforts in the realization of this, and other volumes in the series, is hereby recorded. 

When the H- H dipole-dipole interaction can be measured for a specific pair of H nuclei, studies of the temperature dependence of both the H NMR line-shape and the H NMR relaxation provide a powerful way of probing the molecular dynamics, even in very low temperature regimes at which the dynamics often exhibit quantum tunnelling behaviour. In such cases, H NMR can be superior to quasielastic neutron scattering experiments in terms of both practicality and resolution. The experimental analysis can be made even more informative by carrying out H NMR measurements on single crystal samples. In principle, studies of both the H NMR lineshape and relaxation properties can be used to derive correlation times (rc) for the motion in practice, however, spin-lattice relaxation time (T measurements are more often used to measure rc as they are sensitive to the effects of motion over considerably wider temperature ranges. 

For the partially deuterated benzoic acid (C6D5COOH), the solid state H NMR spectrum is dominated by the intra-dimer H- H dipole-dipole interaction. In a single crystal, both tautomers A and B are characterised by a well-defined interproton vector with respect to the direction of the magnetic field (Fig. 1). Proton motion modulates the H- H dipole-dipole interactions, which in turn affects the H NMR lineshape and the spin-lattice relaxation time. It has been shown that spin-lattice relaxation times are sensitive to the proton dynamics over the temperature range from 10 K to 300 K, and at low temperatures incoherent quantum tunnelling characterises the proton dynamics. A dipolar splitting of about 16 kHz is observed at 20 K. From the orientation dependence of the dipolar splitting, the... 

Besides the initial object of CARS as a tool for the determination of molecular energy levels and its application to combustion studies and related areas, there are several other scientific applications of academic interests. These include spectral lineshape studies, collisional effects, Dicke narrowing, nonadditivity effects, and motional narrowing. CARS turned out to be an excellent method for these studies (Berger et al., 1992 and references therein). 

The motion actually observed by i.r. absorption is a small part of the whole pattern, since for effective absorption one must match both frequency and wavelength in the electromagnetic and crystal osdllations. Absorption occurs essentially at the frequency of the longest waves present in the crystal These waves are damped by anharmonic interaction with the numerous short-wave modes, except at very low temperatures where interaction with ctystal ddects and surfaces takes over. In spite of the complexity of the process one has to expect a simple lineshape and a fairly simple temperature dependence for a wave propagating in one crystal direction. 

Similar to fluorescence depolarization and NMR, two limiting cases exist in which the molecular motion becomes too slow or too fast to further effect the ESR lineshape (Fig. 8) (35). At the fast motion limit, one can observe a narrow triplet centered around the average g value igxx + gyy + giz with a distance between lines of aiso = Axx- -Ayy- -A2,z)l3, where gu and Ajj are principal values of the g-tensor and the hyperflne splitting tensor A, respectively. At the slow motion limit, which is also referred to as the rigid limit, the spectrum (shown in Fig. 8) is a simple superposition of spectra for all possible spatial orientations of the nitroxide with no evidence of any motional effects. Between these limits, the analysis of the ESR lineshape and spectral simulations, which are based on the Stochastic Liouville Equation, provide ample information on lipid/protein dynamics and ordering in the membrane (36). 

M. J. Duer, Solid-State NMR Studies of Molecular Motion , p. 1 J. Higinbotham and I. Marshall, NMR Lineshapes and Lineshape Fitting Procedures , p. 59... 

Fig. 10. Simulated LG-CP dipolar powder lineshapes for motions with different effective asymmetry, as labelled, and effective anisotropy An.1 5...

Fig. 25. Random walk simulations for static 2H NMR powder lineshapes arising from a quadrupole echo 90°x-t-90°v-t-FID pulse sequence for the model of an isotropic 3° jump.36 (a) Jump correlation time, tj = 411 gs correlation time for the motion, xc = 100 ms, echo delays x as given in the figure. Dotted line is the spectrum for an isotropic random jump with xj = xc = 100 ms and an echo delay x — 200 gs. (b) Jump correlation times xj and motional correlation times xc as given in the figure, echo delay x = 100 gs.

The NMR lineshape readily reveals molecular anisotropy and motional averaging at a segmental level. It provides information about fast and intermediate dynamics, and about molecular order (see Section 6.2.5). 

In a miscible blend, a specific interaction would influence chemical shifts and/or lineshapes of component polymers (see Section 10.2.2.1). Molecular motion is also affected by the interpolymer interaction, and is investigated by spin-lattice relaxation, and lineshapes and 2D exchange NMR (see Section 10.2.2.2). 

Shown in Figure 3 are EPR spectra of spin label FDNP-SL in solution and bound to anti-DNP antibody. The more restricted motion of antibody-bound antigen results in broader lineshapes and a larger maximum peak-to-peak splitting, termed A. The three-... 

Bruno G and Freed J 1974 ESR lineshapes and saturation in the slow motional region ELDOR Chem. Phys. Lett, pp 328-32... 

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