Quadrupole echo

Morrow et al. measured the spin-lattice relaxation time Ti and quadrupole echo decay times T ) of headgroup deuterated d4-DMPC as a function of temperature and pressure to yield additional information about changes in the headgroup dynamics. Generally, motions in a LC phospholipid bilayer can... 

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

MAS = H-MAS NMR spectroscopy at 298 K, ADLF = nuclear quadrupole resonance by level crossing (at 77 K), QE = quadrupole echo (static) NMR spectroscopy at low temperature of die solid state, p.wave = microwave spectroscopy of the gaseous state. 

The former is a protein of 14.7 kDa involved in the multienzyme nucleotide excision repair (NER) pathway with a determined NMR solution structure . In this protein, the Zn + possesses rather a structural than a catalytic role. Zn NMR spectra were acquired using a rather sophisticated probe (for details, see Reference 87) and operating at temperatures 5-250 K. Data acquisition was performed with the application of spin-echo methods for enhanced sensitivity . Specifically, experiments were carried out at 25 K using a combination of CP (cross-polarization) and spikelet echo pulse sequences which provide a considerable increase in signal-to-noise ratio (of the order of 30) relative to a classical quadrupole echo pulse sequence. The proton field strength applied to the above measurements was 60 kHz with a matching field of 20 kHz for zinc and a contact time... 

Fig. 9. PHBA/BPT - Xydar, molecular motion observed in 2H quadrupole echo spectra (61.4 MHz)...

The solid-state deuterium NMR experiments were performed on surface-adsorbed material from triethoxyaminopropylsilane-rf, (DAPES) and triethoxy-aminobutylsilane-dj (DABES) prepared as described previously [9]. The samples were heated to 90°C at 10 mmHg for 12 h after adsorption. These deuterated coupling agents have been characterized by IR and NMR spectroscopy. The solid-state deuterium NMR spectroscopy was done on a Varian VXR-200 at 30.7 MHz. A quadrupole echo pulse sequence was used with a 2 s interval, 2 /is 90° pulse, 30 ps echo time, 2 MHz sweep width, and, typically, overnight accumulation. 

Fig. 13. 2H NMR spectra of DMS recorded with the QCMPG pulse sequence (see Fig. 12) at the temperatures shown, from reference 33. For each temperature, the normal quadrupole echo 2H spectrum is shown above the QCMPG spectrum for comparison. Also shown are simulated quadrupole echo and QCMPG spectra, also taken from reference 33.

The 2-site 120° jump motion for the basal molecules switches between these two hydrogen bonding arrangements and clearly requires correlated jumps of the hydroxyl groups of all three basal molecules. On the assumption of Arrhenius behaviour for the temperature dependence of the jump frequencies, the activation energies for the jump motions of the apical and basal deuterons were estimated to be 10 and 21 kj mol-1, respectively. This dynamic model was further supported by analysis of the dependence of the quadrupole echo 2H NMR lineshape on the echo delay and consideration of 2H NMR spin-lattice relaxation time data. 

The temperature dependence of the quadrupole echo 2H NMR lineshape and 2H NMR spin-lattice relaxation time measurements demonstrated that the hydrogen bonding arrangement is dynamic. 

Hollander and Prins investigated the effect of pressure on the glass-transition temperature Tg in atactic polypropylene in the pressure range up to 5000 bar. The decay rate of the deuteron-NMR quadrupole echo was used to monitor the glass transition. In further papers the same authors also studied the methyl group, segmental and chain motions of this molecular system. 

To bypass receiver deadtime effects, wideline spectra are derived by Fourier transformation of the decay of an echo. By use of the Hahn echo and the stimulated echo (Section 2.2.1), wideline spectra of and other spin-5 nuclei can be measured, for example, but not the spectra of dipolar coupled spins and of quadrupolar nuclei like H. The magnetization of nuclei with spin / = 1 can be refocused by the quadrupole echo or the solid echo, and by the Jeener-Broekaert echo or the alignment echo [Slil] (Fig. 3.2.6). 

A quadrupole echo is generated similar to a Hahn echo by two pulses which are separated by half the echo time t = t /2 (Fig. 3.2.6(a)). However, the second pulse is a 90° and not a 180° pulse, and it is shifted in phase by 90° with respect to the first. The quadrupole echo appears at a time t2 = t j2 after the second pulse. The spectra shown in Fig. 3.2.5 have been simulated for the quadrupole echo technique with acquisition of the echo decay during h — t. Clearly, the lineshape strongly depends on type and time scale of the motion. In this way, molecular reorientation with correlation times in the range of 10 s < Tc < 10 s can be characterized by H NMR [Laul, Wehl]. Faster motion with correlation times in the range of 10 s < < 10" s can be investigated... 

Fig. 15. quadrupole echo spectra of p-nitroaniline-MA - /2 recorded using a hole-burning pulse... 

H quadrupole powder patterns are generally fairly straightforward to record. Usually, a solid- or quadrupole-echo pulse sequence (90 v i 90, -t-FID) is used in order that dead time losses do not occur these would otherwise severely distort the lineshapes.2... 

Fig. 24. Calculated 2H quadrupole echo and MAS NMR spectra for a two-site reorientation35 such that the 2H quadrupole coupling tensor unique principal axis moves through 106°, i.e. the motion appropriate for the two-site motion of the methyl groups in deuterated DMS. The simulations assume an inhomogeneous symmetric log-Gaussian distribution of correlation times with a mean correlation time of 5 x 10 5s and a standard deviation ranging from 0 to 3 decades, (a) Quadrupole echo spectra with echo delay time t = 30 /is. (b) MAS spectra.

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.

NMR is ideally suited to explore molecular motions in the polymer. Different types of motion can be discriminated on behalf of their timescale and geometry of exchange. One-dimensional quadrupole echo lineshapes (see Section 6.2.7.1) are particularly sensitive to segmental dynamics [1-6, 9-12], when there is either fast exchange between discrete geometries (with Tc <1/Avq) or when the motion occurs on the intermediate timescale (tc= 1/Aj q). Dynamic processes in the intermediate to slow motional limit (tc > l/Ar Q) are addressed by 2D exchange spectroscopy (see Section... 

One-dimensional quadrupole echo NMR lineshape analysis of powder samples is particularly informative when fast, discrete jumps occur between sites of well-defined geometry as, for example, in a phenyl group undergoing two-site exchange. In this case, the characteristic Pake-pattern is transformed into an axially asymmetric lineshape with an apparent asymmetry parameter r] 9 0 (see Equation (6.2.3)) [1-8]. The asymmetric lineshapes, shown on the left in Fig. 6.2.2, can be derived by considering how the individual components of the principal EFG tensor become averaged by the discrete jumps. Within the molecular frame, and in units of as defined by Equation (6.2.2), the static axially symmetric tensor consists of the components = 1, = — 1/2, and V y = — 112. This traceless tensor satisfies the... 

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