Dipolar modulation

Pulse Measurements. Comparison of 3-pulse PELDOR data for nitroxide biradicals with interspin distances of 15.4 to 24.0 A at X-band and S-band demonstrated the separation of nuclear hyperfine and dipolar interactions and the separation of contributions from dipolar and exchange interactions.20 However, the deeper nuclear modulation at S-band limits the options for interpulse spacings. The inability to observe dipolar modulation in a PELDOR experiment... 

Four-pulse DEER measurements were performed on a dimer of copper-substituted azurin molecules with a Cu(II)-Cu(II) distance of 26 A.27 Experiments were performed at 10 K with pulse lengths of 16 ns for ji/2 and 32 ns for p pulses and a 75 MHz difference between the frequencies of the pump and observe pulses. Analysis of the dipolar frequencies required consideration of orientation selection in both the pump and observe pulses because only a subset of the Pake pattern is represented in the Fourier transform of the experimental data. For this sample the orientation of the interspin vector relative to the g matrices of the two centres was known from high-field EPR. Dipolar modulation could not be detected for a second dimer with a copper-copper distance of 14.6 A.27... 

Pulse measurements. A self-complementary RNA with 12 base pairs was spin labelled at uridine near the middle of the strand.83 Three-pulse PELDOR gave an interspin distance of 35 2 A, which is in good agreement with molecular modelling. For a shorter self-complementary RNA, dipolar modulation was not observed, which was attributed to aggregation or weak duplex formation. 

Fig. 36. Pulse sequence for direct determination of H— C— H molecular torsion angles by heteronuclear dipolar modulation of C DQ coherence. ...

Fig. 10. (a) Pulse sequence for determination ofH—C—C—H torsional angles under magic-angle spinning conditions, through heteronuclear dipolar modulation of the double-quantum coherences. The pulse sequence r uires that the RF carrier is set to... 

The aforementioned methods for determining the CSA are applicable only for samples without dipolar-modulated chemical-shift powder patterns [47]. Under these circumstances, the analysis problem becomes quite difficult. Sample-spinning NMR techniques must be used to measure the heteronuclear dipolar-chemical-shift 2D powder patterns [48,49]. 

ColvinVL, Cunningham K L and Alivisatos A P 1994 Electric field modulation studies of optical absorption in CdSe nanocrystals dipolar character of the excited state J. Chem. Phys. 101 7122... 

Although beyond the scope of the present discussion, another key realization that has shaped the definition of click chemistry in recent years was that while olefins, through their selective oxidative functionalization, provide convenient access to reactive modules, the assembly of these energetic blocks into the final structures is best achieved through cydoaddition reactions involving carbon-het-eroatom bond formation, such as [l,3]-dipolar cydoadditions and hetero-Diels-Al-der reactions. The copper(i)-catalyzed cydoaddition of azides and terminal alkynes [5] is arguably the most powerful and reliable way to date to stitch a broad variety... 

Tjandra, N., Bax, A. Measurement of dipolar contributions to /ch splittings from magnetic-field dependence of J modulation in two-dimensional NMR spectra./. Magn. Reson. 1997, 124, 512-515. 

FIGURE 10.4 Anisotropy averaging in the EPR of TEMPO as a function of temperature. The spectra are from a solution of 1 mM TEMPO in water/glycerol (10/90). The blow-up of the middle 14N (/ = 1) hyperfine line in the 90°C spectrum has been separately recorded on a more dilute sample (100 pM) to minimize dipolar broadening and, using a reduced modulation amplitude of 0.05 gauss, to minimize overmodulation. The multiline structure results from hyperfine interaction with several protons. 

Optimal control offers the possibility to obtain an optimal solution to a given problem, as described by a Hamiltonian and a set of supplementary conditions reflecting the problem, either analytically or numerically. As dipolar recoupling is associated with time-modulated anisotropic parameters and spread of parameters due to different orientation of the individual crystallites, attempts to use optimal... 

The homonuclear dipolar recoupling technique of radio frequency-driven recoupling (RFDR) involves a hard n pulse per rotor period and its recoupling mechanism is based on the modulation of chemical shift difference [35-37]. When the delta-pulse approximation is relaxed so that the pulse width of the 7t pulse is about one-third of the rotor period, the so-called finite-pulse RFDR (fpRFDR) could selectively reintroduce the homonuclear dipole-dipole interaction under fast MAS conditions [38], Because the recoupling mechanism of fpRFDR does not require the presence of chemical shift difference, it can be applied to study samples with a singly labeled site. For the study of amyloid fibrils, the technique of fpRFDR is usually applied in a constant-time framework (see below). 

Under MAS the quadrupole splitting becomes time dependent, Qg = Qg (f) (see Sect. 2.3.4). This influences both the spin-locking behavior [223] and the polarization transfer [224], with the latter being further affected by the periodic modulation of the IS dipolar interaction. The effect of MAS on spin-locking of the S magnetization depends on the magnitude of the so-called adiabaticity parameter ... 

The CP dynamics is affected both by the periodic modulation of Qg and by the periodic modulation of the I-S dipolar interaction [216, 224]. Assuming that the... 

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

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