Adiabatic cross-polarization

Lobo and Ramanathan have combined adiabatic and Hartmann-Hahn cross-polarization for sensitivity enhancement in solid-state separated local field 2D NMR experiments of partially ordered systems The magnetization in double- and zero-quantum reservoirs of an ensemble of spin-1/2 nuclei has been examined and their role in determining the sensitivity of a class of separated local field NMR experiments based on Hartmann-Hahn cross-polarization has been described. Lobo and Ramanathan report that for the liquid crystal system studied, a large dilute spin-polarization, obtained initially by the use of adiabatic cross-polarization, can enhance the sensitivity of the above experiment. The signal enhancement factors, however, are found to vary and depend on the local dynamics. The experimental results have been utilized to obtain the local order-parameters of the system. 

Fig. 3 NMR spectra of 5CB at 298 K showing the signal enhancement from the use of adiabatic cross-polarization (b), in comparison to the single-pulse experiment (a). (Reproduced with permission from J. Chem. Phys., 2008, 128, 114504.)...

One can further increase the amount of transferred polarization if one carries out the cross polarization in an adiabatic fashion. In this experiment, the amplitude of one of the spin-lock fields is usually varied in a tangential shape [33-35]. In addition to the compensation of instabilities in the amplitude and rf field inhomogeneities, one can also obtain a gain in signal by a up to a factor of two. The concept of adiabatic polarization transfer will be discussed in more detail in Sect. 11.3.1. 

Hartmann-Hahn cross polarization between two low-y nuclei has been successfully used to record chemical-shift correlation spectra between 13C and 15 N nuclei. Cross polarization between two low-y nuclei suffers from a high sensitivity to the exact matching condition at one of the side bands of the Hartmann-Hahn condition [101]. Adiabatic methods (APHH-CP) can eliminate most of this sensitivity and lead to high transfer efficiencies [34, 62, 90]. 

M. Baldus, D. G. Geurts, S. Hediger and B. H. Meier, Efficient N- C polarization transfer by adiabatic-passage Hartmann-Hahn cross polarization. J. Magrc Reson.. 4. 1996, 118, 140-144. 

S. Hediger, B. H. Meier and R. R. Ernst, Adiabatic passage Hartmann-Hahn cross polarization in NMR under magic angle sample spinning. Chem. Phys. Lett., 1995, 240, 449-456. 

Bertrand, "Adiabatic J cross-polarization in liquids for signal enhancement in NMR," J. Am. Chem. Soc. 102, 2526-2528 (1980). 

Hediger S, Meier BH, Kurm ND et al (1994) NMR cross polarization by adiabatic passage through the Hartmann-Hahn condition (APHH). Chem Phys Lett 223 283-288... 

Zandomeneghi and Meier " have applied APCP (adiabatic-passage cross polarization) in N-NMR spectroscopy of weakly oriented peptide. They have determined Z>hn RDCs by means of the VAS experiment. 

The excitation of H and Na quadrupole order using adiabatic demagnetization in the rotating frame has been described. Double-quantum crosspolarization between half-integer quadrupolar spin systems, B Na and B-> A1, has been described. Spin-locking of / = nuclei in static and spinning samples has been described and applied to cross polarization from Na to N at a distance of more than 2.6 A. H and Na NMR studies have been... 

Electron transfer (ET) is of course accompanied by rearrangement of the solvent as shown by the horizontal displacement in Figure 26. Tradiational theories for ET fall into two cases. In the nonadiabatic case it is assumed that the rate of ET is controlled by the process of crossing from one electronic state (e.g., LE) to the other (e.g., CT) [60,61]. Alternatively in the weakly adiabatic case, it is assumed that the solvent polarization is always in equilibrium with the changing charge distribution. For this latter case transition state theory is applicable [59]. 

Now, the non-adiabatic electron transitions is examined only when electron matrix element Fif is small (see the criterion (10) and (10a)). It is the criterion of applicability of the perturbation theory on F f, but it is not the criterion of applicability of the concept of non-adiabatic transition between two crossing diabatic terms. As it is known (see, for example, ref. [5]) the true image of terms is changed on taking into account the interaction V. Denote two terms without inter-term interaction as E[(R) and E (R), where R is the generalized nuclear coordinate. If the crystal phonons (or the outer-sphere variables in a polar medium) only participate in the transition, then E[(R) and E (R) are the parabolic terms independent of the value of shift of... 

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