Adiabatic polarization transfer

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. 

The APRR (adiabatic polarization transfer under rotational resonance conditions) experiment is essentially an polarization transfer experiment, with the difference that the spinning rate is not kept constant but is ramped adiabatically through the condition (Fig. 15). This leads to a more complete transfer and a more broadband resonance condition, rendering the experiment less sensitive to distributions of isotropic chemical shielding in disordered systems. 

Encyclopedia of Nuclear Magnetic Resonance, Vol. 9, ed. D.M. Grant and R.K. Harris, John Wiley Sons Ltd., Chichester, UK., 2002 R 182 M. Ernst and B.H. Meier, Adiabatic Polarization-Transfer Methods in MAS Spectroscopy , p. 23... 

Kiefer PM, Hynes JT (2002) Nonlinear free energy relations for adiabatic proton transfer reactions in a polar environment. I. Fixed proton donor—acceptor separation. J Phys Chem A... 

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 ... 

Polarization-transfer experiments which are based on a resonance condition, i.e. where a variable quantity in the experiment is matched to a parameter of the investigated spin system, can be carried out as a transient experiment or as an adiabatic experiment Figure 11.5 illustrates the differences between these two types of experiments. In a transient or sudden" experiment, the density operator is prepared in a state orthogonal to the effective polarization-transfer Hamiltonian (Fig. 11.5a). When the polarization-transfer Hamiltonian is switched on, the density operator starts precessing around the effective Hamiltonian, and usually maximum polarization transfer is reached after a 180° rotation. Since often the size of the effective Hamiltonian at the matching condition depends on... 

In the adiabatic experiment, the density operator is prepared such that it is initially oriented along the starting effective Hamiltonian prepared to be far away from the resonance condition (Fig. 11.5 b). The direction of the effective Hamiltonian is then slowly changed (e.g. by a change in rf amplitude or frequency) to pass through the resonance condition to the final position, again far away from the resonance condition. If the change of the effective Hamiltonian is carried out adiabatically, the density operator will follow the trajectory of the Hamiltonian, and a (complete) polarization transfer can occur. The variation in the size of the effective Hamiltonian at the resonance condition only influences the condition for adiabaticity. Therefore, it is in principle possible to obtain si-... 

The Marcus treatment uses a classical statistical mechanical approach to calculate the activation energy required to surmount the barrier. It assumes a weakly adiabatic electron transfer process and non-equilibrium dielectric polarization of the solvent (continuum) as the source of activation. This model also considers the vibrational contributions of the inner solvation sphere. The Hush treatment considers ion-dipole and ligand field concepts in the treatment of inner coordination sphere contributions to the energy of activation [55, 56]. 

The dynamical theory also provides a framework for the study of the diabatic free energy profiles as functions of the reaction coordinate required in the theory of non-adiabatic electron transfer reactions. We illustrate this new application by calculating the free energy profiles in solvents covering a wide range of polarity. 

Unlike correlation spectroscopy based on spin diffusion, the adiabatic version enables, in principle, almost full exchange of magnetization between the two spins. As a result, the entire signal intensity will reside in the cross-peaks. Violation of the adiabaticity is characterized by the appearance of a diagonal peak and can be expected to occur if the rotation sweep is too fast compared to the interaction between spins. While numerical simulations indicate possible linear dependencies of the polarization transfer coefficient on spin coupling and the rate of the sweep over a range of practical values, the validity of this assumption remains to be tested. Here we present a semi-quantitative example of a relayed polarization transfer process. 

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. 

R. Verel, M. Baldus, M. Nijman, J. W. M. van Os and B. H. Meier, Adiabatic homonuclear polarization transfer in magic-angle-spinning solid-state NMR. Chem. Phys. Lett., 1997, 280. 31-39. 

Kieeee, P. M., Hynes, J. T. (2003) Kinetic isotope effects For adiabatic proton transfer reactions in a polar environment, J. Phys. Chem. A 107, 9022-9039. 

In this context, we must at least briefly mention the REAPDOR experiment [94] (rotational-echo adiabatic-passage double resonance), a technique that is very similar to the BJ3DOR pulse sequence, although it is more efficient, as it is designed especially for quadrupolar nuclei utilizing an adiabatic pulse of length for the polarization transfer (Fig. 25). In this... 

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