Incoherent magnetization transfer

Here the generic term Hartmann-Hahn experiment is used for polarization- or coherence-transfer experiments that are based on the Hartmann-Hahn principle (see Section II), that is, on matched effective fields that are created by a rf irradiation scheme. These experiments may be classified according to the following practical and theoretical aspects (see Fig. 6) that are related to properties of samples, spin systems, coherent magnetization transfer, effective Hamiltonians, multiple-pulse sequences, and incoherent magnetization transfer ... 

Physical parameters Molecular 0.1-1 nm Dipole-dipole interaction Second moment, fourth moment of lineshape Incoherent magnetization transfer characteristic times for cross-polarization and exchange Mesoscopic lnm-0.1 p,m Longitudinal relaxation time Ti Transverse relaxation time T2 Relaxation time Tip in the rotating frame Solid-echo decay time T2e Spin-diffusion constant Microscopic 0.1-lOp.m Molecular self-diffusion constant D Macroscopic 10 p,m and larger Spin density... 

Heteronuclear incoherent magnetization transfer is the transfer of longitudinal magnetization. It can proceed in the laboratory frame and in the rotating frame. The nuclear Overhauser effect (NOE) [Nogl] is a manifestation of polarization tranter in the laboratory frame. In the extreme narrowing limit saturation of dipolar relaxation of the I doublet of a heteronuclear IS two-spin- system leads to an enhancement of the S-spin polarization by a factor... 

We note that the just described pulse sequence is identical to the 2D exchange NMR experiment of solution NMR, which is an isotropic-shift exchange experiment. The same sequence is also used in 2D nuclear Overhauser effect spectroscopy (NOESY) NMR in solution and used extensively in the elucidation of the stmcture of biomacromolecules in solution. There, however, the exchange is based on incoherent magnetization transfer by incoherent cross-relaxation. ... 

Both homonuclear and heteronuclear versions of relayed nOe experiments are known. The homonuclear relayed NOESY experiment involves both an incoherent transfer of magnetization between two spins H and H/ that are not coupled but close in space, and a coherent transfer of magnetization between two spins H(and H that are /-coupled together. The magnetization pathway may be depicted as... 

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

The nuclear Overhauser effect is used to measure distances between nuclei. It is a relaxation effect that, like a chemical reaction, is incoherent in nature as opposed to the scalar coupling that is coherent and is detected as a frequency splitting of resonances in the spectrum. Relaxation can be best viewed at as the flow of population between different states as it would occur in a chemical reaction where one molecule is transformed, e.g., by a reaction of zero-order into a second molecule. The same is true for the NOE where magnetization of spin A by a zero-order reaction is transferred to mag-... 

Figure 5.75. Pulse sequences used for relayed NOE (A) an incoherent transfer of magnetization occurs, followed by a coherence transfer of magnetization (B)a coherence transfer takes place first, and is then followed by an incoherent transfer of magnetization.

This implies that the fluorescence intensity Ip- (t) at a time where all excited atoms oscillate in phase is N times larger than in the incoherent case (Dicke superradiance) [11.36]. This phenomenon of superradiance is used in the photon-echo technique for high-resolution spectroscopy to measure population and phase decay times, expressed by the "longitudinal" and "transversal" relaxation times T and T2. This technique is analogous to the spin-echo method in nuclear magnetic resonance [11.37]. Its basic principle may be understood in a simple model, transferred from NMR to the optical region. 

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