Double Hahn echo

Fig. 9.1.5 [Akol] Timing of rf and gradient signals for the PRESS method. Transverse magnetization of a volume element is selected by formation of a double Hahn echo with selective pulses in the presence of orthogonal field gradients.

Several convenient routes for signal enhancement in quadrupolar SSNMR experiments involve manipulation of the ST populations [46], which results in an enhanced CT population difierence, thereby boosting experimental S/N and reducing experimental times. The apphcation of double-frequency sweeps (DFS) before a Hahn-echo or QCPMG experiment results in the DFS-echo [61,62] and DFS—QCPMG [50,63] pulse sequences, respectively, which Rossini et al. showed are very usefijl for the acquisition of challenging SSNMR spectra [64]. Utilization of... 

Figure 17.10 H MAS (15 kHz) Hahn-echo NMR spectra of pristine (Si02 sample) and fluorinated silica (10F2 and 50F2 samples). The double arrows indicate the chemical shift ranges of non-hydrogen-bonded silanols, physisorbed water and hydrogen bonded silanols. Reproduced by permission j. Phys. Chem. C 2009, 113, pp. 1865218660, American Chemical Society...

The combination of homonuclear Hartmann-Hahn transfer with homonuclear double- or zero-quantum spectroscopy yields the so-called DREAM experiment (double-quantum relay enhancement by adiabatic mixing Berthault and Perly, 1989) and the zero-quantum-(ZQ) TOCSY experiment (Kessler et al., 1990a), respectively. Multiplet-edited HOHAHA spectra can be obtained by adding a spin-echo sequence to the Hartmann-Hahn mbdng period (Davis, 1989a). 

Static spin echo decay spectroscopy also forms the basis for the measurement of magnetic dipole-dipole interactions between two unlike nuclei I and S. While this interaction is refocused by the Hahn spin echo, it can be recoupled by applying a 7i-pulse to the S-spins during the dipolar evolution period [12]. This manipulation inverts the sign of the heterodipolar Hamiltonian, and thereby interferes with the ability of the Hahn spin echo technique to refocus this interaction. The corresponding pulse sequence, termed SEDOR spin echo double resonance) shown in Fig. 4, compares the I-spin echo intensity as a function of dipolar evolution time (a) in the absence and (b) in the presence of the ti(S) pulses. Experiment (a) produces a decay F(2ti)/Fo, which is dominated by homonuclear dipole-dipole interactions, while experiment (b) results in an accelerated decay, reflecting the contribution from the heteronuclear I-S dipole-dipole interaction, which is now re-introduced into the spin Hamiltonian. For multi-spin systems, a Gaussian decay is expected ... 

Slichter s spin echo approach is a variation on the spin echo double resonance (SEDOR) experiment originally proposed by Hahn I03. To see how this experiment works, consider the spin echo sequences shown in Fig 53. The first case, Fig 53a, describes Hahn s original echo experiment 103. In a hetero-nuclear case (always observing the rare spin) any heteronuclear dipolar interactions (along with the chemical shift and inhomogeneous line broadening processes) will be refocused as a result of this sequence. The second example. Fig 53b, is Hahn s SEDOR experiment (74). Here, the heteronuclear dipole-dipole interaction is not refocused because of the application of the second tt pulse to... 

The so-called SEDOR technique (spin-echo double resonance [9, 47, 48], see Fig. 5) allows the determination of internuclear distances, rjs, for isolated spin pairs consisting of a resonating J-spin and a non-resonating S-spin. Hahn s echo [49] of the resonating J-spins is observed with a time between the 7i/2-pulse and the 7T-pulse. A rr-pulse applied to the S-spins after a variable time ti (0characteristic damping of Hahn s echo of the resonating /-spins. The damped amplitude, of Hahn s echo under SEDOR conditions is determined by ... 

The most commonly used 2D nmr technique applied to through-bond interactions is termed (7 ) correlated spectroscopy (COSY), others are total correlation spectroscopy (TOCSY), which allows somewhat longer-range through-bond connectivities to be observed than with COSY, spin echo coherence transfer spectroscopy (SECSY), relayed coherence transfer spectroscopy (RELAY), double quantum spectroscopy (DQNMR) and homonuclear Hartmann-Hahn spectroscopy (HOHAHA). The most commonly used 2D nmr technique applied to through-space interactions is termed nuclear Overhauser effect spectroscopy (NOESY) also used is the closely related rotating-frame NOESY (ROE-SY). 

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