Nuclear coherence transfer echo

Figure 5 Diagram showing contours that arise from broad hyperfine lines detected hy nuclear coherence transfer echoes under conditions of (a) weak and (h) strong hyperfine coupling...

Three different ID ESEEM schemes using the pulse sequence in Figure 5c and die nuclear CTE have been proposed deadtime-free ESEEM by nuclear coherence transfer echoes (DEFENCE) [24], the combination peak (CP) experiment, and the... 

Ponti A, Schweiger A. 1995. Nuclear coherence-transfer echoes in pidsed EPR. J Chem Phys 102 5207-5219. 

In the HYSCORE experiment only nuclear frequencies in different manifolds belonging to the same paramagnetic center are eorrelated with each other. For multinuclear spin systems the assignment of nuelear frequencies is often not straightforward, since some of the correlation peaks may not be observed in the HYSCORE speetrum due to the small intensity of the nuelear transitions in one of the two Ws manifolds. Additional information can be gained if correlations of nuclear frequencies within the same manifold can be obtained. Cross-peaks that represent such correlations can be created by replacing the nonselective transfer n pulse in the HYSCORE sequence by the double nuclear-coherence transfer (DONUT) mixer % - t - n [58]. This DONUT-HYSCORE experiment with the pulse sequence ji/2 -ti nil - echo results in crosspeaks and (twp i, copj). The presence of these cross-peaks in the DONUT-... 

The pulse methods rely on selective irradiation of a particular resonance line with a radio frequency (rf) and observation of the resulting effects in the rest of the spectrum. Among commonly employed methods are 2D correlated spectroscopy (COSY), 2D spin-echo correlated spectroscopy (SECSY), 2D nuclear Overhauser and exchange spectroscopy (NOESY), 2D J-resolved spectroscopy (2D-J), and relayed coherence-transfer spectroscopy (RELAYED-COSY) (Wutrich, 1986). 

At the qualitative level, NMR studies devoted to this issue exploit heteronuclear dipolar recoupling to correlate resolved MAS NMR signals of the nuclear species associated with different network formers. This information comes from the dephasing of the observe nuclei in REDOR, TRAPDOR, or REAPDOR experiments, or from one- or two-dimensional correlation spectroscopy involving coherence transfer via TEDOR (transferred echo double resonance) or... 

M. Bjerring, J. T. Rasmussen, R. S. Krogshave and N. C. Nielsen, Heteronuclear coherence transfer in solid-state nuclear magnetic resonance using a 7-encoded transferred echo experiment. J. Chem. Phys., 2003, 119, 8916-8926. 

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). 

Considering the resolution of the nuclear frequency spectrum, this two-pulse echo experiment is not optimal. The nuclear frequencies are here measured as differences of frequencies of the ESR transitions, so that the line widths correspond to those of ESR transitions. The nuclear transitions have longer transverse relaxation times Tin and thus smaller line widths. In fact, if the second mw pulse is changed from a n pulse to a Ji/2 pulse, coherence is transferred to nuclear transitions instead of forbidden electron transitions. This coherence then evolves for a variable time T and thus acquires phase v r or vpT. Nuclear coherence cannot be detected directly, but can be transferred back to allowed and forbidden electron coherence by another nil pulse. The sequence (jt/2)-x-(Jt/2)-r-(jt/2)-x generates a stimulated echo, whose envelope as a function of T is modulated with the two nuclear frequencies v and vp. The combination frequencies v+ and v are not observed. The modulation depth is also 8 211. The lack of combination lines simplifies the spectrum and the narrower lines lead to better resolution. There is also, however, a disadvantage of this three-pnlse ESEEM experiment. Depending on interpulse delay x the experiment features blind spots. Thus it needs to be repeated at several x values. 

The disadvantage of the fast echo decay in two-pulse ESEEM can be circumvented with the three-pulse ESEEM experiment shown in Figure 5b. In this pulse sequence the first two nil pulses create nuclear coherence that develops during the evolution time T and decays with the transverse nuclear relaxation time 72n which is usually much longer than the corresponding relaxation time 7m of the electrons. The third nJl pulse transfers the nuclear coherence back to observable electron coherence. The modulation of the stimulated echo is given by... 

It is worth reiterating that nuclear coherence, comprising nuclear frequencies of the spin system, is created by the first two m.w. pulses. During evolution time T the nuclear coherence accumulates phase, and the transfer of this nuclear coherence back to electron coherence with the third m.w. pulse causes the stimulated echo intensity to be modulated by the nuclear frequencies, enabling their measurement. 

The remote-echo detector is shown in Figure 11. In this method the electron spin echo at the end of the pulse sequence, which uses Vi < rnuclear coherence generator, is not recorded. Instead, at the time of echo formation an additional nil pulse transfers the electron coherence to longitudinal magnetization. The echo amplitude information can thus be stored for a time interval up to the order of T. After a fixed time delay h < T l, the z-magnetization is read out using a two-pulse echo sequence with a fixed time interval X2 > r. Remote echo detection can be applied to many experiments, including three-pulse ESEEM and HYSCORE, and thus can eliminate blind spots with an appropriate choice of small ri. Note, however, that it may suffer from reduced sensitivity due to the increased sequence time. 

---