Spin-echoes heteronuclear

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

THE HETERONUCLEAR SPIN ECHO CONTROLLING /-COUPLING EVOLUTION AND CHEMICAL SHIFT EVOLUTION... 

Reversing the sign of these operators in the center of a spin echo leads to refocusing of the evolution. This is an easy way to remember how to design a heteronuclear spin echo use a 180° pulse alone to refocus all but 13C chemical shift evolution use a 13C 180° pulse alone to refocus all but lH chemical-shift evolution use simultaneous lH and 13C 180° pulses to refocus all but Jqh evolution. You can go through the full product operator analysis of each kind of spin echo, and you will find that the sign changes shown above are the crucial differences that control what refocuses and what continues to evolve in the second half of the spin echo. 

Figure 2.16. The influence of spin-echoe.s on scalar coupling as illustrated for two coupled spins A and X. (a) A homonuclear spin-echo (in which both spins experience a 180° pulse) allows the coupling to evolve throughout the sequence, (b) A heteronuclear spin-echo (in which only one spin experiences a 180° pulse) causes the coupling to refocus. If both heteronuclear spins experience 180° pulses, the heteronuclear coupling evolves as in (a) (see text).

There are several heteronuclear 2D J(X, H)-resolved experiments for the determination of J(X, H) coupling constants all based upon a heteronuclear spin echo which refocuses the chemical shift evolution of the nucleus but which let through the... 

The spin-echo experiment is particularly simple to set up as it does not require proton pulses or their calibration, a desirable property when the experiment was first introduced but of little consequence nowadays. The same results can, in fact, be obtained by the use of proton 180° pulses rather than by gating of the decoupler [23] (Fig. 4.15b). In this case the A period is broken in two periods of 1/27 separated by the simultaneous application of proton and carbon 180° pulses. These serve to refocus carbon chemical shifts but at the same time allow couplings to continue to evolve during the second A/2 period (Section 2.2). Hence, the total evolution period in which coupling is active is 1/7, as in the decoupler-gating experiment above, and identical modulation patterns are produced. It is this shorter pulsed form of the heteronuclear spin-echo that is widely used in numerous pulse sequences to refocus shift evolution yet leave couplings to evolve. 

Dipole—dipole (heteronuclear) Af (hetero) Spin-echo double resonance (SEDOR) ... 

SPIN-ECHO FORMATION IN HOMONUCLEAR AND HETERONUCLEAR SYSTEMS... 

The basic pulse sequence for the production of a spin-echo is illustrated in Fig. 2.1. The behavior of C vectors in a heteronuclear CH sys-... 

Spin-Echo Formation in Homonuclear and Heteronuclear Systems... 

Figure 2.3 Spin-echo experiment. The behavior of nucleus X in an AX spin system is shown. (A) Application of the second 180° pulse to nucleus X in the AX hetero-nuclear system results in a spin-flip of the two X vectors across the x -axis. But the direction of rotation of the two X vectors does not change, and the two vectors therefore refocus along the —y axis. The spin-echo at the end of the t period along the -y axis results in a negative signal. (B) When the 180° pulse is applied to nucleus A in the AX heteronuclear system, the spin-flip of the X vectors...

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

Several modifications have been proposed for the basic HNN-COSY experiment. For example, frequency separations between amino and aromatic 15N resonances are typically in the range 100-130 ppm and therefore much larger than between imino 15N donor and aromatic 15N acceptor resonances. As has been pointed out by Majumdar and coworkers [33], such 15N frequency separations are too large to be covered effectively by the non-selective 15N pulses of the homonuclear HNN-COSY. They therefore designed a pseudo-heteronuclear H(N)N-COSY experiment, where selective 15N pulses excite the amino and aromatic 15N resonances separately to yield excellent sensitivity [33]. An inconvenience of this experiment is that the resonances corresponding to the amino 15N nuclei are not detected, and a separate spin-echo difference experiment was used to quantify the h2/NN values. A slightly improved version of this pseudo-heteronuclear H(N)N-COSY [35] remedies this problem by the use of phase-coherent 15N pulses such that both amino and aromatic 15N resonances can be detected in a single experiment. 

Rotational-echo double resonance (REDOR), originally introduced by Gullion and Schaefer [102], is a method to recouple heteronuclear spin pairs. The sequence relies on a train of rotor-synchronized n pulses applied to the I spin to interrupt the spatial averaging of the heteronuclear dipolar coupling under MAS to give a nonvanishing dipolar Hamiltonian over a full rotor cycle (Fig. 11.8). Typically, REDOR data are collected by col-... 

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