Soft-COSY experiments

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

Figure 7.16 When soft pulses are used for excitation and mixing in a 2D experiment, it becomes a 2D soft experiment. The spectrum of the 2D soft experiment has reduced frequency ranges in F, and Fj. The excitation ranges of the selective pulse depend on the type of experiment. For example, in a soft COSY-COSY experiment, one multiplet is excited, while in the soft NOESYexperiment the whole resonance region of a group of signals is excited. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Selective excitation of a resolved resonance followed by homonuclear or heteronuclear Hartmann-Hahn transfer can also be advantageous in the preparation period of two-dimensional experiments. For example, two-dimensional COSY, NOESY, TOCSY, and two-dimensional /-resolved subspectra of individual spin systems can be acquired based on this principle (Homans, 1990 Sklenaf and Feigon, 1990 Nuzillard and Massiot, 1991 Gardner and Coleman, 1994). In selective two-dimensional experiments like soft COSY (Briischweiler et al., 1987 Cavanagh et al., 1987),... 

Figure 2.11. Proton-Proton shift correlations of a-pinene (1) [purity 99 %, CDCls, 5 % v/v, 25 °C, 500 MHz, 8 scans, 256 experiments], (a) HH COSY (b) HH TOCSY (c) selective one-dimensional HH TOCSY, soft pulse irradiation at Sh = 5.20 (signal not shown), compared with the NMR spectrum on top deviations of chemical shifts from those in other experiments (Fig. 2.14, 2.16) arise from solvent effects...

Nymphaea caerulea, for seven natural anthocyanins stabilizing a DNA triplex, etc. Sequential analysis of the oligosaccharide structures of the flavonol tamarixetin-7-O-rutino-side has been performed by ID multistep-relayed COSY-ROESY experiments. Selective excitation was performed by Gaussian-shaped soft pulses. 

Fig. 8.2. Some of the most common 2D pulse sequences that can be employed using a proper choice of parameters to record 2D spectra of paramagnetic molecules (A) NOESY, (B) ROESY, (C) COSY, (D) ISECR COSY, (E) zero-quantum (double quantum) COSY, (F) TOCSY, (G) HMQC, (H) HSQC. Sequences (A), (B) and (F) are also used to obtain EXSY spectra. SL indicates a soft spin-lock sequence, while MLEV17 indicates a train of spin-locking hard pulses that optimizes the development of J/j coupling. In the reverse heteronuclear experiment (G) the upper and lower levels refer to H and heteronucleus, respectively. The phase cycles are not indicated. For clarity of discussion, all initial pulses can be thought to be applied along the y axis, in such a way that the coherence after the first 90° pulse is always along x. ...

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