Correlated spectroscopy pulse sequence

Our simple 2-D experiment is actually a very important experiment sometimes simply called COSY (Correlation SpectroscopY), and which we will call COSY for the time being in order to clearly indicate what is being correlated. The pulse sequence for H — H COSY is none other than the one we have already described above in Figure 5.3 two tt/2 proton pulses separated by the required evolution period, q, which is systematically incremented, and the acquisition period, t2. 

Figure 1. Pulse sequences of some typical 2D-NMR experiments. COSY = correlation SpectroscopY, DQFCOSY = Double Quantum Filtered COSY, RELAY = RELAYed Magnetization Spectroscopy, and NOESY = Nuclear Overhauser Effect SpectroscopY.

Exchange correlation spectroscopy (E. COSY), a modified form of COSY, is useful for measuring coupling constants. The pulse sequence of the E. COSY experiment has a mixing pulse )3 of variable angle. A number of experiments with different values of /3 are recorded that eliminate the multiplet components of unconnected transitions and leave only the multiplet components for connected transitions. This simplified 2D plot can then be used to measure coupling constants. 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

Scalar coupled experiments COSY and TOCSY The correlated spectroscopy (COSY) experiment is one of the most simple 2D-NMR pulse sequences in terms of the number of RF pulses it requires [32]. The basic sequence consists of a 90-C-90-acquire. The sequence starts with an excitation pulse followed by an evolution period and then an additional 90° pulse prior to acquisition. Once the time domain data are Fourier transformed, the data appear as a diagonal in... 

Total correlation spectroscopy (TOCSY) is similar to the COSY sequence in that it allows observation of contiguous spin systems [35]. However, the TOCSY experiment additionally will allow observation of up to about six coupled spins simultaneously (contiguous spin system). The basic sequence is similar to the COSY sequence with the exception of the last pulse, which is a spin-lock pulse train. The spin lock can be thought of as a number of homonuclear spin echoes placed very close to one another. The number of spin echoes is dependent on the amount of time one wants to apply the spin lock (typically 60 msec for small molecules). This sequence is extremely useful in the identification of spin systems. The TOCSY sequence can also be coupled to a hetero-nuclear correlation experiment as described later in this chapter. 

In this chapter, the discussion will be focused on the ID TOCSY (TO-tal Correlation SpectroscopY) [2] experiment, which, together with ID NOESY, is probably the most frequently and routinely used selective ID experiment for elucidating the spin-spin coupling network, and obtaining homonuclear coupling constants. We will first review the development of this technique and the essential features of the pulse sequence. In the second section, we will discuss the practical aspects of this experiment, including the choice of the selective (shaped) pulse, the phase difference of the hard and soft pulses, and the use of the z-filter. The application of the ID TOCSY pulse sequence will be illustrated by examples in oligosaccharides, peptides and mixtures in Section 3. Finally, modifications and extensions of the basic ID TOCSY experiment and their applications will be reviewed briefly in Section 4. 

Reports on the application of pulsed field gradient (PFG) assisted pulse schemes for two-dimensional X/Y correlation spectroscopy focused mainly on the adaptation of HSQC sequences which seemed to perform better than HMQC experiments under these conditions.21 Although the generalisation of standard pulse sequences for / C correlation spectroscopy should in principle be straightforward, large spectral ranges and short relaxation... 

Fig. 1. Pulse sequence for the X/Y H PFG-HSQC experiment as employed for 19F/13C correlation spectroscopy in Ref. 21. 90° and 180° hard pulses are denoted by solid and open bars, respectively groups of two solid and one open bars denote 90° 0 — 180° +9o — 90° pulse sandwiches that serve as composite 180° pulses. 2 are delays of length 1 /(2 Jx,v), and r is a short delay of the same length as the gradient pulse (typically 1 ms). Phase cycles are as in the standard HSQC experiment, and the ratio of gradient pulse strengths is set to G2/G1 = Yy/Yx- Decoupling is employed using WALTZ-16 ( H) and GARP (Y) pulse trains.

The acronym COSY stands for Correlated SpectroscopV and this technique is widely used to determine all of the coupling interactions in a single experiment. This proves to be more efficient than the decoupling experiment in which each signal is irradiated in turn to determine its coupling partners. COSY involves a multiple pulse sequence (which we do not need to know anything about in order to use the technique) and is an example of two-dimensional (2D) spectroscopy. 

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