Homonuclear /-coupling evolution

In the following Check its the possible deficiencies of the heteronuclear coherence transfer step in combination with the preparative evolution step are shown. In an attempt to represent a real sample a number of different values for J(C, H) are used to simulate the simultaneous chemical shift and homonuclear coupling evolution. 

Homonuclear coupling evolution during the pulse sequence gives its own effect to the polarization transfer to the desired coherence and therefore affects the quantitativity. For 2D INEPT the crucial point in this respect is the INEPT period. 

Figure 9 Signal loss with respect to homonuclear coupling evolution in INEPT (see Equation (12)).

The loss of polarization is less severe in 2D INEPT compared to HSQC (cf. Figure 3). As Jhh seldom exceeds 10% of the /ch/ the effect of homonuclear coupling evolution to the quantitativity can be regarded as negligible in this particular case, but the error from this source should be understood when high precision in quantification is needed. 

The 180° 13C pulse at the middle of the evolution period interchanges the precession frequencies of the a and /3 spins (see Fig. 9.2, bottom) and effectively decouples the spins during tu and a broadband decoupling sequence, such as WALTZ or GARP, is applied during f2.Thus, the 13C spectrum in the F2 dimension is decoupled, and the H spectrum in the Fx dimension retains homonuclear couplings but is also decoupled from 13C, as illustrated in Fig. 10.106. 

Figure 3 Vector diagrams showing the evolution of (a) H- Sn, (b) H-" Sn and (c) H- n (jc+119, 117 and 115) proton magnetization during the preparation period of the basic four-pulse sequence. A = 1 /(2V(" Sn, H) ). It is assiuned, for the sake of clarity, that magnetization undergoes no chemical shift, homonuclear coupling or other heteronuclear coupling evolutions...

The sequence that achieves this (Fig. 7.15) is a simple variant on the INEPT-based heteronuclear shift correlation sequence of Fig. 6.31 (HETCOR), so the loss in sensitivity is compensated somewhat by the use of a polarisation transfer step. In fact the only difference between the two lies in the net evolution of only shifts or only couplings for the whole of ti- The addition of a proton 180 pulse at the midpoint of ti here serves to refocus proton chemical shifts and heteronuclear coupling constants (so the X-spin 180° pulse of HETCOR becomes redundant) but leaves the proton homonuclear couplings free to evolve. The resulting spectrum therefore contains only proton multiplets in fi dispersed by the corresponding X-spin shifts in f2 (Fig. 7.16) ... 

The sequence below shows the gradient-selected DQF COSY pulse sequence modified by the inclusion of extra 180° pulses to remove phase errors. Note that although the extra 180° pulses are effective at refocusing offsets, they do not refocus the evolution of homonuclear couplings. It is essential, therefore, to keep the gradient pulses as short as is feasible. 

Homonuclear correlation of nuclei of with low natural abundance, spin-echo sequence for scalar coupling evolution, DQF to suppress unwanted coherences, a double quantum frequency scaled dimension in 2D spectra... 

The simplest 2D NMR experiment, which offers enhanced resolution, is the homonuclear /-resolved experiment. ° The experiment is basically a spin-echo sequence, where the echo period is incremented. During this ti period the chemical shift evolution is refocused with a 180° pulse, but homonuclear / couplings are evolved. If we consider a proton H coupled to proton(s) H, the density function of the observed proton magnetization is modulated by these couplings accordingly... 

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