J-resolved spectroscopy

An alternative approach to resolving proton multiplets is to disperse them according to the chemical shift of the carbon nucleus to which the protons are attached, rather than those of the proton themselves [25]. The advantage of this approach lies in the typically greater dispersion of the carbon chemical shifts, although one must tolerate the reduced sensitivity of carbon observation. 

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)  

Correlations through space The nuclear Overhauser effect 

Establishing NOEs and hence spatial proximity between protons. Suitable only for small molecules (Mr KXX)), for which NOEs are positive. Observes steady-state or equilibrium NOEs generated from the saturation of a target. 

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Homonuclear teclmiques such as J-resolved spectroscopy also exist for rotatmg all multiplets tlirough 90°, to resolve overlaps and also give a ID spectrum from which all homonuclear couplings have been removed [26]. 

J-resolved spectroscopy Two-dimensional techniques, both homo- and heteronuclear, that aims to simplify interpretation by separating chemical shift and coupling into the two dimensions. Unfortunately prone to artifacts in closely coupled systems. 

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.

These results suggest that the signals arise from dipole-dipole coupled protons. Kreis et al. confirmed this finding by measurements using one-dimensional zero- and double-quantum filtering, two-dimensional J-resolved spectroscopy, two-dimensional constant time COSY and longitudinal order separation... 

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). 

The pulse sequence for J-resolved spectroscopy is (90) — fj — (180). The FID is observed as an echo, so there needs to be an additional delay ty before the detection period ti, and the complete sequence is (90) — tj — (180) — tj — f2, as shown in Figure 26. For COSY the basic pulse sequence is (90) — ty— (90) — tj and for NOESY it is (90) — ty— (90) — D — (90) — 2, with a pulsed field gradient applied during the delay, D. A large number of other pulse sequences has been designed both to improve detection and to permit other forms of correlation spectroscopy. 

J-Resolved Spectroscopy. These spectra (either homo- or hetero-nuclear) are 2D spectra which allow the identification of coupling patterns and coupling constants. Each atom signal in the ID spectrum plotted on one of the edges of the spectrum shows in the 2D spectrum a number of spots equahng the multiplicity of the ID signal. Coupling patterns for otherwise overcrowded ID spectra can be identified. 

Chapter 7 Separating shifts and couplings J-resolved spectroscopy... 

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