Heteronuclear-resolved spectroscopy

In the heteronuclear version of the experiment, the chemical shift of the X spin is presented in/2 whilst couplings to a second nucleus, typically protons, are presented infi. 

The/i dimension therefore enables an analysis of resonance multipUcity as weU as measurement of the heteronuclear coupling constants (/xh) themselves, as described furtber below. 

To reduce the /i information content of the 2D spectrum to only couplings, it is necessary to make the detected FIDs insensitive to chemical shift evolution during the ti period, which is readily achieved by the use of a spin-echo during ti (Fig. 7.1a) [1,2]. Thus, following initial X spin excitation, simultaneous 180 proton and carbon pulses are applied at the midpoint of ri, such that X spin chemical shifts will refocus but tbe beteronuclear coupling will continue to evolve. There is no mixing step in /-resolved experiments because magnetisation or coherence transfer to other spins is not employed. The sequence 

Separation of heteronuclear couplings (usually H X) from chemical shifts. Used to determine the multiplicity of the heteroatom or to provide direct measurement of heteronuclear coupling constants. 

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Figure 5.9 Pulse sequence for spin-flip heteronuclear /-resolved spectroscopy.

What is the difference between homo- and heteronuclear 2D /-resolved spectroscopy ... 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

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.

The most recent developments in 2D NMR of solids are the heteronuclear chemical shift correlation spectroscopy (421), 2D exchange NMR, which enables very slow molecular reorientations to be monitored (422), and heteronuclear. /-resolved 2D NMR (423). 

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