Evolution of heteronuclear coupling

A perfect inversion pulse simply inverts z-magnetization or, more generally, all z-operators Iz — -Iz. If the pulse is imperfect, it will generate transverse magnetization or other coherences. Inversion pulses are used extensively in heteronuclear experiments to control the evolution of heteronuclear couplings. 

As might be expected at this point, two processes are ongoing in the second half of the evolution period. First, having applied a 180° pulse at ti/2, we should expect that magnetization will be refocused in a spin-echo at time = tj. Second, since the decoupler has been gated off for the second half of the evolution period, the spin echo will be J-modulated by the evolution of heteronuclear couplings during the second ti/2 interval. 

Now a simple, general and pratical method is available for direct measurement of V(ch) coupling constants [73]. ID and 2D heteronuclear single quantum multiple bond correlation (HSQMBC) experiments are easily performed in a routine manner. These experiments are based on the evolution of heteronuclear single quantum coherence (SQC) in... 

The first 90° proton pulse rotates the longitudinal magnetization to the y axis [Figure 5.77(b)]. The 180° pulse at the center of the evolution period removes the overall effects of heteronuclear coupling which would have otherwise existed at the end of the evolution period. The second nonselective... 

Evolution of the Hr magnetization under the heteronuclear long-range scalar coupling nJCH... 

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.

Fig. 10.13. 2D J-resolved NMR spectrum of santonin (4). The data were acquired using the pulse sequence shown in Fig. 10.12. Chemical shifts are sorted along the F2 axis with heteronuclear coupling constant information displayed orthogonally in F . Coupling constants are scaled as J/2, since they evolve only during the second half of the evolution period, t /2. 13C signals are amplitude modulated during the evolution period as opposed to being phase modulated as in other 13C-detected heteronuclear shift correlation experiments.

Fig. 11.8 (a) REDOR pulse sequence for the determination of dipolar couplings between 13C and 15N. Initially 13C polarization is generated by cross polarization from the protons. During the following evolution period n pulses are used to prevent the averaging of the heteronuclear dipolar... 

Fig. 11.16 The pulse sequence used to monitor the evolution of carboncarbon double-quantum coherence over a single rotor period in the presence of the proton-carbon heteronuclear dipolar coupling (a). The evolution of the double-quantum coherence between the Cl 4 and Cl 5 carbons in the retinal of bacteriorhodopsin in the ground state (b). The observed evolution is consistent with a C14-C15 torsion angle of 164° (reproduced with permission from Ref. [172]).

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