J‐modulated ADEQUATE experiments using different kinds of refocusing pulses

Abstract

Owing to the recent developments concerning residual dipolar couplings (RDCs), the interest in methods for the accurate determination of coupling constants is renascenting. We intended to use the J-modulated ADEQUATE experiment by Kövér et al. for the measurement of (13)C - (13)C coupling constants at natural abundance. The use of adiabatic composite chirp pulses instead of the conventional 180 degrees pulses, which compensate for the offset dependence of (13)C 180 degrees pulses, led to irregularities of the line shapes in the indirect dimension causing deviations of the extracted coupling constants. This behaviour was attributed to coupling evolution, during the time of the adiabatic pulse (2 ms), in the J-modulation spin echo. The replacement of this pulse by different kinds of refocusing pulses indicated that a pair of BIPs (broadband inversion pulses), which behave only partially adiabatic, leads to correct line shapes and coupling constants conserving the good sensitivity obtained with adiabatic pulses.

Additional pitfalls of using 1,1-ADEQUATE for structure elucidation

“… J ‐Modulated 1,1‐ADEQUATE spectrum of 2 . Data were acquired at 600 MHz for the same sample used for the acquisition of the data shown in Fig.…”

Inversion of 1JCC correlations in 1,n‐ADEQUATE spectra

“…The choice of 1 J CC coupling constants for the calculation of the amplitude transfer curves shown in Fig. was based on a J ‐modulated ADEQUATE spectrum of retrorsine ( 1 ) . The measured 1 J CC couplings constants are shown superimposed on the structure below; the experimental spectrum is shown in Supplemental Fig.…”

Application of 1,n-ADEQUATE and Modified Variants to Structure Elucidation and Spectral Assignment Problems

“…49 Extensive DFT calculations were also performed on 13 C chemical shifts as well as the calculation of 1 J CH , n J CH , 1 J CC , and n J CC coupling constants. The structure of coniothyrione was reinvestigated using 1.7 mm MicroCryoProbe capabilities that included the acquisition of HSQMBC, 50 -52 1,1-ADEQUATE, 12 -14 Jmodulated ADEQUATE, 27,28,43 and single optimized inverted 1 J CC 1,n-ADEQUATE 16 spectra for which the 1 J CC / n J CC coupling delays were optimized for 60/6 Hz. The optimization of the experiment was a compromise to accommodate the large 1 J CC C3-C4 coupling of 81.7 Hz measured using a J-resolved ADEQUATE experiment.…”