Spectral Simplification and “Pseudo Decoupling” in One-Dimensional Proton NMR Spectra Using Pulsed-Field Gradients

Abstract

High-resolution proton NMR spectra of complex molecules are generally crowded due to the homonuclear spin–spin couplings. Knowledge of chemical-shift frequencies from such crowded spectra are of utmost importance in identifying the spin system. These shifts would have been easily measurable had it been possible to achieve proton decoupling. Since the pioneering work of Aue et al. (1) , two- dimensional J-resolved spectroscopy seems to have offered the best route to ‘‘broadband-decoupled’’ proton spectra. Subsequently, various attempts have been made to improve upon the technique (2–9) , of which the most promising solution seems to be ‘‘purged J spectroscopy’’ ( 8, 9) . Recently, a new proton ‘‘decoupling’’ method has been proposed (10) which uses the classic spin-echo sequence without any purging pulse. A new era has started in the field of NMR with the use of pulsed-field gradients, which in turn, is a result of technological progress in the design of high- doublequality shielded gradients. We have reported the use of these gradients for editing one-dimensional proton NMR spectra (11) , where the gradients are used for suppressing artifacts arising from $180^o$ pulse imperfections and for z filtering. It was shown that the experiment can be used to identify unambiguously singlets and triplets overlapping with other multiplets. This was done by suppressing the signals arising from doublets and quartets by matching the interval between the two $90^o_x$ pulses with \tau= 1/2J and with the use of gradient pulses (Fig. 1a) . This process leaves the coherences of the singlets unaffected. For triplets, the central line is unaffected and the outer lines are inverted (Fig. 2a) . We report here the extensions of this pulse sequence to filter singlets and triplets without any distortions (Fig. 1b) and to achieve ‘‘pseudo-decoupling’’ of the triplets in the proton NMR spectra (Figs. 1c and 1d).