the spins, and the gradient pulse pairs G 1 , G 2 and G 4 , G 5 , High-resolution proton NMR spectra of complex molesandwiching 180Њ pulses, eliminate the artifacts that might cules are generally crowded due to the homonuclear spinbe created due to 180Њ pulse imperfections. spin couplings. Knowledge of chemical-shift frequencies
In the pulse sequence of Fig. 1b, for an uncoupled spin I from such crowded spectra are of utmost importance in iden-(singlet), the equilibrium magnetization I z is transformed tifying the spin system. These shifts would have been easily measurable had it been possible to achieve proton decouinto 0I y after the first 90Њ x pulse, the second 90Њ x pulse conpling. Since the pioneering work of Aue et al. (1), twoverts it back into I z which is unaffected during the gradient dimensional J-resolved spectroscopy seems to have offered pulse G 3 . The last 90Њ x pulse transforms it into 0I y , which the best route to ''broadband-decoupled'' proton spectra. becomes I y by the end of the subsequent refocusing t period. Subsequently, various attempts have been made to improve This echoed signal is detected during acquisition. For a couupon the technique (2-9), of which the most promising pled spin, 0I y magnetization after the first 90Њ x pulse evolves solution seems to be ''purged J spectroscopy'' (8, 9). Reunder the J couplings to other spins. For a two-spin system cently, a new proton ''decoupling'' method has been pro-I and S, the spin I evolves under the coupling with spin S posed (10) which uses the classic spin-echo sequence withgiving rise to the terms 0I y cos(pJt) and 2I x S z sin(pJt). out any purging pulse. A new era has started in the field of The first term goes to zero for t Å 1/2J. The second term NMR with the use of pulsed-field gradients, which in turn, after the second 90Њ x pulse is converted into 02I x S y , a mixis a result of technological progress in the design of highture of zero, and double-quantum coherences. The doublequality shielded gradients. We have reported the use of these quantum coherences are dephased by the gradient pulse, G 3 , gradients for editing one-dimensional proton NMR spectra while the zero-quantum coherences, which are insensitive to (11), where the gradients are used for suppressing artifacts the magnetic field gradients, are suppressed by the random arising from 180Њ pulse imperfections and for z filtering. variation of the delay D, which is comparable with the recip-It was shown that the experiment can be used to identify rocal of the zero-quantum frequencies (13). Thus, at this unambiguously singlets and triplets overlapping with other point, all the signals arising from the two-spin system (doumultiplets. This was done by suppressing the signals arising blet) are completely suppressed. Similarly, it can be shown from doublets and quartets by matching the interval between that the signals arising from a spin coupled to three other the two 90Њ x pulses with t Å 1/2J and with the use of equivalent spins (quartet) are also suppressed by the end of the delay D. For the spin coupled to two other equivalent gradient pulses (Fig. 1a). This process leaves the coherences of the singlets unaffected. For triplets, the central line is spins (triplet), it may be noted that after the third 90Њ x pulse, unaffected and the outer lines are inverted (Fig. 2a). We the central line is aligned along the y axis, while the outer report here the extensions of this pulse sequence to filter lines have opposite phase (11) (Figs. 1a and2a). These singlets and triplets without any distortions (Fig. 1b) and to outer lines are brought in phase with the central line by achieve ''pseudo-decoupling'' of the triplets in the proton allowing them to precess by an additional period of t Å NMR spectra (Figs. 1c and1d).
1/2J. This echoed triplet signal is detected during the acquisition. Thus, this pulse sequence suppresses doublets and The evolution of the coherences under the influence of radiofrequency pulses and t delays can be followed by the quartets, passing only singlets and undistorted triplets. product-operator formalism (12) and by a simple vector It is clear from the Fig. 1c that, until the third 90Њ x pulse, picture. The spins under consideration are all homonuclear the pulse sequence is the same as that given in Fig. 1b andspin-1 2 nuclei. The 180Њ pulses in the middle of the first and hence as described in the previous paragraph; by the end of the delay period D, the signals arising from the two-spin second t or t/2 periods in all the pulse sequences (Figs. 1b to 1d) serve to refocus the chemical-shift evolution of system (doublet) and from spins coupled to three other 113