Purging sandwiches and purging pulses in polarization transfer experiments. The equivalence of DEPT and refocused INEPT with a purging sandwich

Abstract

A purging sandwich of proton pulses, 90”(‘H)-&180”(‘H)-t-90”(‘H), inserted into NMR pulse sequences has recently been successfully applied for improving experimental 1D and 2D NMR techniques (1). These include suppression of J cross-talk in 13C subspectral editing by extension ofthe SEMUT (2) and DEPT (3) pulse sequences to their GL versions (4), broadband homonuclear decoupling (5, 6) and separate determinations of heteronuclear coupling constants (7, 8) in 2D heteronuclear shift correlation and J-resolved spectroscopy, respectively. Similarly introduction of a single purging 90°(‘H) pulse just prior to acquisition may be useful for improving or modifying the acquired FID. An illustrative example is provided by the refocused INEPT (9, 10) pulse sequence which with a final 90”(‘H) purging pulse (INEPT+) (II) completely eliminates the multiplet and phase anomalies observed in proton coupled INEPT/refocused INEPT 13C or “N spectra (II, 12). DEPT and INEPT are generally considered two independent variants on the theme of polarization transfer with otherwise no coherence. In this communication we demonstrate that insertion of a purging sandwich 90°-~/2-1800-~/2-900(~o) into refocused INEPT (IO) results in a pulse sequence identical to DEPT (f3 = 90” gives the equivalent to EPT (13)). Furthermore, we point out that pulse sequences recently reported as new methods for polarization transfer (14) actually must be considered simple extensions of already reported sequences using a 90”(‘H) purging pulse or purging sandwich; e.g., DEPT with a 90°(‘H) purging pulse or INEPT+ with a purging sandwich. Thus, the results of such sequences are most easily understood by simply considering the effect of the purging pulse with a particular phase (or of the purging sandwich) on a pulse sequence already analyzed. Using operator techniques (I, 2, 4, II, 12, 15) it is demonstrated how the effects of purging sandwiches and 90” purging pulses easily account for the equivalence and results of these pulse sequences. DEPT (3) has been observed and claimed to be superior to INEPT in several aspects; e.g., DEPT yields undistorted multiplets in coupled spectra and is markedly less sensitive to variations in J values. A detailed analysis of INEPT, DEPT, and extensions of these techniques, provided by Sorensen and Ernst (II), points out the spectral distortions resulting from these experiments. Although DEPT has been claimed to yield accurate edited 13C subspectra (3, 16) it is clearly far from being insensitive to variations in J (2, 1 I). However, J cross-talk for DEPT (2) arising from the J range