Excitation, polarization transfer INEPT

The other two spectral editing sequences are insensitive nuclei enhanced by polarization transfer (INEPT) and distortionless enhancements by polarization transfer (DEPT) (see Figure 12). Both of these sequences involve FF excitation, followed by... 

The selectivity of INEPT can be achieved in the most straightforward way, as suggested by Bax and coworkers301 302, by replacing all the hard non-selective proton pulses of INEPT with selective ( soft ) pulses (this pulse sequence is sometimes denoted as SPINEPT). To retain full sensitivity, the selective pulses must cover the selected multiplet and its 29Si satellites, i.e. the excitation band should be approximately equal to the width of the multiplet plus the value of /(29Si—XH) to be used for polarization transfer. For the choice of suitable selective pulses, see Reference 135. 

Most NMR spectroscopists have on some occasion examined the Periodic Table of NMR-active nuclei with 13C, I70, etc., and contemplated that at first glance essentially the whole Periodic Table is potentially open to study by NMR. Solution-state NMR studies of nuclei with small magnetic moments exist, but often when papers report data concerning such nuclei the information is obtained not by simple direct excitation but indirectly using polarization transfer techniques such as INEPT and DEPT, and/or by using reverse detection methods.1 As many of these indirect methods are not available in the... 

One-dimensional subspectra also may be obtained by combining selective excitation and broadband homonuclear Hartmann mixing with het-eronuclear polarization-transfer steps like INEPT, DEPT (distortionless enhancement by polarization transfer), or heteronuclear Hartmann-Hahn transfer (Doss, 1992 Gardner and Coleman, 1994 Willker et al., 1994). Related experiments with multiple-step selective Hartmann-Hahn mixing in combination with heteronuclear coherence transfer were used by Kupce and Freeman (1993a). 

Fig. 1. Pulse sequences for ID- X, "Y H) polarization transfer experiments. If not stated otherwise, narrow and wide bars denote 90° and 180° hard pulses, narrow and wide ellipsoids 90° and 180° shaped pulses. Essential phase cycles for selection of the polarization transfer signal are given on top of the pulses, if no phase is indicated, pulses are applied along the x-axis A denotes a fixed delay of length (n/(X,Y))" (a) UPT (, 0 = 90°), (b) unrefocused INEPT, (c) unrefocused selective INEPT with soft pulses, (d) INEPT with selective excitation via H, "Y cross-polarization si denote WALTZ-17 spinlock pulse trains which were applied for a period t— (2/( H,Y)) ...

To improve the sensitivity the selective J-resolved pulse sequence may be combined with a refocused INEPT polarization transfer experiment. In contrast to the original heteronuclear J-resolved experiment the first excitation pulse is executed on the F2 channel with the spin-echo sequence sandwiched between two incremental delay. Coupling evolves during the second incremental delay before the refocused INEPT unit creates in-phase coherence for the nuclei which are coupled to the selected proton nucleus allowing decoupling on the F2 channel during data acquisition. 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

Figure 3. Polarization transfer excitation using INEPT. Figure courtesy of Varian Associates.

---