Inverse INEPT

The inverse INEPT (Bodenhausen and Ruben, 1980) and inverse DEPT (Brooks et ai, 1984) experiments utilize such an approach. In the inverse INEPT experiment, successive 90° pulses are applied to the C nucleus, followed by a H read pulse. Protons not coupled to the C nucleus are suppressed by presaturation of the entire H-NMR spectrum before the polarization transfer, so only those signals will be detected that are generated by polarization transfer from the C nucleus. 

FIGURE 12.16 Pulse sequence for the triple resonance 3D NMR experiment HNCO. H and N denote H and 15N, C denotes 13C=0, and K denotes 13C . Pulses at times 1, 2, and 3 constitute an INEPT sequence that transfers coherence from H to. V, where it precesses during q. Pulses at times 6, 7, and 8 represent an HMQC sequence that creates multiple quantum coherence in C (where it precesses during and transfers coherence back to N. Pulses 10 and 11 are an inverse INEPT sequence that transfers coherence back to H for detection during f3.The other 180° pulses refocus heteronuclear spin couplings. Note that coherence is not transferred to spin K. 

Figure 2.9 Pulse sequence for the INEPT experiment. (B) Effect of pulses on H magnetization. Application of the pulse sequence shown results in population inversion of one of the two proton vectors of the CH doublet and therefore causes an intensification of the corresponding C lines.

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Fig. 1. Pulse sequences for determining spin-lattice relaxation time constants. Thin bars represent tt/2 pulses and thick bars represent tt pulses, (a) The inversion-recovery sequence, (b) the INEPT-enhanced inversion recovery, (c) a two-dimensional proton-detected INEPT-enhanced sequence and (d) the CREPE sequence. T is the waiting period between individual scans. In (b) and (c), A is set to (1 /4) Jm and A is set to (1 /4) Jm to maximize the intensity of IH heteronuclei and to (1/8) Jm to maximize the intensity of IH2 spins. The phase cycling in (c) is as follows 4>i = 8(j/),8(-j/) 3 = -y,y A = 2(x),2(-x) Acq = X, 2 —x), X, —X, 2(x), —x, —x, 2(x), —x, x, 2 —x),x. The one-dimensional version of the proton-detected experiment can be obtained by omitting the f delay. In sequence (d), the phase 4> is chosen as increments of 27r/16 in a series of 16 experiments.

The physics behind SPI (Selective Polarization Inversion)77 or SPT (Selective Polarization Transfer)78,79 experiments is described and explained in every current NMR textbook, since it provides a nice introduction to understanding some of the more common current experiments (e.g. INEPT or 2D homo- and heteronuclear correlation experiments). The two names, SPI and SPT, are used indiscriminately for the same experiment, although in general SPI might be considered a special case of an SPT experiment with maximum polarization transfer achieved by inversion78. 

FIGURE 51. Measurement of 29Si spin-lattice relaxation time in 1,1,3,3-tetramethyldisilazane using conventional inversion-recovery (top, measuring time 6.5 h, T = 37.6 1.4 s) and INEPT enhanced version (bottom, measuring time 45 min, T = 38.1 0.9 s) with the phase of penultimate proton pulse +y. Reproduced by permission of Academic Press from Reference 357... 

FIGURE 9.10 Summary of the INEPT experiment as applied to a coupled H-13C system. Top Pulse sequence. Bottom Depiction of the behavior of and 13C magnetizations, as described in the text. In practice, the 90° l3C pulse may be applied coincident with the final H pulse but is shown slightly displaced to illustrate in (g) and (h) the behavior of the 13C magnetization after the population inversion has been established in (f). 

Is the probe direct, or inverse The former is good for direct observation with or without INEPT enhancement. The latter will give poor signal-to-noise in direct experiments since the sample does not fill the coil space, but is much preferred for indirect detection via, for example, a heteronnclear mnltiple qnantum coherence (HMQC) or heteronnclear single qnantum coherence (HSQC) experiment. 

Three pulse schemes which were proposed for indirect Ti measurements and produce the same results as the inversion-recovery sequence are shown in Fig. 12. The first method is an inversion-recovery experiment on the insensitive "Y spin, with a subsequent DEPT transfer to the observed nucleus "X. Although successfully applied in model studies, the experiment suffers from the necessity of long relaxation delays and was considered to be too insensitive for application to and insensitive metal nuclei. Higher sensitivity can be achieved with double polarization transfer methods which start with "X magnetization. TWo sequences were employed on the basis of double DEPT (Fig. 12(b)) or INEPT transfer (Fig. 12(c)). In order to remove effects of... 

F%. 12. Pulse sequences for indirect Ti measurements " (a) Inversion recovery with subsequent DEPT transfer, (b) Inversion recovery with double DEPT transfer and a series of 180°(X) pulses to suppress cross-relaxation effects, (c) Inversion recovery vrith double INEPT transfer. 

Figure 4.22. The evolution of proton vectors during the INEPT sequence. Following initial evolution under Jxh the I80°(H) pulse flips the vectors about the x-axis and the ISOTC) pulse inverts their sense of precession. After a total evolution period of I/2Jhx the vectors are antiphase and are subsequently aligned along z by the WyfH) pulse. This produces the desired inversion of one half of each H-X doublet for all resonances.

The processes that occur during the evolution period are probably the most important in describing the effect of the complete pulse sequence. During this period coherence can evolve, coherence can be selectively manipulated or coherence transfer can occur. Coherence manipulation can be the inversion of the coherence order (WATERGATE experiment) or in a l S spin system a phase shift depending upon signal multiplicity (APT or SEMUT experiment). In the case of heteronuclear IS spin systems the creation of antiphase coherence and subsequent polarization transfer using a INEPT or a DEPT unit can be used in multiplicity edited experiments or heteronuclear 2D correlation experiments. In transient NOE experiments such as ROE and TROESY, coherence... 

The DEPT sequence shown below was introduced to overcome problems associated with the INEPT sequence [5.65]. Further development have led to the inverse DEPT sequence [5.66] used in IR detected 2D experiments [5.68] in addition to direct detection... 

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