INEPT spectra

As mentioned already, the INEPT spectra are typified by the antiphase character of the individual multiplets. The INEPT C-NMR spectrum of 1,2-dibromobutane is shown, along with the normal off-resonance C-NMR spectrum, in Fig. 2.12. Doublets show one peak with positive phase and the other with negative phase. Triplets show the outer two peaks with positive and negative amplitudes and the central peak with a weak positive amplitude. Quartets have the first two peaks with positive amplitudes and the remaining two peaks with negative amplitudes. 

Figure 6. 29Si NMR INEPT spectra of alkyne 6k (n = 3) (upper trace) and of cation 9k (lower trace) which demonstrate the low field shift of the 29Si NMR resonance AS 29Si upon ionization.

Fig. 4. (a) 300 MHz proton spectrum and (b)-(e) selective reverse INEPT spectra of 28% menthone (Aldrich) in acetone-ds, measured using a 5 mm sample in the 10 mm broadband probe of a Varian Associates XL300 spectrometer using the sequence of fig. 1. The sample contains substantial quantities of isomenthone, seen clearly in the methyl region of trace (a). Spectra (b) to (e) used selective excitation of carbon sites 6, 7, 2 and 8, respectively, with delays 2r of 3.85, 3.85, 1.92 and 1.54 ms. 32 transients were used for each trace no spin lock pulses or 180° pulses were used. Traces (b) to (e) have a vertical scale lOOOx that of trace (a). No homodecoupling was used during acquisition. 

Non-refocused INEPT spectra can be used to determine both 29Si—XH and 1II—1II couplings (but not their signs) in a manner similar to that described earlier for SPT spectra. The INEPT spectra are measured with several r values chosen almost arbitrarily and then a simulation program119 is used to determine the exact values of the coupling constants120,121. 

FIGURE 16. 29Si INEPT spectra (not refocused) of bis(trimethylsilyl)adenine 11 measured without decoupling (bottom) and with selective decoupling of NH proton (top) during acquisition. The selective decoupling identifies the silicon with chemical shift S = 6.88 as Si—NH while that at iS — 14 77 js ]—silicon. Reproduced by permission of John Wiley Sons, Ltd... 

FIGURE 43. Assignment of the two lines in the 29Si NMR spectmm of methyl 3o, 12a-bis(trimethyl-siloxy)-5/i-cholenate, 37, by selective INEPT. Top trace 29Si INEPT spectmm two middle traces selective INEPT spectra measured with selective excitation of H lines indicated by arrows in the bottom trace with partially assigned ll NMR spectmm (25 mg of the sample in 0.7 ml of CDCI3, H frequency 200 MHz, 29Si frequency 39.7 MHz, 5 mm broad-band probe, selective pulse by DANTE train, r = 70 ms, A = 149 ms). Reproduced with permission of Collection of Czechoslovak Chemical Communications from Reference 304... 

A number of "X, ""Y coherence transfer experiments involving polarization transfer from or to quadrupolar nuclei were performed by using both INEPT and the universal polarization transfer (UPT) scheme (Fig. 1(a)) which is a generalization of DEPT. The use of the INEPT method is unproblematic. It can be applied without modification for polarization transfer from any number of equivalent spin-half nuclei to a nucleus of arbitrary spin, and a report on successful measurement of C, D INEPT spectra also demonstrated its utilization for magnetization transfer from a quadrupolar to a spin-half nucleus. Application of the UPT scheme requires appropriate adjustment of the variable pulses <(> and 9 to the spin and number of the... 

Fig. 20. 2D- Li, P H INEPT spectra of a mixtixre of stereoisomeric phos-phavinyKdene carbenoids at -40°C and -60°C Tbe disappearance of the cross-peak for one isomer indicates that this species engages at the higher temperature in dynamic intermolecular exchange which leads to a loss of the spin-spin coupling.

Figure 15 Selective 2D J-S INEPT spectra and cross sections of a(l-3) linkage of trisaccharide 37.

Figure 4.24. Experimental carbon-13 INEPT spectra of methanoic acid recorded with the phase of the last proton pulse set to (a) y and (b) -y. Subtracting the two data sets (c) cancels natural magnetisation that has not been generated by polarisation transfer and equalises the intensity of the two lines.

Figure 4.26. Relative multiplet line Intensities in coupled INEPT spectra, (a) conventional multiplet intensities and those from INEPT (b) without and (c) with suppression of natural magnetisation.

Figure 4.27. Natural abundance N INEPT spectra of adenosine-5 -monosulphate 4.5. Lower traces show results with the INEPT sequence and upper traces with the refocused-INEPT sequence. Delays were calculated assuming (a) J = 90 Hz and (b) J = 10 Hz. For (a) the lower trace displays the 1 0 -1 pattern of the NH2 group whilst in (b) all resonances display antiphase two-bond H- N couplings. Spectra are referenced to nitromethane.

In practice, the appearance of decoupled INEPT and DEPT 29Si-NMR spectra are usually the same. However, coupled INEPT and DEPT spectra differ dramatically. Coupled DEPT spectra essentially appear as greatly enhanced standard acquisition spectra the multiplicity, phase, and relative intensities of multiplets using DEPT are the same as those obtained from normal FT-NMR techniques. In contrast, coupled INEPT spectra contain several distinctive distortions (1) the outer lines of multiplets in INEPT spectra are much enhanced compared to relative multiplet intensities obtained using standard acquisition or DEPT-NMR techniques (2) the central line of odd line multiplets in INEPT has zero intensity and (3) the two halves of a multiplet in INEPT are 180° out of phase. Thus, a triplet and a quartet in INEPT would appear as 1 0 —1 and 1 1 —1 patterns, respectively, instead of the normal 1 2 1 and 1 3 3 1 patterns seen with DEPT (see Section IV,A). 

As shown for dodecamethylcyclohexasilane (Section III,A), long-range couplings (provided they are resolvable) can be used for polarization transfer. From (MeO)4Si, for example, one can obtain well-resolved proton coupled spectra (Fig. 6) and decoupled spectra (Fig. 7). The coupled spectra vividly show the alteration of the relative intensities of the (MeO)4Si multi-plet in comparing the standard acquisition spectrum or DEPT spectrum to the INEPT spectrum. The relatively greater enhancement of the outer lines in coupled INEPT spectra is shown by the observation of 11 lines (including the zero-intensity central line) in the INEPT spectrum (Fig. 6a), compared to nine lines seen in the DEPT and standard acquisition spectra (Figs. 6b and c). Multiplet intensities in the DEPT and standard acquisition spectra are nearly identical, as expected. 

There are two cases in which the DEPT technique offers advantages over INEPT. First, in coupled spectra, DEPT gives normal multiplet phases and intensities, which may aid interpretation and are certainly more familiar in appearance than those obtained from INEPT. In addition, coupled DEPT spectra tend to be better resolved than the corresponding INEPT spectra, especially for complex spin systems (see Fig. 3). This is probably a consequence of DEPTs lesser J dependence compared to INEPT. Second, for samples in which large J variations are present or suspected, DEPT is the method of choice because it is less likely to suppress signals than INEPT (see Fig. 5). 

Doddrell et al. (12) derived the expression for the theoretical enhancement of decoupled INEPT spectra, d [Eq. (1), for t = Topt], Furthermore, Burum... 

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