INEPT multiplet intensities

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.

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. 

In polarization transfer experiments such as INEPT, the recorded multiplets do not have the standard binomial intensity distributions of... 

The simulated INEPT 13C spectrum of 2-chlorobutane is shown in Figure 12.14. By comparing this to the corresponding undecoupled 13C spectrum (Figure 8.11), note how each multiplet is both intensified and divided at its center into positive and negative lines that no longer show the same relative intensities as in the undisturbed multiplet. The appearance of these multiplets is quite similar to the multiplet effect CIDNP spectra we saw in Section 11.9, and as was true there, the middle leg of a multiplet with an odd number of lines vanishes. 

Figures Pt H NMR spectra of [Pt(C5H5 N)(PPh3)Cl2] recorded in THF (bottom) normal spectrum (middle) with INEPT enhancement from P, yijys = 40.48/21.50 2, note the antiphase multiplet (top) with refocused INEPT enhancement from P, note slight loss of intensity due to signal dephasing during the longer pnlse sequence...

The DEPT experiment [33] (Distortionless Enhancement by Polarisation Transfer) is the most widely used polarisation transfer editing experiment in carbon-13 spectroscopy, although its application is certainly not limited to the proton-carbon combination. It enables the complete determination of all carbon multiplicities, as does the refocused INEPT discussed above, but has a number of distinct advantages. One of these is that it directly produces multiplet patterns in proton-coupled carbon spectra that match those obtained from direct observation, meaning methylene carbons display the familiar 1 2 1 and methyl carbons the 1 3 3 1 intensity patterns this is the origin of the term distortionless . However, for most applications proton decoupling is applied during acquisition and multiplet structure is of no consequence, so the benefits of DEPT must lie elsewhere. 

INEPT unit refocuses the antiphase coherence (I Sy Sx). The detected signal intensity is correlated with the effective polarization transfer and can be reduced by the underlying I Sy antiphase coherence if the refocusing is not prefect. In the coupled spectrum these antiphase coherences produce multiplet anomalies. 

The following Check it demonstrates that the refocusing unit as an extension of the simple coupled INEPT experiment causes severe multiplet anomalies which can be simply circumvent by applying an additional 90° pulse to the I spin. This new sequence is called INEPT+. The complete discussion of the INEPT+ sequence and the product operator analysis of the refocused INEPT are described elsewhere [5.59]. The results for the observable coherences without the underlying intensity factors are summarized for the refocused INEPT and the INEPT+ experiment in Table 5.15. 

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). 

The expected intensities for any multiplet from a proton coupled INEPT sequence can be determined similarly by multiplying the terms of the appropriate binomial expansion (intensities expected for a normal H-coupled... 

J(HH) is of the same order. As largely reported for older HSQMBC experiments, the standard INEPT transfer can be replaced by other schemes such as INEPT-BIRD, CPMG, or CPMG-BIRD elements (Fig. 27B-D). In all these versions, pure IP multiplets would be obtained but with a different signal intensity dependence as a function of the chosen transfer element. As shown previously for PIP experiments, the key element is the adiabatic 2-filter that removes any AP contribution due to J(HH) and unmatched "J (CH) couplings evolution. The importance of the adiabatic z-filter is illustrated with the superior performance of the 8-Hz PIP module over conventional, CLIP, and z-filtered HSQMBC experiments acquired under the... 

The multiplets encountered in INEPT spectra do not have the normal binomial intensity distribution of 1 1 for doublets, 1 2 1 for triplets, 1 2 2 1 for quartets, etc. Instead, the intensities obtained are shown in the form of a Pascal triangle (Figure 5.18) in which the outer peaks of the multiplets are generally more intense than the inner ones. Figure 5.19 shows the NOE-... 

The zero sum of intensities in a multiplet in these differential polarization transfer experiments means that any decoupling that causes all the lines to coalesce results in exact cancellation of intensities. Several methods have been devised to avoid this problem when decoupling is required. One is pulse-interrupted precession,which results in a net polarization transfer, as does the insertion of a properly chosen delay period in the INEPT sequence (as in Figure 12) so that all members of a multiplet appear positively enhanced, i.e., INEPT with refocusing (INEPTR). The DEPT (Distortionless Enhancement Polarization Transfer) sequence gives undistorted relative intensities of the multiplet components. 

---