INEPT-INADEQUATE

Summary Many cyclic and linear transition metal-silicon compounds have been obtained by the elimination of alkali halides. The structures of these complexes were determined by INEPT and INEPT-INADEQUATE NMR spectroscopy. 

Kuroda and coworkers240 used a 2D INEPT-INADEQUATE sequence with a sym-metrization similar to the simple INADEQUATE used by Hengge and Schrank218 as mentioned above. Their pulse sequence... 

FIGURE 30. 29Si—29Si ID INEPT-INADEQUATE spectra top spectrum for 17, J = 108 Hz bottom spectrum for 18, J = 1.2 Hz (no experimental details given). Reproduced by permission of Wiley-VCH from Reference 238... 

The INEPT-COS-INADEQUATE (Figure 8 B), which is in terms of sensitivity equivalent to the 1,1-ADEQUATE,37 outperforms (relative sensitivity of 0.325) the INEPT-INADEQUATE experiments only for HiCCHm (m = 0, 1, 2, 3) segments. Particularly encouraging theoretical predictions for HCCH and HCC segments were obtained for the COS-DEPT pulse sequence35 (not shown) with double the relative sensitivity (0.65) for HCCH segments and the same sensitivity (0.325) for HCC moieties. These results, however, were not confirmed experimentally by the authors. 

Figure 15 (A) A 2D DJM-INEPT-INADEQUATE spectrum of Me-/J-D-xylopyranoside. (B) F, traces...

Figure 18 Section of a H13C INEPT-INADEQUATE spectrum of 5 at natural abundance and 13C-labeled 6. From Ref. 62, reproduced by permission of the American Chemical Society.

The NMR measurements were performed in CgDj, partly mixed with toluene in order to increase solubilities. Chemical shifts are referred to TMS or CFCI3. All experiments were carried out on a Bruker 300 MSL spectrometer. Si spectra were recorded at 59.6 MHz using INEPT and INEPT-INADEQUATE pulse sequences among others. F Spectra were recorded at 282.4 MHz and H spectra at 300.1 MHz. 

The major disadvantage to this experiment is its extremely low sensitivity. An alternative is INEPT-INADEQUATE (sometimes called ADEQUATE), which uses proton observation and pulsed field gradients over pathways such as H-C-C. The resulting spectrum resembles that of 2D INADEQUATE, but can contain peaks only when at least one of the paired carbons has an attached proton. Although such technical refinements ameliorate the problem of sensitivity, this family of experiments has not been widely used. 

The 2D INADEQUATE or INEPT-INADEQUATE experiment requires additional spectrometer time. It is usually an experiment of last resort, although specific structures may be particularly amenable to this technique—for example, when there are several quaternary carbons that prevent COSY analysis. 

Figure 5.76, The proton-delected INEPT-INADEQUATE spectrum of menthol 5.11 (50 mg in CDCI3) recorded at 400 MHz using the optimised 1,1-ADEQUATE sequence of [97], The step-wise tracing of the carbon skeleton of the cyclohexane ring is illustrated in which carbon connectivities are identihed in horizontal traces (alternative routes involving the diastereotopic 3, 4 and 6 partners also exist but, for clarity, are not shown).

Figure 5.77. The gradient-selected INEPT-INADEQUATE sequence for identifying carbon-carbon connectivities through proton observation (time delays in the sequence have been removed for clarity). The sequence selects for H- C- C fragments for which the gradient ratios are 1 —1 1. Despite the complexity of the sequence, its operation may be understood by recognising discrete, simpler segments within it that have defined roles to play, which in this case are INEPT and INADEQUATE steps.

The first step in increasing the signal intensity in INADEQUATE experiments is to implement a polarization transfer step. The INEPT-INADEQUATE experiment [5.162] starts with the INEPT sequence creating antiphase coherence for the sensitive nucleus with a subsequent coherence transfer step before continuing the basic INADEQUATE experiment. In Check it 5 A. 3.2 the INEPT-IN ADEQUATE pulse sequence is created and the results compared with the unrefocused ID INADEQUATE experiment. 

The operation of the proton-enhanced sequences is best understood by reference to the original INEPT-INADEQUATE sequence shown in simplified form in Fig. 5.80 in which the individual component parts have been identified and may be followed in a stepwise manner. Thus, initial transfer from proton to carbon is via the INEPT sequence introduced in Chapter 4 followed by the generation of DQC as for the INADEQUATE above. This again evolves to encode c after which gradient selection begins. The DQ then returns to single quantum, and the C- C-coupling is refocused by the spin-echo. The final step is INEPT in reverse to transfer from carbon back onto proton, followed by... 

INEPT-INADEQUATE insensitive nuclear enhancement by polarization transfer... 

The substitution pattern is recognizable by the Si-2D-INEPT-INADEQUATE-NMR spectra (Fig. 3). 

Si INEPT-INADEQUATE NMR spectroscopy afforded further evidence for the nortricyclene structure of 7 as the coupling constants (Fig. 4) are consistent with the proposed atom arrangement. 

MHz frequency as shown. A proton spectrum occurs over a chemical shift range of 10 ppm, which corresponds to 2.5 kHz at 500 MHz. As seen in Figure 3.22, all of the protons in the sample would see 98%-100% of the power of the 500 MHz radiation delivered and all would be excited simultaneously. A pulse programmer is used to control the timing and shape of the RF pulses used to excite the sample. Square wave pulses are commonly used, but multipulse experiments and 2D NMR experiments with other pulse shapes are performed. There are hundreds of pulse sequences and 2D experiments that have been developed, with curious names like attached proton test (APT), DEPT, INEPT, INADEQUATE, COSY, and many more, some of which will be discussed later in the chapter. Each pulse sequence provides specific and unique NMR responses that enable the analyst to sort out the NMR spectrum and deduce the chemical structure of a molecule. 

FIGURE 3. 53.7-MHz Si- Si 2D-INEPT-INADEQUATE spectrum (a) Tetracycle 5a in CeDe-(b) Tricycle 6 in CDCI3 (40 °C). and projection of the 2D spectrum on the F2 axis (top). Signals connected by solid lines correspond to one-bond interactions ( /) those connected by dotted lines denote long-range couplings ("/). Reproduced by permission of VCH Verlagsgesellschaft from Reference 12... 

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