Pulse sequence INADEQUATE

All disilanes and trisilanes were synthesized with methods described in the literature [6]. 29Si NMR spectra were recorded with a BRUKER MSL 300 spectrometer, using solutions of the silanes in C6D6 (app. 50 %). 29Si29Si coupling constants were measured with the standard INADEQUATE pulse sequence. 

Fig. 2.48. INADEQUATE pulse sequence (a I) and pulse-driven motion (a-f) of carbon-13 magnetizations S0 for UC —12C singlet and S2 for 13C — 13C doublet signals in the rotating frame. Proton broadband decoupling is applied throughout the experiment.

The AB and AX systems of all 13C —13C bonds appear in one spectrum when the INADEQUATE pulse sequence (Fig. 2.48) is applied. Complete interpretation usually becomes difficult in practice due to signal overlapping, isotope shifts and AB effects (Section 2.9.4). A separation of the individual 13C— 13C two-spin systems by means of a second dimension would be desirable. It is the frequency of the double quantum transfer (d e) in Fig. 2.48 which introduces a second dimension to the INADEQUATE experiment. This double quantum frequency vDQ characterizes each 13CA — I3CX bond, as it depends on the sum of the individual carbon shieldings vA and vx in addition to the frequency v0 of the transmitter pulse located in the center of the spectrum if quadrature detection is applied [69c, 71] ... 

ID INADEQUATE LBR pulse sequence generates a theoretical sensitivity enhancement of 41% compared to a regular ID INADEQUATE. The same gain is also provided by ID INADEQUATE CR.5 As the latter pulse sequence contains more delays, ID INADEQUATE LBR is more robust when it comes to the mismatch between the set and actual / values. An experimental comparison, based on 11 sucrose signals, of all three ID INADEQUATE pulse sequences... 

Typical 13C rectangular pulses with y Blmax = 20 kHz (12.5 and 25 /is for 90° and 180° pulses) generate large off-resonance effects for chemical shift range of 200 ppm required for 13C. These effects can lead to a significant attenuation or complete loss of signal in multi-pulse INADEQUATE pulse sequences. Practical implementations of INADEQUATE pulse sequences must, therefore, include some sort of compensation of off-resonance effects. Several approaches have been reported to date. 

For the HCCH and HCC segments, which are of particular interest to polyaromatic compounds, high relative sensitivity (0.5 and 0.25) was also predicted for the DEPT2-INADEQUATE pulse sequence (Figure 8 C). The main reason why this pulse sequence performs so well is that it incorporates refocusing of AP proton-carbon coherences prior to the creation of DQ carbon coherences. Its performance is also boosted by a smaller number of pulses (6 H and 7 C vs. 11 H and 10 C) and delays compared to those of the COS-INADEQUATE pulse sequence. 

Very recently, Cadars et al. have presented a new modification of refocused INADEQUATE pulse sequence with the acronym REINE (REfocused INADEQUATE spin-Echo).64 As shown in Figure 12, the REINE joins two known approaches. The additional spin-echo period in the REINE experiment allows the detection of the /-modulation for each 2D peak. 

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. 

R.K. Harris, S.A. Joyce, CJ. Pickard, S. Cadars, L. Emsley, Assigning carbon-13 NMR spectra to crystal structures by the INADEQUATE pulse sequence and first principles computation a case study of two forms of testosterone, Phys. Chem. Chem. Phys. 8 (1) (2006) 137-143. 

C-NMR chemical shift data of cellulose 7a have been determined by Hesse-Ertelt et al by INADEQUATE and RAI techniques applied to uniformly C-labelled bacterial celluloses of different Gluconacetobacter xylinus strains. The robustness of the refocused INADEQUATE MAS NMR pulse sequence for probing through-bond connectivities has been demonstrated by Cadars et al in a large range of solid-state applications. The authors present a detailed account that combines product-operator analysis, numerical simulations and experiments of the behaviour of a three-spin system during the refocused INADEQUATE pulse sequence. 

Figure 5.27. The INADEQUATE pulse sequence. Since at the end of the pulse sequence, the intense magnetization Mq of the single nucleus is 90° out of phase with respect to the weaker magnetizations, M and M", of the adjacent nuclei, the latter can be detected without interference from the former. As M and M" are out of phase in (f), an additional sequence 1/(4 J)- 180J-1/(4 J) can be applied to refocus them.

The multi-receiver PANACEA experiment is designed aroxmd the INADEQUATE pulse sequence [32, 33] and also incorporates ID spectra, multiplicity edited 2D HSQC-s and 3D J-HMBC experiments. In addition, PANACEA does not require the conventional field/frequency lock and can be recorded in pure liquids. 

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