INEPT scheme

In summary, two rules of thumb emerge for optimum conduction of "X Y correlations (i) indirect detection via the HMQC scheme is the method of choice if the ratio Jx/jy is high and/or large spectral widths in FI have to be covered (ii) in all other cases, relaxation and line width aspects turn the scale 2D experiments are preferably performed by observing the nucleus with the narrower lines, with the use of HMQC, HSQC or polarization transfer schemes depending on the individual sample properties, and ID-INEPT schemes may be competitive for systems with few "Y nuclei. The situation of case (i) is typically encountered for correlations between highly receptive nuclei as or and low-y transition metals ( Fe, Rh, Ag, Os). Case (ii) applies to correlations between C and heteronuclei with comparable resonance frequencies ( Si, Pt, Hg, ° Pb), but also to borderline cases such as P, C or where despite considerable en-... 

The capabilities of some of these schemes are shown in Fig. 20, featuring a series of 2D 27A1-31P HETCOR spectra of A1P04-14, including 27A1 31P R-INEPT (MAS-based, acquired at 9.4 T and 18.8 T), [31P]27A1 HMQC (following the approach of Massiot et al. [254], also acquired at 9.4 T and 18.8 T), as well as... 

In the case of the 7-hydroxy-substituted compounds 44 (Scheme 2), 54 different derivatives were investigated by 13C NMR spectroscopy and, in some cases, also by 1SN NMR spectroscopy <1995JST(335)273>. With the help of proton-coupled 13C NMR spectra, semi-selective INEPT (insensitive nuclei enhanced by polarization transfer) experiments, and heteronuclear multiple bond correlation (HMBC) two-dimensional 2D-NMR spectra, all shifts could be unequivocally assigned. While the C-7 shifts did not allow the existing tautomeric situation to be determined, a clear decision could be made by H NMR spectroscopy in this respect. The 1SN NMR spectra revealed an equilibrium between the N(4)H and N(3)H tautomeric forms, which is fast on the NMR timescale. 

Magnetization is initially transferred from 1Hn(i) to 15N(/) spin. Unlike in the HNCA-TROSY scheme, the desired coherence is transferred from the 15N(i) spin to the 13C7(/ — 1) spin of the preceding residue. To this end, nearly uniform fNc( 15 Hz) scalar coupling is used. As 2/NC is negligibly small, the coherence is transferred exclusively to the 13C (/ — 1) nucleus. Finally, the 13C —13C INEPT is used to transfer magnetization from 13C (i— 1) to... 

The first part, before the t period, of the experiment is identical to the HN(CO)CA-TROSY scheme. This step chooses solely the sequential pathway in an HN(CO)CA-TROSY manner. The chemical shift of the 13C nucleus is recorded during the t evolution period. The back-transfer route is, however, quite different. We transfer the desired coherence from 13C directly back to the 15N nucleus and remove the second 13C -> 13C INEPT step found in HN(CO)CA-TROSY and replace it with the HNCA like back-transfer step. The antiphase 2./N<> coupling then refocuses simultaneously with VNc during the 13C 15N back-INEPT step. Thus, the HN(CO)CANH-TROSY... 

As in the case of the HN(CO)CA-TROSY scheme, the HN(CO)CANH-TROSY experiment can be readily expanded to a four-dimensional HN(CO)CANH-TROSY experiment without increasing the overall length of the pulse sequence. This can be accomplished by labelling the 13C (f) chemical shift during additional incremented time delay, implemented into the 13C — 13C INEPT delay. As a result, a well-dispersed 13C (i- 1), 13C (i- 1), 15N(i), Hn(0 correlation map is obtained with minimal resonance overlap albeit with the inherent sensitivity loss by a factor of y/2. 

Poly(hydrosilane)s are stable compounds and can be manipulated in the air only for a short period since they are oxygen sensitive. In order to study the oxidation products, a xylene solution of poly(phenylhydrosilane)(Mw = 2340, Mw/Mn = 1.72) was refluxed (140 °C) for 12 h in a system exposed to the air [15]. Only minor changes were observed by GPC analysis whereas FTIR showed characteristic absorptions due to siloxane-type structures on the polymer backbone. A detailed NMR analysis, based on H NMR, Si INEPT and H- Si HMQC spectroscopies, indicated that the oxidized material contains the units 7-10 shown in Scheme 8.2. In particular, units 7,8 and 9+10 were present in relative percentages of 27%, 54% and 19%, respectively, which mean that more than 70% of the catenated silicons were altered. It has also been reported that silyl hydroperoxides and peroxides are not found as products in the autoxidation of poly(phenylhy-drosilane) [16]. 

Phase cycling scheme for the selective reverse INEPT pulse sequence of fig. 1. Phases are shown in multiples of 90° subscripts indicate that a given phase or bracketed block of phases should be repeated the stated number of times, e.g., the notation (01)2 (13)2 indicates the sequence 0101 1313. Phases in the sequence of fig. 1 other than those listed above remain unchanged in successive transients. 

A remaining limitation of the pulse sequence shown - the restriction of the accessible spectral range of Y nuclei which results as a consequence of the need to apply -pulses to the Y-channel - is circumvented by the sequence shown in Fig. lib43 which uses INEPT for the /X and a HMQC scheme for the... 

The idea of back transformation of a three-dimensional NMR experiment involving heteronuclear 3H/X/Y out-and-back coherence transfer can in principle be carried to the extreme by fixing the mixing time in both indirect domains. Even if one-dimensional experiments of this kind fall short of providing any information on heteronuclear chemical shifts, they may still serve to obtain isotope-filtered 3H NMR spectra. A potential application of this technique is the detection of appropriately labelled metabolites in metabolism studies, and a one dimensional variant of the double INEPT 111/X/Y sequence has in fact been applied to pharmacokinetics studies of doubly 13C, 15N labelled metabolites.46 Even if the pulse scheme relied exclusively on phase-cycling for coherence selection, a suppression of matrix signals by a factor of 104 proved feasible, and it is easily conceivable that the performance can still be improved by the application of pulsed field gradients. 

Reproducibility of the results depends strongly on the signal-to-noise ratio achieved. Therefore, whenever polarization transfer can give some sensitivity enhancement, use of INEPT (or other scheme) for T measurement would increase the precision and save experimental time. It is also possible to measure the relaxation times indirectly through measurements of 29Si satellites in 3H NMR spectra355, but application of the INEPT variant of such measurements is much more straightforward. 

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