INEPT-HMQC

The sensitivity of these experiments can be increased by employing INEPT polarization transfer to nucleus Y in HETCOR or to nucleus X in HMQC or HSQC in place of the first 90° pulse (as in a number of 3D experiments). The INEPT-HMQC combination ... 

Fig. 4. Special pulse schemes for 2D- X, "Y H) correlations. The same notation as before is used A denotes a fixed delay of length ( /( H,Y(X)) l (a) HMQC sequence for indirect detection of spin-1 nuclei. (b) INEPT-HMQC. (c) INEPT-HETCOR. (d) HMQC-TOCSY si denotes an MLEV spinlock sequence of duration t which is framed by trim pulses. ...

Indirect 2D NMR methods are generally more facile routes to obtaining chemical shifts for species in solution. The spectra are measured under the conditions (concentration, spectral parameters and accumulation times) characteristic of the nucleus through which the silver is being indirectly observed. DEFT, INEPT, HMQC and HSQC are the techniques most commonly employed. 1H, 13C, 19F and 31P are most frequently used as the observing nuclei in the current literature. Several examples of indirect observation are given here. 

Both direct and indirect methods (INEPT, HMQC, broad due to the quadrupole moment of the metal. 

J splittings cannot be directly resolved. In addition to the obvious advantage of providing a map of chemical bonds between the spins, /-based transfers do not require spin-locking and are not disturbed by molecular motions. The major drawback of polarization transfer through J coupling is that the delays involved in the pulse sequences, such as insensitive nuclei enhanced by polarization transfer (INEPT) [233] or heteronuclear multiple-quantum coherence (HMQC)... 

The details of this analysis, which is a generalization of a previous formalism for n = 1 [236], are beyond the scope of this review. Overall, the efficiency of R-INEPT and HMQC experiments has been found to be comparable, at least in applications involving 31P and 27Al. A major advantage of R-INEPT is that it can be combined with MQMAS or STMAS into a simple 2D heteronuclear correlation experiment, MQ/ST-J-HETCOR (Sect. 7.3). 

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... 

Fig. 20 2D HETCOR spectra of A1P04-14 (a) 27A1 31P R-INEPT at 9.4 T, (b) 27A1 31P HMQC at 9.4 T, (c) 27A1 31P R-INEPT at 18.8 T, (d) 27A1 31P HMQC at 18.8 T, (e) 27A1 31P SPAM-MQ-/-HETCOR with R-INEPT at 18.8 T, and (f) 27AI 3IP SPAM-MQ-D-HETCOR obtained using cross-polarization at 18.8 T. The total experimental times were 5 h (a), 5 h (b), 2 h (c), 2 h (d), 10 h (e) and 7 h (f). (Reproduced with permission from [235])...

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]. 

During the structural elucidation of the first flavone-xanthone dimer (swertifranche-side) through a series of ID or 2D NMR techniques including COSY, phase-sensitive ROESY, reversed-detected HMQC, HMBC, and selective INEPT experiments, H and C signals appeared as two peaks indicating two conformers and classically, the coalescence of the split signal was observed as the temperature was increased. 

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... 

Selective excitation and selective detection have been recently reviewed by the recognized leaders in this field135 who viewed 29Si NMR applications from a broader perspective. In this review we focus on those selective experiments that have found wider use in studies of organosilicon compounds, selective INEPT and HEED as representatives of the two classes (excitation and detection) of selective experiments. Some other selective experiments are mentioned in other sections (e.g. SPT, selective decoupling, SPINEPTR, V>-BIRD HMQC, DQF COSY). 

Later we will see how these couplings can be exploited in experiments that enhance the sensitivity of 13C spectra (INEPT), measure the number of hydrogens attached to each carbon (APT and DEPT), and correlate 13C chemical shifts with H chemical shifts using a second dimension (2D-HETCOR, -HMQC, -HSQC, and -HMBC). But for detecting a simple 13C spectrum, we need a way to suppress these 13C- H couplings so we can observe a single line (singlet) for each 13 C resonance. 

FIGURE 12.16 Pulse sequence for the triple resonance 3D NMR experiment HNCO. H and N denote H and 15N, C denotes 13C=0, and K denotes 13C . Pulses at times 1, 2, and 3 constitute an INEPT sequence that transfers coherence from H to. V, where it precesses during q. Pulses at times 6, 7, and 8 represent an HMQC sequence that creates multiple quantum coherence in C (where it precesses during and transfers coherence back to N. Pulses 10 and 11 are an inverse INEPT sequence that transfers coherence back to H for detection during f3.The other 180° pulses refocus heteronuclear spin couplings. Note that coherence is not transferred to spin K. 

Is the probe direct, or inverse The former is good for direct observation with or without INEPT enhancement. The latter will give poor signal-to-noise in direct experiments since the sample does not fill the coil space, but is much preferred for indirect detection via, for example, a heteronnclear mnltiple qnantum coherence (HMQC) or heteronnclear single qnantum coherence (HSQC) experiment. 

For Si, a new pulse sequence, a combination of INEPT and HMQC, was proposed. Here first an INEPT transfer from protons to Si is used to polarize the Si nuclei. Subsequently the HMQC sequence is applied to correlate Si with This technique is especially useful for heteronuclei which show no or negative NOE enhancements. The method was demonstrated on decamethylte-trasiloxane as a model compound and applied to a polymer silicon oil as an example [9]. correlations will become important in the field of silicon... 

HSQC. The Heteronudear Single Quantum Correlation (HSQC) experiment is an alternative to HMQC that accomplishes a similar objective. The experiment generates, via an INEPT sequence, single quantum (or " N) coherence, which evolves and then is transferred back to the proton frequency by a second INEPT sequence, this time in reverse. The main difference from the HMQC result is that HSQC spectra do not contain H- H couplings in the C ( Jj) dimension. As a result, HSQC cross peaks tend to have improved resolution over analogous HMQC cross peaks. HSQC is preferred when there is considerable spectral overlap. 

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