HETCOR experiments

An alternative way of acquiring the data is to observe the signal. These experiments are referred to as reverse- or inverse-detected experiments, in particular the inverse HETCOR experiment is referred to as a heteronuclear multiple quantum coherence (HMQC) spectmm. The ampHtude of the H nuclei is modulated by the coupled frequencies of the C nuclei in the evolution time. The principal difficulty with this experiment is that the C nuclei must be decoupled from the H spectmm. Techniques used to do this are called GARP and WALTZ sequences. The information is the same as that of the standard HETCOR except that the F and F axes have been switched. The obvious advantage to this experiment is the significant increase in sensitivity that occurs by observing H rather than C. 

The basic pulse sequence employed in the heteronuclear 2D shift-correlation (or HETCOR) experiment is shown in Fig. 5.40. The first 90° H pulse bends the H magnetization to the y -axis. During the subsequent evolution period this magnetization processes in the x y -plane. It may be considered to be made up of two vectors corresponding to the lower (a) and higher (/3) spin states of carbon to which H is coupled. These two... 

H/ C One-Bond Shift Correlations by Hetero-COSY (HETCOR) Experiment... 

COSY and HETCOR experiments are extremely useful in the structure elucidation of complex organic molecules. The geminal and vicinal protons and their one-bond C-H connectivities are first identified from the HETCOR spectrum, and then the geminal couplings are eliminated from the COSYspectrum, leaving vicinal connectivities. By careful interpretation of the COSY and the one-bond HETCOR spectra, it is then possible to obtain information about the carbon-carbon connectivities of the protonated carbons ( pseudo-INADEQUATE information). In this way the carbon-carbon connectivity information of protonated carbons is obtainable through a combination of COSY and HETCOR experiments. 

The 27Al-29Si RAPT-CPMG-HETCOR experiments were performed at 14.1 T on a Varian NMR System 600 spectrometer, equipped with a triple resonance MAS probe. 

Figure 1. Pulse sequence for 27Al-29Si HETCOR experiment with sensitivity enhancement by RAPT and CPMG data acquisition.

Fig. 19 Pulse sequences and coherence transfer pathways used in MQMAS- >-HETCOR experiments. In MQMAS-/-HETCOR, CP is replaced by /-based transfer...

Information about the surface and interface structures in hexadecylamine-capped CdSe NC of 2 nm size has been obtained by a variety of 1H, 13C, 113Cd, and 77Se NMR techniques [342]. The 77Se CP-MAS-NMR spectrum showed five partially resolved peaks from surface or near-surface Se environments. It was possible to obtain 2D heteronuclear correlation (HETCOR) spectra between 1H and the other three nuclei despite the inherent sensitivity limitations (the 77Se- 3I-I HETCOR experiment required 504 h ). The latter experiment indicated that the methylene protons of the hexadecylamine chain interact with the surface Se atoms via a tilt of the chain toward the surface. The surface Se atoms were not seen to interact with thiophenol present, and it was suggested that thiophenol binds to a selenium vacancy at the surface. 

Derivatives 47-51 were prepared from the iV-oxide 46 using zinc as the deoxygenating agent (Scheme 11). The use of Zn in NH4CI solution led to the deoxy derivatives 47 and 48 in moderate yields. The reduction was clearly observed by NMR HETCOR experiments (HMQC and HMBC). When the reduction of the derivative 46 (R = CH2C1) was carried out under the same conditions, compound 51 was generated via a Zn-promoted reductive dimerization process <2001EJM771>. 

Proton-carbon HETCOR experiment has recently been done on a liquid crystal97 to observe long-range proton-carbon correlations. In this... 

Fig. 8. Pulse schemes for 2H 2D NMR spectroscopy of weakly ordered, deuterated solutes (a) 2H Q-COSY, (b) 2H-13C HETCOR experiment with 13C decoupling, (c) 2H-2H COSY 2D experiment. (Reproduced by permission of American Chemical Society.)...

The carbon and DEPT (distortionless enhanced polarization transfer) spectra are shown in Figure 10. The HETCOR (heteronuclear two-dimensional proton-carbon correlation) spectrum is shown in Figure 11. The carbon assignments are listed in Table 5. Long-range HETCOR experiments were used to make the assignments for the thiophene carbons. 

Frequency-switched Lee-Goldburg (FSLG) 13C-1H heteronuclear correlation (HETCOR) experiments... 

We introduce the use of solid-state 2D FSLG 13C-3H HETCOR experiments for correlating intermolecular interactions in miscible, hydrogen bonded polypeptide blends in this section. The pulse sequence that is used... 

The miscibility scale of PG/PLV (50/50) has been shown to be about 3 nm by the H Tip experiments.72,73 These values are consistent with each other because the value of 2-3 nm is the upper-limit of the domain size.54 Thus, it can be said that the 2D FSLG 13C-3H HETCOR experiments do not conflict with the 111 Tip experiments reported previously. These results show that the PG and PLV chains in the blend are sufficiently close to each other to make intermolecular hydrogen bonds. 

Heteronuclear correlation (HETCOR) is an experiment establishing a correlation between the chemical shift of proton and that for another nucleus (i.e. C, P, Nor Si). The experiment relies on the heteronuclear dipolar interactions and uses a CP to transfer magnetization from protons to a rare spin.4,6 We have used 2D HETCOR experiment to characterize support/counter-cation/cluster interaction in the supported mesoporous catalysts.21... 

The l3C— H COSY (HETCOR) experiment correlates l3C nuclei with directly attached (i.e., coupled) protons these are one-bond ( /CH) couplings. The frequency domains of FI (v,) and F2 (v2) are of different nuclei, and so there is no apparent diagonal or symmetry. 

Fig. 20. Experimental setup for applications of the SPECIFIC CP experiment in the context of triple-resonance solid-state NMR experiments. After an initial broadband adiabatic CP step from protons to the I nuclei, SPECIFIC transfer to the observed 5 nucleus occurs during the mixing time tm. The resulting signal represents a dipolar and chemical shift-filtered spectrum and can be controlled by variation of the carrier frequencies and the radiofrequency during the SPECIFIC transfer. A conventional HETCOR experiment is obtained by the introduction of an evolution time t. (Adapted from Baldus et al.215 with permission.)...

Pulse Sequence. The basic pulse sequence for the HETCOR experiment can be considered as a derivative of the pulse sequence used for the CP experiments (Figure 46). The main difference between the CP and HETCOR pulse... 

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