HMQC

A number of variations of both HSQC and HMQC have been developed. Both methods are widely used and are of comparable value. The HMQC experiment uses fewer pulses, hence can be somewhat shorter and is less dependent 

FIGURE 12.10 Pulse sequence for the heteronuclear multiple quantum coherence experiment. See text for discussion of the state of the spin system at the times indicated. 

As usual, suitable phase cycling and/or use of pulsed field gradients is critical to avoid the detection of undesired coherences. 

The pulse sequences described by Sklenar and Bax (1987), one by Zuiderweg (1990), and others, circumvent these problems and can be effectively utilized with macromolecules. However, because virtually all alkaloids may be categorized as small molecules, pulse sequences such as HMQC (Bax and Subramanian 1986) and others discussed below may be employed without considering problems inherent to large molecules. 

4 Inverse-Detected One-Bond Heteronuclear Shift Correlation Experiments 

At present, several experiments are available for inverse-detected one-bond heteronuclear shift correlation. The HMQC experiment described by Bax and Subramanian (1986) has probably been most widely employed. Alternatives, however, are available in the form of DEPT-HMQC (Kessler et al. 1989b) and the HSQC or so-called Overbodenhausen experiment (Boden-hausen and Ruben 1980). For alkaloids with highly congested proton spectra, DEPT-HMQC may be a useful alternative to HMQC, because it allows the acquisition of edited correlation spectra. For investigators interested in correlation of protons to alkaloidal nitrogen atoms via one or two bonds, HSQC or a doubly refocused variant may be the preferred choice. 

Quite probably, HMQC (Bax and Subramanian 1986) is the most widely employed inverse-detected heteronuclear chemical shift correlation experi- 

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A H(detected)- C shift correlation spectrum (conmion acronym HMQC, for heteronuclear multiple quantum coherence, but sometimes also called COSY) is a rapid way to assign peaks from protonated carbons, once the hydrogen peaks are identified. With changes in pulse timings, this can also become the HMBC (l eteronuclear multiple bond coimectivity) experiment, where the correlations are made via the... 

Fig. 6. Spectra of 2-methyl-5-bromopentaiie acquired using a Bruker 300AMX spectrometer (a) TOCSY using a 5-mm dual probe and (b) HMQC...

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. 

HC HMQC (heteronuclear multiple quantum coherence) and HC HSQC (heteronuclear single quantum coherence) are the acronyms of the pulse sequences used for inverse carbon-proton shift correlations. These sensitive inverse experiments detect one-bond carbon-proton connectivities within some minutes instead of some hours as required for CH COSY as demonstrated by an HC HSQC experiment with a-pinene in Fig. 2.15. 

Two-dimensional C//correlations such as C//COSY or HC HMQC and HSQC provide the Jqh connectivities, and thereby apply only to those C atoms which are linked to H and not to non-protonated C atoms. Modifications of these techniques, also applicable to quaternary C atoms, are those which are adjusted to the smaller Jqh and Jqh couplings (2-25 Hz, Tables 2.8 and 2.9) Experiments that probe these couplings include the CH COLOC (correlation via long range couplings) with carbon-13 detection (Fig. 2.16) and HC HMBC (heteronuclear multiple bond coherence) with the much more sensitive proton detection (Fig. 2.17)... 

CH or HC COSY (HMQC) CH bonds CH COLOC or HC HMBC. Jch and Jch relationships between carbon and protons... 

HMQC Heteronuclear multiple quantum coherence, e.g. inverse CH correlation via one-bond carbon proton-coupling, same format and information as described for ( C detected) CH COSY but much more sensitive (therefore less time-consuming) because of H detection... 

H-Detected Heteronuclear Multiple-Quantum Coherence (HMQC) Spectra... 

Figure 5.51 The pulse sequence employed for the HMQC experiment.

In contrast to the HMQC experiment, which provides connectivity information about directly bonded interactions (i.e., one-bond... 

The heteronuclear multiple-quantum coherence (HMQC) spectrum, H-NMR chemical shift assignments, and C-NMR data of podophyllo-toxin are shown. Determine the chemical shifts of various carbons and connected protons. The HMQC spectra provide information about the one-bond correlations of protons and attached carbons. These spectra are fairly straightforward to interpret The correlations are made by noting the position of each crossf)eak and identifying the corresponding 8h and 8c values. Based on this technique, interpret the following spectrum. 

The HMQC spectrum, H-NMR chemical shift assignments, and C-NMR data of vasicinone are shown. Consider the homonuclear correlations obtained from the COSY spectrum in Problem 5.14, and then determine the carbon framework of the spin systems. 

The HMQC spectrum of podophyllotoxin shows heteronuclear crosspeaks for all 13 protonated carbons. Each cross-peak represents a one-bond correlation between the C nucleus and the attached proton. It also allows us to identify the pairs of geminally coupled protons, since both protons display cross-peaks with the same carbon. For instance, peaks A and B represent the one-bond correlations between protons at 8 4.10 and 4.50 with the carbon at 8 71.0 and thus represent a methylene group (C-15). Cross-peak D is due to the heteronuclear correlation between the C-4 proton at 8 4.70 and the carbon at 8 72.0, assignable to the oxygen-bearing benzylic C-4. Heteronuclear shift correlations between the aromatic protons and carbons are easily distinguishable as cross-peaks J-L, while I represents C/H interactions between the methylenedioxy protons (8 5.90) and the carbon at 8 101.5. The C-NMR and H-NMR chemical shift assignments based on the HMQC cross-peaks are summarized on the structure. 

The HMQC spectrum of vasicinone shows nine cross-peaks representing seven protonated carbons, since two of them (i.e., A and B, and C and D) represent two methylene groups. The C-4a and )8 methylene protons (8 2.70 and 2.20) show one-bond heteronuclear correlations with the carbon resonating at 8 29.4 (cross-peaks A and B), while the C-So and )3 methylene protons (8 4.21 and 4.05) exhibit cross-peaks... 

D-TOCSY- H- N-HMQC/HSQC H spin systems, N shifts... 

D-NOESY- H- C-HMQC/HSQC Sequential assignment of the spin systems, in particular, identification of nOes between side chains... 

D H- C HMQC-NOESY H- C Identification of nOes between side... 

Assignment of spin system TOCSY-TOCSY TOCSY-HMQC TOCSY-TOCSY 3D/4D HCCH-TOCSY HCCH-COSY HC(C)NH-TOCSY... 

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