Heteronuclear multiple-quantum coherences

From the standpoint of experimental complexity, the HMQC experiment for direct correlation purposes is much simpler than the HSQC experiment described below. The HMQC experiment has its origins in the work of Bax, Griffey, and Hawkins in 

The HMQC/GHMQC experiments are quite useful and were treated in an early review by Martin and Crouch [71]. Relative to the single quantum variant of the experiment discussed below, the effective Fj resolution of the multiple quantum experiment suffers due to homonuclear coupling modulation during the evolution period, which leads to broadened responses being observed in the Fj dimension. The difference in the effective resolution of the HMQC vs. HSQC experiments was noted in a review on applications of inverse-detection in alkaloid chemistry by Martin and Crouch [70] and has since been treated in more detail by Reynolds and others [117—119]. On this basis, the HSQC/GHSQC experiments discussed in the following section should be preferentially used on a routine basis in the opinion of the authors. 

Heteronuclear Single Quantum Coherence Chemical Shift Correlation Techniques 

The gradient or GHSQC version of the experiment apphes a pair of gradients rather than the three gradients used in the GHMQC experiment (see Fig. 8.19). Gradients are applied in the ratio of 4 1 for heteronuclear correlation. 

The first gradient, Gl, is applied during the evolution period while the second gradient, G2, is applied during the final refocusing delay following the 180° pulse sandwich just prior to acquisition. More sophisticated variants of the experiment 

---

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... 

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. 

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... 

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. 

C-NMR, COSY, HMQC (heteronuclear multiple quantum coherence), and HMBC (heteronuclear multiple bond correlation).48 Furthermore, the structure of trimer was confirmed by X-ray crystallography.48 The incorporation of 13C into the indole 3a position proved valuable in these structural determinations and in documenting the ene-imine intermediate. For example, the presence of a trimer was readily determined from its 13C-NMR spectrum (Fig. 7.7). 

HMQC-TOCSY Heteronuclear multiple quantum coherence-total correlation IAA Instrumental activation analysis... 

During the delay A, the proton coherence associated with long-range heteronuclear couplings is fully converted into anti-phase (AP) magnetization. The second 90° pulse applied on the 13C channel converts this AP magnetization into heteronuclear multiple quantum coherences (2HxCy). 

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)... 

In addition to well-resolved one-dimensional (ID) 1H and 13C spectra, which are usually sufficient for monitoring synthetic steps, HR-MAS techniques can be applied to two-dimensional (2D) homonuclear and heteronuclear experiments, which allow a wealth of structural information to be obtained. H,13C HMQC (heteronuclear multiple quantum coherence) spectra are particularly useful in the analysis of solid support-bound oligosaccharides, since the anomeric protons exhibit characteristic resonances. Such a spectrum of a polymer-bound trisaccharide glycal is shown in Figure 8.4. 

This scheme is applied in the so-called X-half-filter technique (Fig. 17.4a, c), with the only difference of an additional 90 (X) pulse with constant phase [16, 17]. Instead of generating heteronuclear multiple quantum coherence 2 Iy Sy, which cannot readily be detected (in case of selecting the 1H-X pairs), one now always ends up with proton antiphase coherence, but with the same phase alternation ... 

All one-bond H- C connectivities were established by a heteronuclear multiple-quantum coherence (HMQC) experiment. Partial structures including a tetraene system, a phenyl group and a diol moiety as shown in Figure 27A were determined by a COSY experiment. 

NMR data are heteronuclear multiple quantum coherence and heteronuclear multiple bond coherence readouts the carbon atoms at sites of glycosylation are given in bold. 

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

Inverse-detected experiments have had the greatest effect in making 15N NMR experiments feasible for small samples. These experiments take advantage of the higher sensitivity of NMR to facilitate the observation of insensitive nuclei like 13C and 15N. The H-13C heteronuclear multiple quantum coherence (HMQC) and the related heteronuclear multiple-bond correlation (HMBC) experiments are important in contemporary natural products... 

HMBC ( -Detected Multiple-bond Heteronuclear Multiple Quantum Coherence Spectrum) NMR of 26a allowed one to show that the isopropyl group (Ha, Hfc) is connected to the olefmic carbon (Cc), whereas methyl (IF) and methylene (H-C, H- ) groups are bonded to another olefmic carbon (Cd) and fullerene carbon (Ch) through quaternary carbon (Ce) shown in 26a and 26b (Scheme 8). 

---