HMQC and HSQC

In the heteronuclear experiment category, the experiments of interest are the heteronuclear multiple quantum correlation (HMQC) experiment, the heteronuclear single quantum correlation (HSQC) experiment, and the heteronuclear multiple bond correlation (HMBC, including the gradient-selected version gHMBC) experiment. Both the HMQC and HSQC produce similar results, but each has its own unique advantages and disadvantages. 

An ffMQC or ffSQC experiment carried out with an inverse probe will provide the same information as a ffETCOR experiment carried out with a normal probe, but will do so in far less time. Even carrying out the ffMQC or ffSQC experiment on a normal probe is more efficient than the ffETCOR experiment. This advantage stems from the higher signal-to-noise ratio we obtain when we detect signal from the ff (with its higher gyromagnetic ratio) instead of from C or 

The principal technical hurdle that must be overcome when using ff detection to correlate ff and C (or resonances is that the bulk of the ff signal being detected corresponds to ff s boimd to C (or N). Only through cancellation of 98.9% of the ff signal is the ffMQC or ffSQC experiment able to generate useful data. 

Because a large fraction of the total detected signal must be removed to leave the small fraction of the signal of interest, RE stability is critically important. The use of bandpass RE filters in the ff and C channels improves the quality of the data by preventing the ff lock channel and other RE sources from disturbing the ff and C spin populations. 

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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)... 

HMQC and HSQC are about equally sensitive, the sensitivity being proportional to (Yx)5 2, but as the HSQC pulse sequence uses more than twice as many pulses as HMQC, the latter is preferred (especially on older spectrometers). Both use two polarization transfer steps while HETCOR uses only one transfer. If the coupling constants vary within the sample, HETCOR might be a better choice for lower losses in polarization transfers. Since the sensitivity of HETCOR is proportional to (yx)3 2(YY), the choice of nucleus for detection is not trivial. 

Pulsed field gradient variants of the above sequences have been described HMQC and HSQC have in their gradient versions a higher sensitivity as they allow the use of higher spectrometer gain and usually lead to cleaner spectra with fewer artifacts. For a review see Reference 299. 

EXAMPLES OF ONE-BOND INVERSE CORRELATION (HMQC AND HSQC) WITHOUT 13C DECOUPLING... 

In many cases the inorganic chemist will be interested in the NMR spectrum of a nuclide of low sensitivity (denoted S or indirectly detected spin) in a compound containing a high-sensitivity nucleus (denoted I or detector spin). If scalar coupling exists between the two nuclides, then the 2D HMQC and HSQC techniques offer an alternative to direct observation. The distinction between HMQC and HSQC is that TS -magnetization is stored as either multiple... 

Use of HMQC and HSQC for sensitivity enhancement is most appropriate where the detector has a high y relative to the insensitive nuclide. HMQC/HSQC caimot compensate for low natural abundance of the insensitive spin since only the satellites from coupling to S are detected. 

The HSQC (Heteronuclear Single Quantum Coherence) experiment is another widely used inverse detection experiment. It provides essentially the same information as HMQC, but relies on a different sequence of pulses to effect the transfer of magnetization between H and the heteronucleus. A direct comparison of HMQC and HSQC in the study of a natural product has indicated some advantages of the latter-sequence, which may provide improved sensitivity and narrower crosspeaks for improved resolution. ... 

It is also important that LP not be abused. A sufficient number of increments must be taken from which the FID s can confidently be extended. A total of 64 increments has, for example, been found to be insufficient, while LP s have successfully been carried out with 96 increments. A good practice is to acquire at least 128 increments for accurate prediction. A second concern is that LP not be extended too far (e.g., 128 points predicted to 4,096). W. F. Reynolds (2002) has found that, as a general rule, data presented in the phase-sensitive mode can be predicted fourfold (e.g., 256 data points can be predicted to 1,024), while absolute-value data can be extended twofold, 256 points to 512. A significant exception to the fourfold rule for phase-sensitive experiments concerns the H-detected, heteronuclear chemical-shift correlation experiments. In marked contrast to COSY and HMBC spectra, for which the interferograms are frequently composed of many signals, those of HMQC and HSQC spectra constitute only one (due to the directly attached C). LP s up to sixteen-fold can be performed in these experiments (Sections 7-8a and 7-8b). 

The two principal H-detected, direct, heteronuclear chemical-shift correlation experiments are HMQC and HSQC. The X-nucleus-detected counterpart is HETCOR, The and X-nucleus spectral widths are reduced in each of these experiments. It is important to remember that the latter should be decreased to contain only the signals of protonated X nuclei. Quaternary carbons, for example, do not participate in these experiments, and their signals should not be included in the reduced spectral windows. 

Since the X nucleus in HMQC and HSQC experiments is usually carbon, X refers to... 

Bazzo and co-workers have proposed an optimised 2D HMBC method of higher sensitivity for the measurement of the /hc couplings. Parella and Belloc have presented simple ID HMQC and HSQC pulse schemes which allow the Juc sign and magnitude measurement for samples at natural carbon abundance. 

Isotope edited experiments 3D HCCH-COSY 3D HCCH-TOCSY ID spin-state selective HMQC and HSQC-based 3D MUSIC CBCANH 3D MUSIC CBCA(CO)NH 2D Pro-HSQC... 

The X,"Y correlation techniques described so far do not allow to determine the number of magnetically equivalent detected "X nuclei. Such information is frequently of importance for structure elucidation for example, it would permit to determine the number of phosphine ligands in a metal complex from a P-detected phosphorus-metal shift correlation. A remedy to this problem has been found by recording fully coupled correlation spectra in which the number of X spins can be derived from the multiplet structure in FI. This is easily achieved in both HMQC and HSQC pulse schemes by omitting the refocusing 180° pulse during ti, and various correlations have been performed in this For an interpretation of the results,... 

Figure 6.7. A comparison of experimental crosspeaks taken from HMQC and HSQC spectra acquired under identical conditions of high fi resolution (2.5 Hz/pt). The upper ID trace is taken from the conventional ID proton spectrum, and the vertical traces are fj projections from the 2D spectra. The additional broadening in the HMQC spectrum arises from unresolved homonuclear proton couplings in fi.

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. 

There are two approaches to pulse sequence classification depending on the user s occupation. For the chemist who has to solve a structural question or characterize a new compound it is the spectra obtained from the pulse sequence that is of primary importance. The NMR spectroscopist is usually more concerned with the pulse sequence structure and choice of experimental parameters and whether a particular pulse sequence can be improved or modified to solve a specific problem. These two different approaches lead to confusion in pulse sequence nomenclature such that names are often a combination of the purpose of the experiment and the sequence layout. For example the commonly used acronyms HMQC, HSQC and HMBC imply a consistent abbreviation system yet HMQC and HSQC describe the coherence state during the evolution time whilst HMBC denotes an experiment to correlate nuclei using multiple bond heteronuclear scalar coupling. 

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