Inverse detection heteronuclear multiple quantum coherence

The inverse detection heteronuclear multiple quantum coherence (HMQC) experiment is another approach to two-dimensional NMR techniques, which consists of a transfer of chemical shift and coupling information from relatively insensitive nuclei such as and some metals, to more sensitive nuclei such as H. The advantage of this method is a substantial increase in the sensitivity obtained, due to the greater natural abundance of H (Kingery et al., 2001). 

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... 

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... 

In order to carry out complete structural elucidation of unknown compounds (especially for complex molecules), the RF probe should enable a variety of heteronuclear NMR techniques to be performed. In particular, inverse detection H-15N and 1H-13C experiments such as heteronuclear multiple quantum coherence (HMQC) [29,30] and heteronuclear single quantum coherence (HSQC) [31] find almost ubiquitous application in myriad research environments. Although the microliter-scale probes described above feature both heteronuclear and homonuclear capabilities, no commerical product is... 

For smaller quantities of compounds more sensitive inverse detected techniques are available, such as HMQC ( IH-I C one bond correlation via heteronuclear multiple quantum coherence, analogous to HETCOR) and HMBC (proton detected heteronuclear multiple bond correlation spectroscopy) (15). The last provide, in addition to the intraresidue multiple bond correlations, interresidue correlations between the anomeric carbon and the aglycone protons.We follow this general strategy for the structural determination of tri terpenoid saponins of Bupleurum fruticosum (16) andArdisia japonica (9). 

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

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. 

The other common inverse-detection method, heteronuclear multiple quan-turn coherence (HMQC) relies on multiple-quantum coherence transitions during the pulse sequence. Due to the multiple-quantum coherence transitions it is more laborious to theoretically follow the course of magnetization, and the cross peak will be broader in the Fi dimension due to the /hh evolution. Unlike HSQC, HMQC can also be optimized for Jch couplings. This heteronuclear multiple bond correlation experiment, or HMBC, ° ° has lower sensitivity than HMQC/HSQC experiments, and the Jch correlations can appear as artefacts in the spectrum. However, the cross peak volume should follow the concentration of analyte, so with proper method validation HMQC and HMBC should also be applicable for quantification. 

The inverse-detected 2D NMR experiments that have been discussed to this point have all been discrete, single-purpose experiments, e.g. correlating protons with their directly bound heteronucHde (typically or N). There are another class of inverse-detected 2D NMR experiments that are generally referred to as hyphenated 2D experiments. These are experiments that first establish one type of correlation, followed by an additional experiment segment that then pursues a further spectroscopic task. Predecessors of the inverse-detected variants of these experiments were the HC-RELAY (proton—carbon heteronuclear relayed coherence transfer) experiments pioneered by Bolton [151—155]. Examples of these include, but are by no means hmited to HXQC-COSY and -TOCSY [156—158], -NOESY [159], -ROESY [160], and more recent gradient variants [161] etc., where X = S (single) or M (multiple) quantum variants of the experiments. 

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