Accordion HMQC

While good results were obtained with the ADSQC technique, the authors note that artefacts of a triplet nature were observed in F. as suggested by the analysis of Zangger and Armitage in their report of the accordion-HMQC experiment (see p. 44). - To address this potential deficiency, Hadden et al. utilized randomization of the variable delay table to provide essentially complete suppression of the artefacts observed in the ADSQC. The modified experiment has been given... 

Fig. 4. Accordion HMQC pulse sequence developed by Zangger and Armitage. - This experiment was developed to facilitate the study of metallothioneins in which cadmium has been inserted as an isomorphic replacement for the zinc atoms normally bound to metallothioneins. By using the accordion HMQC experiment, Zangger and Armitage were able to observe correlation responses to the seven cadmium atoms in a single experiment rather than having to perform several conventional experiments with varied optimization.

With the development of the ADSQC. RDSQC, and accordion-HMQC experiments described above, it is likely that there will be more variants to follow. Although the variability of one-bond heteronuclear couplings is a minor problem in direct heteronuclear shift correlation experiments, the large degree of... 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

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