Accordion excitation

In addition to discriminating between two- and three-bond X-H couplings, the 7, 7-HMBC experiment makes use of what is known as accordion excitation. In this technique, critical vector dephasing and refocusing times are varied so that the experiment is optimized for a range of two- and three-bond X-H couplings and not for just an average "7xh ... 

G.E. Martin, C.E. Hadden, Application of accordion excitation in H—long-range heteronuclear shift correlation experiments at natural abundance, Magn. Reson. Chem. 38 (2000) 251-256. 

Fig. 25. Pulse sequence for the broadband HMBC experiment proposed by Meissner and Sprensen." A series of experiments are acquired in which the delay, A, is varied and used in lieu of an accordion-optimized delay. This series of experiments is in some senses analogous to the processed result of the 3D-HMBC experiment proposed by Furihata and Seto. The authors used a series of four A values of 440.7, 343.4, 290.3, and 242.2 ms and claim uniform excitation to 20Hz. The authors also comment that the application of broadband HMBC requires higher numbers of transients/ti increment than conventional HMBC or accordion-optimized experiments, which is not surprising given the likelihood of considerable losses of magnetization due to relaxation proces,ses during the very long A delays that are employed to achieve broadband excitation in this approach.

In 2001, Meissner and Sprensen described methods for the measurement of Jhh anO "Jch couplings employing the broadband excitation approach (see Section 3.3.7) with the report of the broadband XLOC and broadband J-HMBC experiments, respectively. In a further extension of the idea of accordion optimization, Williamson ef reported the development of the J-IMPEACH-MBC pulse sequence. This approach is similar to the EXSIDE experiment previously described by Krishnamurthy and the J-resolved HMBC experiments of Furihata and Seto"- in its use of J-scaling to render small, long-range heteronuclear couplings conveniently measurable. Finally, Williamson e/ also described a new method, G-BIRDr-HSQMBC, that was reported in their survey of the available methods. 

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