ACCORD-HMBC experiment

Aside from the advantage of the ACCORD-HMBC spectrum shown by Fig. 11. Martin and co-workers - examined various aspects of the accordion-optimization process on the character of the spectrum. Modulation in the ACCORD-HMBC experiment is affected by both the optimization range selected and the number of increments of the evolution time (ni) used to digitize the indirectly detected time domain, t]. F skew is governed by a scaling factor, N, which is defined by... 

In addition to the importance of parameter selection on F skew, the ACCORD-HMBC experiment has also been shown to exhibit strong coupling artifact responses similar to those observed in heteronuclear 2D J-resolved spectra. - Finally, the triplet character of responses in the second frequency domain superimposed over the Fi skew was mathematically accounted for by Zangger and Armitage in their development of the ACCORD-HMQC experiment. "... 

There have been no applications of the ACCORD-HMBC experiment reported in the literature to date. 

Fig. 14. IMPEACH-MBC (IMproved PErformance ACcordion-optimized Heteionucicar multiple bond correlation) pulse sequence developed by Martin and co-workers. The experiment is a further modification of the ACCORD-HMBC experiment that utilizes a constant time variable delay in lieu of a simple variable delay. The con.stant time variable delay introduces the interval, D/2 — 180° "C — D/2, which precedes the variable delay interval, vd. As the evolution lime ti is incremented, the interval vd is decremented in the usual fashion. However, at the same time, the Dll (ct A) intervals are incremented in a manner to keep the overall duration of the period D + vd a constant time interval. Hence, homonuclear modulation, which plagues ACCORD-HMBC experiments, is suppressed by the constant time of the intcrv al D + vd. In contrast, evolving heteronuclear couplings arc refocused at time D by the 180° - C pulse located at DU. These couplings then evolve during the variable interval vd to be sampled in the usual, accordion manner. By using this approach, the constant time variable delay pulse sequence element is of constant duration for homonuclear components of magnetization while serving as a variable delay for heteronuclear components.

Fig. 8.25 Pulse sequence schematic for the ACCORD-HMBC experiment pioneered by Wagner and Berger [144]. The experiment begins with a gradient dual-stage low-pass J-filter to suppress unwanted direct correlation responses. A variable duration delay, Vd, follows, the duration of which is decremented from...

Homonuclear coupling modulation during the variable delay in the ACCORD-HMBC experiment prompted the development of a constant time variable delay... 

Fig. 8.26 Results of an aggressively optimized 2—25 Hz ACCORD-HMBC experiment performed on strychnine (2) are presented here. Such data might be used when it becomes necessary to resort to relatively uncommon four-bond ( J/ch) long-range couplings to solve a structural problem. In the 10 Hz optimized...

While uncontrolled F response skew of the type encountered in the ACCORD-HMBC experiment is undesirable [145], user-defined Fj skew can be a useful determinant of response authenticity. A further generation accordion-optimized long-range experiment, CIGAR-HMBC, was developed to provide this flexibility. The constant time variable delay from the IMPEACH-MBC experiment was further modified as follows ... 

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