Basic HMBC Experiment

3 Accordion-optimized Long-range Heteronuclear Shift Correlation Methods. 

The idea of accordion-optimization is by no means new [143]. The idea of applying this method to the optimization of the long-range delay of HMBC-type experiments, however, was only reported in 1998 by Wagner and Berger [144] in their description of the ACGORD-HMBC experiment. The ACCORD-HMBG pulse 

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

GHMBC spectrum of strychnine (2) a total of four four-bond long-range couplings are observed. In contrast, the 2-25 Hz ACCORD-HMBC experimental data shown identified 17 four-bond long-range couplings as shown by 6. (Reproduced with permission - Wiley-VCH). 

The variable delay, D, is halved about a 180° C pulse which serves to refocus Jch components of magnetization at D. These same components of magnetization then experience a variable delay, Vd, during which they are sampled. The accordion 

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Basic Theory-Analysis of the Basic HMBC Experiment 296... 

BASIC THEORY-ANALYSIS OF THE BASIC HMBC EXPERIMENT... 

The second drawback of the basic HMBC experiment is due to the actual range of /CH spin coupling constants, which vary from 1 to 50 Hz. Usually HMBC spectra are recorded with a compromise by setting the corresponding delay in the sequence to about 70 ms to allow an optimum transfer for correlation signals where the spin coupling constant is about 7 Hz. Under these conditions, several important connectivities may yield... 

Recently, a robust, sensitive, and versatile HMBC experiment for rapid structure elucidation has been proposed. The suggested IMPACT-HMBC experiment eliminates the weaknesses of the basic HMBC experiment and the overall performance of the pulse sequence is improved significantly. In addition, it can be recorded with short recovery times, which is useful in routine analysis by NMR when the experimental time is limited. 

In most cases where long-range couplings are exploited to solve a structural problem corresponding information on heteronuclear one-bond interactions is needed as well. Instead of suppressing this valuable information with a low-pass filter as in the basic HMBC experiment, we additionally modified our pulse sequence - the second non-selective 90° pulse is... 

Recent results have been summarized in a number of articles, especially in this journal.17-19 This review will focus on newly developed HMBC pulse sequences and application on small to medium-sized organic molecules. After a short introduction into basic theory, a selection of pulse sequences and a look at the large variety of applications will complete the overview of the HMBC experiment. Finally, a brief summary and outlook to future perspectives of HMBC will be given. 

An interesting alternative to three dimensional NMR techniques is to suppress one of the evolution times while retaining the basic 3D pulse sequence. The spectral resolution is no longer increased, which is usually not a problem with smaller molecules, but the extra information is still available. The gs-HMQC-TOCSY experiment represents one such experiment. The combination of the HMQC method with the TOCSY sequence leads, in principle, to a 3D technique. However, if the tj evolution period of the TOCSY part is omitted, a 2D sequence is obtained which provides a 13C edited TOCSY spectrum. Starting from each HMQC cross-signal, one finds in the same row additional signals which are caused by a TOCSY transfer. When the structure elucidation is difficult, this experiment can fruitfully complement the HMBC experiment. 

Nearly simultaneously with the report of the BIRD-HMBC experiment, Meissner and Sprensen described another modification of the basic long-range hetero-nuclear shift correlation experiment that they refer to as broadband HMBC. The authors employ the gradient dual-stage low-pass J-filter used in the accordion-optimized experiments described above (Section 3.3.5) followed by a delay. A, which replaces the accordion-optimized delay or a normal fixed delay for the evolution of long-range heteronuclear components of magnetization. Instead, Meissner and Sprensen acquire a series of several experiments with different... 

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