BIRD-HMBC experiment

Figure 12 Comparison of Vch artefacts intensity illustrated with ID rows taken from a BIRD-HMBC (A), (D) and (G) a G-BIRD-HMBC (B), (E) and (H) and a double tuned G-BIRD-HMBC (C), (F) and (I) experiments showing the Vch artefacts and nJCH responses of C-6 at 135.6 ppm (A), (B) and (C), C-l at 67.2 ppm (D), (E) and (F) and C-10 at 27 ppm (G), (H) and (I) of the 1,3-butadiynyl (tert-butyl) diphenylsilane molecule dissolved in CDCl3. For the BIRD-HMBC and G-BIRD-HMBC experiments, the delays S were adjusted to aV-value of 190 Hz, as an average value for the extreme range of coupling constants for this molecule (125-260 Hz). For the double tuned G-BIRD-HMBC, the /ch nnax and /ch nnin values were set to 240 and 145 Hz, respectively. The corresponding values for the S and S delays were 3.13 and 2.17 ms, adjusted toj values of 160 and 230 Hz, respectively. For both G-BIRD-HMBC experiments, 192 is BIP 720-100-10 pulses have been used for 13C inversion. The same vertical scale is used for all spectra. Residual /ch signals are denoted with arrows.

Additional heteronuclear long-range experiments include the BIRD-HMBC experiment and the broadband HMBC experiment [52]. Experimental variants capable of simultaneously recording direct and long-range heteronuclear correlations include the HMSQ and MBOB experiments [53]. 

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... 

Several new pulse sequences have been designed for the detection of hetero-nuclear long-range couplings. This includes a new 2D pulse sequence (BIRd-HMBC) proposed by Burger et and new broadband XLOC and /-HMBC experiments suitable for the measurement of /hh and /hx couplings in small molecules, designed by Meissner and Sorensen. ... 

The original COLOC sequence did not attempt to suppress l J(C, H) correlation peaks using a low-pass J filter, which is part of the HMBC experiment (section 5.8.3). The authors who proposed the COLOC sequence [5.170] stated that there would be no practical improvement using additional 1J(C, H) suppression elements [2]. However, several modified COLOC experiments have been published to suppress H(C, H) correlation peaks and spectrum artefacts that are introduced by pulse imperfections. The "BIRD-COLOC" sequence [3] uses two BIRD units to suppress the one-bond IH, correlation peaks [5.182]. The alternative "TANGO-COLOC [5.183] is similar to the "BIRD-COLOC" experiment except that a TANGO unit replaces the first BIRD unit. The S-COLOC sequence [5.94] utilizes the COLOC-TANGO sequence plus an additional 180° pulse to enhance the constant-time procedure. 

The gradient BIRDr element [5.209] combines the BIRD element, Check it 5.8.1.3, with additional coherence selection using rf gradients. Check it 5.8.1.5 examines the efficiency of this element in suppressing one-bond correlation peaks in a HMBC experiment. 

There have been no reported applications of the J, J-HMBC experiment published to date. There has, however, been one report of a COSY-type artefact observed in -J. J-HMBC spectra of a cyclopentafurnanone. The COSY-type responses observed are displaced in Fj as a function of the choice of Jscaie-Removing the bipolar gradients flanking the BIRD pulse in the A2 interval of the STAR operator and superimposing a CYCLOPS phase cycle on the BIRD pulse completely suppresses the COSY-type response artefacts associated with the -J, J-HMBC experiment. ... 

Bigler and co-workers illustrated the application of the HMSC experiment using strychnine (14) as a model compound. Excellent discrimination in the processed data was obtained between the direct and long-range correlation responses. Results obtained with the HMSC experiment were compared to conventional HMBC and statically-optimized ACCORD-HMBC data. The sensitivity and lineshape were comparable to the HMBC data and superior to the statically-optimized ACCORD-HMBC data due to the evolution of antiphase coherence into in-phase coherence prior to detection, resulting in FlDs that are not optimal for magnitude calculated presentation. This observation is analogous to that made with the BIRD-HMBC data above (see Section 3.3.6). 

Fig. 8.28 The (/, -HMBC experiment is the most sophisticated accordion-optimized long-range heteronuclear shift correlation experiment reported to date [148]. The experiment uses a pulse sequence operator known as a STAR (selectively tailored F, accordion refocusing) to selectively manipulate two-bond and three-bond long-range correlations to protonated carbon or nitrogen resonances. A. STAR operator used in the (/, J-HMBC experiment. The experiment takes advantage of the ability of a BIRD(x,x,x) pulse to refocus the one-bond heteronuclear coupling of a protonated carbon. By doing this, the coupling to this proton...

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