J-HMBC experiment

Fig. 19. Pulse sequence for the J, J-HMBC experiment described by Krishnamurthy ei This experiment represents the most refined version of the accordion-optimized experiments to be developed thus far and allows the differentiation of Jxh from Jxh long-range correlations to protonated heteroatoms ( - C and N). The experiment further modifies the concept of the constant time variable delay used in the IMPEACH-MBC and CIGAR-HMBC experiments to even more selectively manipulate various components of magnetization. This is done using the pulse sequence operator given the acronym STAR (Selectively Tailored Accordion F Refocusing) (.see also Fig. 20). Differentiation of various components of heteronuclear long-range magnetization is accomplished within the STAR operator, with the balance of the pulse sequence similar to that of the IMPEACH-MBC and CIGAR-HMBC experiments.

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. ... 

Fig. 21. Schematic representation of the differentiation of "Jch from Jch long-range correlation responses in the "J, J-HMBC experiment as a function of the operation of the STAR operator shown in Fig. 20.

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. 

Tokunaga, and co-workers reported using the J-HMBC experiment to measure the long-range two- and three-bond couplings in a series... 

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...

J-HMBC experiment because one-bond carbon-carbon coupling constants (Vcc) affect the carbon atoms adjacent to labelled positions and may give rise to peaks cancelation. Widmalm et al proposed two alternative modifications of J-HMBC sequence, that suppress the interference from one-bond scalar couplings. They demonstrated that for site-... 

The multi-receiver PANACEA experiment is designed aroxmd the INADEQUATE pulse sequence [32, 33] and also incorporates ID spectra, multiplicity edited 2D HSQC-s and 3D J-HMBC experiments. In addition, PANACEA does not require the conventional field/frequency lock and can be recorded in pure liquids. 

Proton resonances for aU residues were assigned using a combination of COSY, TOCSY, HSQC, and HMBC experiments. The large values of y(NH/H-C(/9)) and the small values of J(H-C(a)/H-C(y9) were indicative of antiperiplanar and synclinal arrangements respectively, around those bonds. In addition, medium-range NOE connectivities H-C(/ )i/NH +i, H-C(a)i/NH, + i, NHi/NH +i were consistent... 

Figure 8 Comparison of nJch intensity illustrated with ID rows taken from HMBC experiments recorded using a one-step low-pass J filter (denoted LP), a two-step LPJF (denoted LP2), a three-step LPJF (denoted LP3), a four-step LPJF (denoted LP4), and a five-step LPJF (denoted LP5) showing the nJch response of C-2 (left) and the nJCn response of C-3 (right) of the 1,3-butadiynyl (tert-butyl) diphenylsilane molecule dissolved in CDClj. The measured signal-to-noise (obtained using the sino macro) are indicated on the top of each peak.

Figure 13 Timing diagram for the clean HMBC experiment with an initial second-order and terminal adiabatic low-pass 7-filter.42,43 The recommended delays for the filters are the same than for a third-order low-pass J filter. <5 and 8 are gradient delays, where 8 — <5 + accounts for the delay of the first point in the 13C dimension. The integral over each gradient pulse G, is H/2yc times the integral over gradient G2 in order to achieve coherence selection. The recommended phase cycle is c/)n = x, x, x, x 3 — 4(x), 4(y), 4( x), 4(—y) with the receiver phase c/)REC = x, x.

For D-HMBC experiments, we usually omit the low pass J-filter (the first 90° pulse for C nucleus in the HMBC pulse sequence) aiming to suppress the cross peaks due to the direct Jc-h correlation, but it can be implemented if desirable. In the D-HMBC spectra, the cross peaks between directly bonded C and H do not, in most cases, hinder the easy analysis of the spectra, because these cross peaks appear as singlets. On the contrary, these peaks even contribute to easy NMR spectral analysis when HMQC spectral data are not in hand. 

The HMBC experiment is just an HMQC experiment with the 1/(2J) delay set for a J value of about 10 Hz (typical for two- and three-bond 7ch) rather than 150 Hz (typical... 

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. 

With respect to the pulse sequence layout, the HMBC experiment is essentially a HMQC experiment incorporating a low-pass filter to suppress the one-bond correlation peaks. The low-pass filter, consists of a delay d2 = 1/(2 U(C, H)) and a 90° pulse, which transfers the U(C, H) coherence into a multiple quantum state. In a second period coherences which are generated by JCC, H) evolution are also transferred to a multiple quantum state by a 90° l C pulse but with a different phase in relation to the first 90° pulse of the low-pass filter. A combination of appropriate receiver phase cycling and pulse phase cycling enables the exclusive detection of J(C, H) correlation peaks in the 2D experiment. 

Load the configuration file ch5732.cfg and run a simulation. Inspect the spectrum for residual 1 J(C, H) correlation peaks and compare their intensity with the same peaks of the phase cycled HMBC experiment of Check it... 

The LR-J-HSMQC experiment is an excellent example of the development of pulse sequences optimised for a wide range of nj(i3c, IH) values e.g. the one size fits all approach . The HMBC experiment is very sensitive to variations in the value of H) the HSQMC sequence attempts to overcome this drawback by combining the HSQC and HMQC steps. As such, the authors [5.224(a)] acknowledge the common goal between the HSMQC sequence and the J-compensated APT sequence CAPT (see Check its 5.2.6.3 and 5.2.6.4). 

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