HMQC-COSY pulse sequence

Figure 24 schematically illustrates the assembly of 2D-HMQC and 2D-COSY pulse sequences to form a 3D-HMQC-TOCSY pulse sequence the preparation period (relaxation delay and first pulse) in the COSY pulse sequence is replaced by the pulse sequence elements (excluding data acquisition) from the HMQC sequence to create the 3D pulse sequence on the bottom of the figure. To perform the 3D experiment, an array of ni2 FlDs is collected with 2 incremented this experiment is repeated nil times with ti incremented. The resulting 3D time-domain spectmm is Fourier transformed, first with respect to 3, then with respect to 2, and finally, with respect to 1 to produce the spectmm in Figure 24(c). 

HC HMQC (heteronuclear multiple quantum coherence) and HC HSQC (heteronuclear single quantum coherence) are the acronyms of the pulse sequences used for inverse carbon-proton shift correlations. These sensitive inverse experiments detect one-bond carbon-proton connectivities within some minutes instead of some hours as required for CH COSY as demonstrated by an HC HSQC experiment with a-pinene in Fig. 2.15. 

The pulse sequences for HMQC-COSY and HMQC-NOESY experiments are presented in Fig. 6.10. The 3D HMQC-COSY spectrum of a N labeled tripeptide is shown in Fig. 6.11. Since the coherence transfer involved in this experiment is N([Pg.362]

Similar spectra can be obtained more rapidly and with less sample if the data are acquired through the proton signals, which are much more intense. Basically, the H NMR data are acquired and the H- C coupling constant used as the delay in a pulse sequence, which enables us to obtain the carbon spectrum. This method of obtaining the data is called inverse-mode , since the carbon atoms are detected through their attached hydrogen atoms rather than by direct detection, with obvious benefits in the sensitivity and the time taken to obtain a spectrum. HMQC and HMBC are both examples of inverse-mode spectra and this method is so much quicker than CH COSY that an entire HMQC spectrum can be obtained in much less time than it takes to obtain the proton-decoupled C... 

Fig. 8.2. Some of the most common 2D pulse sequences that can be employed using a proper choice of parameters to record 2D spectra of paramagnetic molecules (A) NOESY, (B) ROESY, (C) COSY, (D) ISECR COSY, (E) zero-quantum (double quantum) COSY, (F) TOCSY, (G) HMQC, (H) HSQC. Sequences (A), (B) and (F) are also used to obtain EXSY spectra. SL indicates a soft spin-lock sequence, while MLEV17 indicates a train of spin-locking hard pulses that optimizes the development of J/j coupling. In the reverse heteronuclear experiment (G) the upper and lower levels refer to H and heteronucleus, respectively. The phase cycles are not indicated. For clarity of discussion, all initial pulses can be thought to be applied along the y axis, in such a way that the coherence after the first 90° pulse is always along x. ...

With suitable modification of the pulse sequences the para-hydrogen effect can be used to enhance the signals in many NMR experiments, for example, COSY, HMQC, HSQC, EXSY and Overhauser spectroscopy. ... 

Figure 2 Basic four-pulse sequence for the proton detected HMQC-COSY experiment [91...

Several ID and 2D NMR techniques have become standard tools for structure elucidation studies of the resveratrol oligomers. The most commonly employed of these include the one-bond i3C- H HETCOR and HMQC, as well as, the two- and three-bond C- H FLOCK [106], COLOC [67,72,88,106], andHMBC [53,71,78] pulse sequences. Typical 2 Vc-h coupling constant values used for these experiments range between. 7=8-12 Hz. Other commonly used H- H correlation methods include standard COSY [64] and NOESY [67,72,78] pulse sequences however, the most widely employed experiment concerning proton-proton... 

General. Mps uncorr. HNMR and C-NMR 250MHz and 62.5MHz, respectively for 1, 400 MHz and lOOMHz respectively, for 2 2D-NMR data ( H- H COSY, HMQC, HMBC, NOESY) standard pulse sequences. CD3OD and pyiidine-dS were used as solvent, respectively for 1 and 2, with IMS as int. standard EI-MS 70ev, direct int. FT-IR KBr CC silica gel (coarse silica gel, 100 200 mesh), D-101 resin (16 S0mesh) TLC precoated silica gel plates (Merk, silica gel 60 F254). 

For natural abundance H, correlation experiments, the 2D HMQC pulse sequence as described by Bax et al. 3 s used. A proton spectral width of 5(X)0 Hz and a carbon-13 spectral width of 25000 Hz were used, with 2K X 512 data points in the t2 and tl dimensions respectively. H, P correlation experiments were obtained using a hetero-TOCSY sequence, with an isotropic mixing time of 67 ms. The spectral width was set to 4000 Hz for protons and 607 Hz for phosphorus, with typically 2K X 256 data points in the two dimensions respectively. The proton 2D DQF-COSY and TOCSY experiments were recorded with standard pulse sequences the data size was 2K X 512, with spectral width of 5500 Hz in both dimensions. The NOESY sequence with a jump-return excitation pulse was used for optimal imino proton detection . Mixing times from 100 to 300 ms were used. The spectral widths were 11000 Hz in both dimensions. 

Fig. 16 Two-dimensional spectra from different authors, all using D-DNP and fast acquisition schemes, (a) C- H-HSQC NMR spectra of a 1 1 1 1 mixture of o-, m-, and p-xylene and toluene, comparing a spectrum recorded by Frydman s ultrafast method (left) with a conventionally recorded spectrum (right) [150]. (Copyright Wiley-VCH. Reproduced with permission), (b) DNP enhanced proton COSY spectrum of 1.5 mM Ala-Gln at 9.4 T using an ultrafast COSY method by Kockenberger and coworkers. (Reproduced from [151] by permission of the PCCP Owner Societies), (c) 2D small flip angle C- H-HMQC pulse sequence by Ludwig et al. [66, 153]. (d) 2D-HMQC spectrum of aspirin recorded in 30 s with 64 increments after 90 min of polarization at 1.3 K (kindly contributed by C. Ludwig)...

Figure 24 Illustration of the assembly of 2D-C0SY and 2D-HMQC pulse sequences to form an HMQC-COSY 3D-NMR experiment. Hypothetical spectra from a CaH-CbH-CcH spin system the ID and spectra, (a) 2D-C0SY spectrum, (b) 2D-HMQC spectmm, and (c) the 3D-HMQC-COSY spectrum showing selected M COSY planes at the shifts of Ca, Cb, and Cc in the dimension. Reprinted with permission from Rinaldi, P. L. Monwar, M. Encyclopedia of Spectroscopy and Spectrometry, 2nd ed. Elsevier, Oxford, UK, 2010. Copyright 2010 Elsevier.

---