Gradient HMQC

The carbon-13 NMR spectra of benazepril hydrochloride were obtained using a Bruker Advance Instrument operating at 75, 100, or 125 MHz. The sample was dissolved in D20, and tetramethylsilane (TMS) was added to function as the internal standard. The 13C-NMR spectra are shown in Figures 10-13, and the gradient HMQC NMR spectra are shown in Figures 14 and 15. The assignments for the observed resonance bands associated with the various carbons are provided in Table 4. 

Figure 14. Gradient HMQC NMR spectrum of benazepril hydrochloride in D20.

The first of the proton-detected experiments is the Heteronuclear Multiple Quantum Correlation HMQC experiment of Bax, Griffey and Hawkins reported in 1983, which was first demonstrated using 1H-15N heteronuclear shift correlation [42]. The version that has come into wide-spread usage, particularly among the natural products community, is that of Bax and Subramanian reported in 1986 [43]. A more contemporary gradient-enhanced version of the experiment is shown in Fig. 10.14 [44],... 

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

The HSQC experiment is based on single rather than multiple quantum coherence during the evolution time, t. The contemporary multiplicity-edited gradient HSQC pulse sequence is shown in Fig. 10.15. Relative to the much simpler HMQC pulse sequence, the HSQC... 

Gradient enhanced versions of the ID HMQC-TOCSY and ID HSQC-TOCSY have also been published recently [31, 55]. 

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

Reports on the application of pulsed field gradient (PFG) assisted pulse schemes for two-dimensional X/Y correlation spectroscopy focused mainly on the adaptation of HSQC sequences which seemed to perform better than HMQC experiments under these conditions.21 Although the generalisation of standard pulse sequences for / C correlation spectroscopy should in principle be straightforward, large spectral ranges and short relaxation... 

Comparison of the results of the one-dimensional gradient supported 31P/15N 1H -se-HSQC experiment with phase-cycled HSQC and HMQC experiments gave relative S/N-ratios of 0.75 0.92 1 which was, under consideration of the suppression of one of the two possible coherence transfer pathways by the field gradients and the longer duration of the pulse sequence, interpreted in terms of a very good performance.25 The main benefits of the PFG-es-HSQC sequence were seen, however, in the excellent level of artefact suppression which allowed one to observe correlations via very small couplings even in cases where the active isotopomer is present in low natural abundance and its lines are normally obscured by residual parent signals (Fig. 3). 

Fig. 3. Sections of two-dimensional 31P/15N H correlation spectra of the azido-substituted monophosphazene derivative shown. The 2D spectra were recorded by using a conventional 31P/15N HMQC pulse scheme with phase-cycling (left), and the gradient-enhanced enhanced sensitivity HSQC pulse sequence of Fig. 2 (right). The onedimensional spectra on top of the 2D-maps were acquired with the lD-versions of both pulse sequences. The right spectrum is distinguished by a substantially lower artefact level and displays an additional clearly visible correlation of the 31P with nitrogen atom N3. Reproduced from Ref. 25 by permission of Elsevier Ltd.

Chapter 6 has been almost completely rewritten. There is more emphasis on pulse sequences and on the use of inverse detection (e.g., HMQC and HMBC experiments). Some experiments from the Fifth Edition have been eliminated (e.g., /-Resolved), and others have been added. The chapter has been renamed Correlation NMR Spectrometry to better reflect the emphasis of the chapter. Because of this name change, the DEPT experiment has been moved to Chapter 5 the APT experiment has been eliminated. Gradient field NMR is presented as a recent development. Problems are assigned. 

Load the spectrum of the gradient-assisted, inverse detected, 2D CH-HSQC-TOCSY experiment acquired with the echo-antiecho technique, D NMRDATA GLUCOSE 2D CH GCHICOTO 001999.RR. Check and if necessary correct its calibration in both dimensions. Set up a layout as for the basic HSQC spectrum. Compare the spectrum with the spectra of the basic HSQC and HMQC experiments. Use the same rows or columns to identify the additional TOCSY-peaks. 

HPLC analysis was carried out using a 250 x 4.6 mm id Spherisorb ODS-2 column at 35°C with elution using isocratic D20/phosphate buffer at pH 2.5 for 5 min, followed by a linear gradient of acetonitrile to 50% after 50 min with UV monitoring of the eluent peaks at 225 nm. The NMR measurements were carried out with solvent suppression at a H NMR frequency of 600 MHz. The assignment of the resonances were based on those of standard compounds such as A-A-A-OH and Y-Y-Y-OH using standard 1-dimensional (ID) spectroscopy and 2-dimensional (2D) heteronuclear correlation methods such as and H-15N HMQC spectra. 

Lee, W., Revington, M. J., Arrowsmith, C. and Kay, L. E. (1994). A pulsed field gradient isotope-filtered 3D 13C HMQC-NOESY experiment for extracting intermolecular NOE contacts... 

In 1989, the first applications of multidimensional NMR were applied to humic substances (Buddrus et al., 1989).This study involved the application of 13C detected J-resolved (J-Res) spectroscopy. The study was successful in that it showed multidimensional NMR was applicable to the study of humic substances. However, in 1989 the lack of various modern experiments and the corresponding hardware (mainly probes fitted with pulse field gradients) made applying NMR to humic materials very challenging. In 1997, Simpson et al. demonstrated that the more sensitive inverse-detected NMR experiments were applicable to NOM (Simpson et al., 1997). In this manuscript COSY,TOCSY and HMQC were applied (Simpson et al.,... 

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