Heteronuclear phase sensitive

C/ H-Shift Correlation For most heteronuclear phase sensitive 2D spectra, all cross peaks should appear in positive absorption. Exceptions are spectra obtained with DEPT modified experiments, designed to discriminate between different carbon multiplicities via positive and negative cross peak intensities. 

If phase-sensitive spectra are not required, then magnitude-mode Pico) (or absolute-mode ) spectra may be recorded by combining the real and imaginary data points. These produce only positive signals and do not require phase correction. Since this procedure gives the best signal-to-noise ratio, it has found wide use. In heteronuclear experiments, in which the dynamic range tends to be low, the power-mode spectrum maybe preferred, since the S/N ratio is squared and a better line shape is obtained so that wider window functions can be applied. 

Notably, two isomeric products can be generated. The usual infrared (IR) and mass spectra as well as H and 13C NMR chemical shifts could not define which isomer was formed. The authors used different NMR techniques, such as 2-D heteronuclear multiple bond correlation (HMBC) experiments and phase-sensitive nuclear overhauser enhancement spectroscopy (NOESY) measurements to elucidate the product s structure. 

The proton-decoupled proton spectra allowed a distinction to be made between homo-and heteronuclear spin couplings, and Laurie and coworkers also demonstrated nulling of residual solvent resonances during the 2D /-resolved NMR of uridine in aqueous solution, wrote software for 45° tilting of the 2D spectra, and developed experimental protocols for multiple data-acquisition and processing, and a method for acquisition of the 2D /-resolved spectra in phase-sensitive mode. Lately, the 2D /-resolved technique has been less used, as it yields little evidence for spectral assignments. 

It is also important that LP not be abused. A sufficient number of increments must be taken from which the FID s can confidently be extended. A total of 64 increments has, for example, been found to be insufficient, while LP s have successfully been carried out with 96 increments. A good practice is to acquire at least 128 increments for accurate prediction. A second concern is that LP not be extended too far (e.g., 128 points predicted to 4,096). W. F. Reynolds (2002) has found that, as a general rule, data presented in the phase-sensitive mode can be predicted fourfold (e.g., 256 data points can be predicted to 1,024), while absolute-value data can be extended twofold, 256 points to 512. A significant exception to the fourfold rule for phase-sensitive experiments concerns the H-detected, heteronuclear chemical-shift correlation experiments. In marked contrast to COSY and HMBC spectra, for which the interferograms are frequently composed of many signals, those of HMQC and HSQC spectra constitute only one (due to the directly attached C). LP s up to sixteen-fold can be performed in these experiments (Sections 7-8a and 7-8b). 

DFT calculations have been used to construct new Karplus curves for 2./ m. 3/Hh (as mentioned before) 2/CH, Veil- Vnccc. 3/hcoc> and 3/hcsc- A major focus of this investigation62 was DFT computations for the exocyclic hydroxymethyl group of aldohexopyranoside derivatives, particularly in methyl a- and /i-D-gluco- and -galacto-pyranosides, for which coupling constants were also determined experimentally by 2D H -13C heteronuclear zero- and double-quantum, phase-sensitive /-HMBC NMR. DFT on methyl /1-D-glucopyranoside yielded 3/hcoc values that fitted the equation ... 

When we collect a 2-D NMR spectrum, both the second frequency dimension data (fj or Fj) and the first frequency dimension data (f2 or F2) may be phase sensitive. (Note that fj and f2 appear to be reversed but this naming convention derives from the order of their time domain precedents, tj and t2, in the NMR pulse sequence.) Zero-and first-order phasing of the second dimension of a 2-D NMR data set is required in many cases. Some experiments, most notably the gradient-selected heteronuclear multiple bond correlation (gHMBC) experiment, use the absolute value of the signal and hence do not require phasing. 

The heteronuclear multiple quantum correlation (HMQC) and heteronuclear single quantum correlation (HSQC) experiments both correlate H resonances with the resonances of some other nuclide, usually C or N. Both the HMQC and HSQC experiments can be run in the phase-sensitive or nonphase-sensitive mode. As in... 

A wide range of proton and carbon NMR techniques, almost exclusively using Fourier Transform (FT) spectroscopy, are presented in applications to support proposed chemical structures, with multinuclear NMR being used where appropriate. Polarisation transfer experiments (e.g. DEPT) are used to indicate carbon multiplicity and the use of two-dimensional techniques, which facilitate signal assignment, is seen more frequently in applications. Typical techniques include H- H correlation (COSY and related phase-sensitive experiments) proton-carbon heteronuclear correlation (including inverse... 

As noted in the introduction to the previous section, phase-sensitive HMBC data can be used to measure long-range heteronuclear coupling constants. 

Figure 1 Heteronuclear multiple quantum correlation (HMQC) pulse sequences (a) sequence for small J(I,S) values (b) for larger, resolved J(1,S) values and phase-sensitive presentation (c) zero or double quantum variant for the determination of the /-spin-multiplicity (d) with refocusing and optional 5-spin...

Fig. 14. (Left) Heteronuclear, correlated absolute-value spectrum of guanosine 2 -monophosphate. The p2 dimension contains the H-coupled P spectrum and the F, dimension contains the P-coupled H spectrum. The large peaks at 4.6 ppm (F,) are unsuppressed axial peaks. (Right) Phase-sensitive slices for each phosphorus transition b and c in the 2 -GMP two-dimensional spectrum. Spectra a and d are simulated for comparison with the experimental slices b and c. From Bolton and Bodenhausen (1979). Copyright 1979 American Chemical Society.

Several modifications have been proposed for the basic HNN-COSY experiment. For example, frequency separations between amino and aromatic 15N resonances are typically in the range 100-130 ppm and therefore much larger than between imino 15N donor and aromatic 15N acceptor resonances. As has been pointed out by Majumdar and coworkers [33], such 15N frequency separations are too large to be covered effectively by the non-selective 15N pulses of the homonuclear HNN-COSY. They therefore designed a pseudo-heteronuclear H(N)N-COSY experiment, where selective 15N pulses excite the amino and aromatic 15N resonances separately to yield excellent sensitivity [33]. An inconvenience of this experiment is that the resonances corresponding to the amino 15N nuclei are not detected, and a separate spin-echo difference experiment was used to quantify the h2/NN values. A slightly improved version of this pseudo-heteronuclear H(N)N-COSY [35] remedies this problem by the use of phase-coherent 15N pulses such that both amino and aromatic 15N resonances can be detected in a single experiment. 

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