Direct heteronuclear chemical shift correlation

Direct heteronuclear chemical shift correlation Conceptually, the 2D J-resolved experiments lay the groundwork for heteronuclear chemical shift correlation experiments. For molecules with highly congested 13C spectra, 13C rather than XH detection is desirable due to high resolution in the F% dimension [40]. Otherwise, much more sensitive and time-efficient proton or so-called inverse -detected heteronuclear chemical shift correlation experiments are preferable [41]. 

Section 7-8 Direct Heteronuclear Chemical-Shift Correlation Via Scalar Coupling 257... 

The two principal H-detected, direct, heteronuclear chemical-shift correlation experiments are HMQC and HSQC. The X-nucleus-detected counterpart is HETCOR, The and X-nucleus spectral widths are reduced in each of these experiments. It is important to remember that the latter should be decreased to contain only the signals of protonated X nuclei. Quaternary carbons, for example, do not participate in these experiments, and their signals should not be included in the reduced spectral windows. 

Heteronuclear chemical-shift correlation experiments can be performed by detecting either protons or the X nuclei (Section 7-8). All of the comments that were made there for the direct, heteronuclear chemical-shift correlation experiments apply equally well to their indirect (or longer range—e.g., two- and three-bond correlation) counterparts. 

Direct Heteronuclear Chemical Shift Correlation Methods... 

Figure 6-21 illustrates this procedure for an adamantane derivative. The H frequencies are on the vertical axis and the C frequencies are on the horizontal axis. The respective spectra are illustrated on the left and at the top. The 2D spectrum is composed only of cross peaks, each one relating a carbon to its directly bonded proton(s). There are no diagonal peaks (and no mirror symmetry associated with a diagonal), because two different nuclides are represented on the frequency dimensions. Quaternary carbons are invisible to the technique, as the fixed times A and A2 normally are set to values for one-bond couplings. This experiment often is a necessary component in the complete assignment of H and resonances. Its name, HETeronuclear chemical Shift CORrelation, usually is abbreviated as HETCOR, but other acronyms (e.g., HSC, for Heteronuclear Shift Correlation, and H, C-COSY, also are used. The method may be applied to protons coupled to many other nuclei, such as Si, and P, as well as C. 

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). 

Heteronuclear chemical shift correlation methods establish the direct link between protons and the respective, directly attached carbons (or nitrogens). In the case of methylenes with inequivalent (anisochronous) protons, the "multiplicity of the carbon in question is irrefutably obvious. For isotropic methylenes and other resonances, the multiplicity of the resonance (CH, CH2 or CH3) in question may be less obvious. Early work by Kessler and co-workers addressed this issue via the development of the DEPT-HMQC experiment. [120] Multiplicity editing is also available for experiments such as GHSQC. An extra pair of delays and pulses, with the flip angle of the proton pulse being adjustable, allow the acquisition of data in... 

Heteronuclear single-quantum correlation (HSQC) permits to obtain a 2D heteronuclear chemical shift correlation map between directly bonded H and X-heteronuclei (commonly and N). It is widely used because it is based on proton-detection, offering high sensitivity when compared with the conventional carbon-detected 2D HETCOR experiment. Similar results are obtained using the 2D HMQC experiment [77],... 

In homonudear shift-correlation experiments like COSY we were concerned with the correlation of chemical shifts between nuclei of the same nuclear species, e.g., H with H. In heteronuclear shift-correlation experiments, however, the chemical shifts of nuclei belonging to different nuclear species are determined (e.g., H with C). These may be one-bond chemical shift correlations, e.g., between directly bound H and C nuclei, or they may be long-range chemical shift correlations, in which the interactions... 

The order in which various NMR data are acquired is largely one of user preference. Acquisition of the proton reference spectrum will invariably be undertaken first. Whether a user next seeks to establish homo- or heteronuclear shift correlations is where individual preferences come into play. Many spectro-scopists proceed from the proton reference spectrum to either a COSY or a TOCS Y spectrum next, while others may prefer to establish direct proton-carbon chemical shift correlations. This author s preference is for the latter approach. From a multiplicity-edited HSQC spectrum you obtain not only the carbon chemical shifts, which give an indication of the location of heteroatoms, the degree of unsaturation and the like, but also the number of directly attached protons, which eliminates the need for the acquisition of a DEPT spectrum [51, 52]. The statement in the prior sentence presupposes, of course, that there the sensitivity losses associated with the acquisition of multiplicity-edited HSQC data are tolerable. 

The direct heteronuclear shift correlation experiments exploit the one-bond ( Vch) heteronuclear coupling as the basis of establishing chemical shift correlations. The concept of using multiple quantum coherence was developed by Muller in 1979 [83] that of using single quantum coherence came out of the work of Bodenhausen and Ruben in 1980 [111]. 

Nuclear magnetic resonance (NMR) has proved to be a very useful tool for structural elucidation of natural products. Recent progress in the development of two-dimensional 1H- and 13C-NMR techniques has contributed to the unambiguously assignment of proton and carbon chemical shifts, in particular in complex molecules. The more used techniques include direct correlations through homonuclear (COSY, TOCSY, ROESY, NOESY) [62-65] and heteronuclear (HMQC, HMBC) [66. 67] couplings. 

Because, in (RSn)i20i4(OH)6 X2, the anions do not interact directly with any tin atom but with the /U-2-OH that constitute the charged poles, the exchange of the anions or their ionic dissociation cause only small changes on the Sn chemical shifts (Table 2.3.1). H- H NOESY/ROESY (homonuclear Overhauser correlation spectroscopy) and HOESY (heteronuclear NOESY) have been used to... 

Correlations anticipated in various homonuclear ( H- H) and heteronuclear ( C- H) 2D NMR experiments are conceptualized in Eigure 5.1. A hypothetical model compound (the chemical shifts are not accurate and are for illustrative purposes only) with three aromatic protons and four side-chain protons on its three side-chain carbons is used to illustrate the information available from each experiment. A set of five experiments, in addition to the standard ID proton and carbon spectra, are useful for characterizing any model compound or lignin. The correlation spectroscopy (COSY) experiment correlates directly coupled protons (Figure 5.1a). 

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