Heteronuclear 2D /-resolved spectra

The most common way to record heteronuclear 2D /-resolved spectra is the gated decoupler method, so called because the decoupler is gated, i.e., switched on during the preparation period (for nOe) during the first... 

Many variations of this experiment are known. Some of the pulse sequences used for recording heteronuclear 2D/resolved spectra are shown in Fig. 5.8. In a modified gated decoupler sequence (Fig. 5.8b), the decoupler is off during the first half of the evolution period and is svdtched on during the second half. Any C resonances that are folded over in the F, domain may be removed by employing the fold-over corrected gated decoupler sequence (FOCSY) (Fig. 5.8c) or the refocused fold-over corrected decoupler sequence (RE-FOCSY) (Fig. 5.8d). 

Figure 5.10 (A) Selective spin-flip pulse sequence for recording heteronuclear 2D / resolved spectra. (B) Its effect on magnetization vectors. The selective 180° pulse in the middle of the evolution period eliminates the large one-bond coupling constants, /< ...

Polarization transfer techniques like INEPT and DEPT have been used to enhance sensitivity in heteronuclear 2D /-resolved spectra. In combination with the semiselective sequence just described, INEPT has been used to suppress long-range Jen couplings and to measure the one-bond couplings (Fig. 5.15) (Rutar, 1984). Driven equilibrium pulses for fast restora-... 

Pure 2D absorption line shapes are readily obtained in heteronuclear 2D /-resolved spectra. The incorrect setting of 90° and 180° pulses can, however, cause ghost peaks that can be removed by a phase cycling procedure, appropriately named Exorcycle (Rutar, 1984b). A... 

In homonuclear 2D /-resolved spectra, couplings are present during <2 in heteronuclear 2D /-resolved spectra, they are removed by broad-band decoupling. This has the multiplets in homonuclear 2D /-resolved spectra appearing on the diagonal, and not parallel with F. If the spectra are plotted with the same Hz/cm scale in both dimensions, then the multiplets will be tilted by 45° (Fig. 5.20). So if the data are presented in the absolute-value mode and projected on the chemical shift (F2) axis, the normal, fully coupled ID spectrum will be obtained. To make the spectra more readable, a tilt correction is carried out with the computer (Fig. 5.21) so that Fi contains only /information and F contains only 8 information. Projection... 

In homonuclear-shift-correlated experiments, the Ft domain corresponds to the nucleus under observation in heteronuclear-shift-correlated experiments. Ft relates to the unobserved or decoupled nucleus. It is therefore necessary to set the spectral width SW, after considering the ID spectrum of the nucleus corresponding to the Ft domain. In 2D /-resolved spectra, the value of SW depends on the magnitude of the coupling constants and the type of experiment. In both homonuclear and heteronuclear experiments, the size of the largest multiplet structure, in hertz, determines... 

Heteronuclear two-dimensional /-resolved spectra contain the chemical shift information of one nuclear species (e.g., C) along one axis, and its coupling information with another type of nucleus (say, H) along the other axis. 2D /-resolved spectra are therefore often referred to as /,8-spectra. The heteronuclear 2D /-resolved spectrum of stricticine, a new alkaloid isolated by one of the authors from Rhazya stricta, is shown in Fig. 5.1. On the extreme left is the broadband H-decoupled C-NMR spectrum, in the center is the 2D /-resolved spectrum recorded as a stacked plot, and on the right is the con tour plot, the most common way to present such spectra. The multiplicity of each carbon can be seen clearly in the contour plot. 

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. 

Carbon-proton connectivities can be determined using several methods. The number of protons directly attached to the carbon in question will split the carbon resonance according to the 2nl + 1 rule seen in proton NMR. There tends to be, however, much overlap of the multiplets in fully proton-coupled carbon spectra, sometimes such that it is very difficult to distinguish between the various multiplets. Routine carbon spectra are therefore measured fully proton decoupled for simplicity. Information regarding the exact number of protons attached to the carbons can be acquired from APT, DEPT or INEPT experiments. In APT spectra, the carbons bearing an odd number of protons (CH, CH3) can be distinguished from carbons with no or two attached protons (quaternary C, CH2). DEPT and INEPT experiments can distinguish between all four types of carbons (primary, secondary, tertiary and quaternary). Heteronuclear 2D /-resolved spectroscopy can also be used to obtain the multiplicities of the carbons, as well as Vc-h ... 

The matrix obtained after the F Fourier transformation and rearrangement of the data set contains a number of spectra. If we look down the columns of these spectra parallel to h, we can see the variation of signal intensities with different evolution periods. Subdivision of the data matrix parallel to gives columns of data containing both the real and the imaginary parts of each spectrum. An equal number of zeros is now added and the data sets subjected to Fourier transformation along I,. This Fourier transformation may be either a Redfield transform, if the h data are acquired alternately (as on the Bruker instruments), or a complex Fourier transform, if the <2 data are collected as simultaneous A and B quadrature pairs (as on the Varian instruments). Window multiplication for may be with the same function as that employed for (e.g., in COSY), or it may be with a different function (e.g., in 2D /-resolved or heteronuclear-shift-correlation experiments). 

Information about the surface and interface structures in hexadecylamine-capped CdSe NC of 2 nm size has been obtained by a variety of 1H, 13C, 113Cd, and 77Se NMR techniques [342]. The 77Se CP-MAS-NMR spectrum showed five partially resolved peaks from surface or near-surface Se environments. It was possible to obtain 2D heteronuclear correlation (HETCOR) spectra between 1H and the other three nuclei despite the inherent sensitivity limitations (the 77Se- 3I-I HETCOR experiment required 504 h ). The latter experiment indicated that the methylene protons of the hexadecylamine chain interact with the surface Se atoms via a tilt of the chain toward the surface. The surface Se atoms were not seen to interact with thiophenol present, and it was suggested that thiophenol binds to a selenium vacancy at the surface. 

Heteronuclear coupling constants (1,b7c,h) are most commonly measured from heteronuclear 2D experiments. The 3/c H couplings can be easily extracted from /-resolved spectra as well as from f or F2 proton coupled HSQC spectra. The undesired evolution of "/CH during q can be eliminated with use of an appropriate bilinear rotation decoupling (BIRD) pulse, such as BIRDd,x in. /-resolved spectroscopy35 and 111RD in Fi-coupled HSQC.36 Spin-state selective excitation techniques, S3E and S3CT37 38 (spin-state-selective coherence transfer), can also be used for the measurement of... 

In addition to the importance of parameter selection on F skew, the ACCORD-HMBC experiment has also been shown to exhibit strong coupling artifact responses similar to those observed in heteronuclear 2D J-resolved spectra. - Finally, the triplet character of responses in the second frequency domain superimposed over the Fi skew was mathematically accounted for by Zangger and Armitage in their development of the ACCORD-HMQC experiment. "... 

The heteronuclear J-resolved 2D experiment produces completely separate information in both domains, X nucleus chemical shift in F2 (since broadband decoupling in t2 removes the effect of J coupling) and AX coupling in F. This is particularly valuable, for example, in C- H cases since complete coupling patterns can be extracted for each without overlap of adjacent patterns. Information can be extracted easily, in contrast to direct observation of highly overlapped coupled ID spectra. 

Quinting and Cai [62] carried out high-resolution C-NMR and proton NMR measurements to determine the tacticity of poly(n-butyl methacrylate) (PBMA) with particular focus on the peak assignments for the n-butyl side chain. Free-radical and anionic PBMA were examined, with the former being predominantly syndiotactic and the latter isotactic. Proton NMR resonances for the n-alkyl chain of these polyacrylics show a combination of effects from configurational sensitivity and homonuclear scalar interactions. A combination of J-resolved proton NMR and proton- C-heteronuclear correlated 2D-NMR spectra was used to characterise the long-range chemical shift effects due to tacticity. 

The third class are /-resolved experiments. These produce spectra with the peaks at the frequencies along /2 corresponding to the resonances observed in the ID spectrum of the detected nucleus. In homonu-clear 2D /-spectroscopy, the peaks are dispersed into the fi dimension based on homonuclear /-coupling. In heteronuclear 2D J-spectroscopy, the peaks are dispersed into the dimension based on heteronuclear /-coupling (e.g. detection of in fi and at the shift of each a multiplet, resulting from all of the resolved /(-p couplings, is observed in / ). 

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