2D-Hetcor

Table Chemical shift assignments for sy -[(= SiO)Re(= CtBu)(= CHtBu)(CH2tBu)] through the combined use of ID and 2D HETCOR NMR data...

Fig. 20 2D HETCOR spectra of A1P04-14 (a) 27A1 31P R-INEPT at 9.4 T, (b) 27A1 31P HMQC at 9.4 T, (c) 27A1 31P R-INEPT at 18.8 T, (d) 27A1 31P HMQC at 18.8 T, (e) 27A1 31P SPAM-MQ-/-HETCOR with R-INEPT at 18.8 T, and (f) 27AI 3IP SPAM-MQ-D-HETCOR obtained using cross-polarization at 18.8 T. The total experimental times were 5 h (a), 5 h (b), 2 h (c), 2 h (d), 10 h (e) and 7 h (f). (Reproduced with permission from [235])...

Nitrogen-14, with its natural abundance of 99.6%, is one of the most ubiquitous and, until recently, least studied NMR-active nuclei. Due to the integer spin number (/ = 1), its single-quantum transitions are affected by first-order quadrupolar broadening, which in most materials is on the order of a few megahertz. A new class of 2D HETCOR protocols has been recently developed, which makes it possible to indirectly observe well-resolved 14N sites via their spin-1/2 neighbors and obtain the related parameters of the quadrupolar tensors. 

Heteronuclear correlation (HETCOR) is an experiment establishing a correlation between the chemical shift of proton and that for another nucleus (i.e. C, P, Nor Si). The experiment relies on the heteronuclear dipolar interactions and uses a CP to transfer magnetization from protons to a rare spin.4,6 We have used 2D HETCOR experiment to characterize support/counter-cation/cluster interaction in the supported mesoporous catalysts.21... 

Later we will see how these couplings can be exploited in experiments that enhance the sensitivity of 13C spectra (INEPT), measure the number of hydrogens attached to each carbon (APT and DEPT), and correlate 13C chemical shifts with H chemical shifts using a second dimension (2D-HETCOR, -HMQC, -HSQC, and -HMBC). But for detecting a simple 13C spectrum, we need a way to suppress these 13C- H couplings so we can observe a single line (singlet) for each 13 C resonance. 

Figure 9.6 shows a 2D HETCOR spectrum with the ID 13C spectrum displayed at the top (horizontal or F2 dimension) and the ID lH spectrum displayed vertically on the left side (vertical or Fi dimension). From any peak (resonance) in the lH spectrum, we can follow a horizontal line until we encounter a spot or blob of intensity in the 2D data matrix. These clusters of intensity represent correlations in the 2D spectrum and are called crosspeaks. From the crosspeak we move up along a vertical line and run into the 13C peak in the ID 13C spectrum corresponding to that proton s personal carbon atom (the one it is directly bonded to). In this way we can pair up each proton peak in the lH spectrum with a carbon peak in the 13C spectrum—a process called chemical-shift correlation. 

GENERAL APPEARANCE OF INVERSE 2D SPECTRA 11.2.1 2D HETCOR versus 2D HSQC/HMQC... 

Figure 18 Comparison of (a) z-filtered and (b) the ordinary correlated, INEPT based, 2D HETCOR spectra of Me3SiC(0)CH3...

Any HETCOR pulse sequence can readily be extended by a Hahn-echo pulse train [20] which is particularly useful in the case of Sn/ H HEED-HETCOR of tin-nitrogen compounds. This enables one to measure y( Sn, N), /( N, H) (together with the relative signs) and A / N( Sn), all at the same time (see Figure 13). Numerous other modifications of 2D HETCOR experiments are feasible, many of which are based on schemes of selective excitation and/or... 

Since they display a number of structural features frequently encountered in triterpenic natural products, the spectral study of these metabolites may be of general interest to phytochemists.The structures of 1-5 were deduced from ElMS and detailed NMR analysis including 2D (HETCOR) and... 

Figure 3.25. Na- P 2D HETCOR spectra of Na3P309 A. Obtained with the normal HETCOR pulse sequence and B. with MQ-MAS incorporated in the pulse sequence to obtain higher resolution from Wang, De Paul and Bull (1997) with permission of the copyright owner.

A closer inspection of the 2D HETCOR cross-sections in the dimension suggests that the isotropic chemical shift of this resonance is correlated with the extent of Al-O-P connectivity, resulting in a tentative chemical shift scale for Q m Ai) species (see Table 1) [83,84]. Replacement of a P by an Al in the second coordination sphere results in upheld shift effects of 10 to 15 ppm. 

It is possible to separate and assign the H signals of the PPs using 2D HETCOR spectra, because the correlations are clearly resolved in the two-dimensional spectra for proton-carbon pairs. The 2D HETCOR technique allows resolution of H CRAMPS to be tied to the higher resolution associated with - C chemical shifts. - ... 

Fig. 32. 2D HETCOR NMR spectra of silk fibroins (A) Tussah Antheraea...

Thus, the 2D HETCOR technique allows the resolution of H CRAMPS to be tied to the higher resolution associated with C chemical shifts. However,... 

D HETCOR NMR has the disadvantage of having a small scaling factor, which leads to a larger error in H chemical shifts. Accordingly, it is necessary to confirm the observation mentioned above with a well-defined simple model polypeptide of Bombyx mori, [Ala-Gly]i2, which is described in the following subsection. 

From 2D HETCOR measurements, it is easily deduced that the H chemical shifts of both the L-alanine and glycine residues in [Ala-Gly]i2-I are close to those in Bombyx mori-l. Furthermore, the H" chemical shifts of the glycine residue in [Ala-Gly]i2-I and Bombyx mori-l are also close to those of PG II (6 = 3.7). On the other hand, the H chemical shift of the L-alanine residue in [Ala-Gly]i2-I (.b = 1.5) agrees with the side-chain H chemical shift of Bombyx mori-l (b = 1.6). Thus, all signals of the H CRAMPS spectra of Bombyx mori-I can be assigned [<5 = 10-8, H b = 6.9, phenyl protons mainly of L-tyrosine residue b = 3.9, H (mainly of Ala and Gly residues) b = 1.6, side-chains (mainly H of Ala residue)]. 

Now it is clear that the H chemical shift reflects the conformation of model polypeptide [Ala-Gly] 12 and natural silk fibroins such as Tussah Antheraea pernyi and Bombyx mori silk fibroins. It is confirmed that the well-defined [Ala-Gly] 12 is a suitable model for the structural study of natural silk fibroins (silk I and silk II forms) using high-resolution solid-state NMR. As a result, the H peak assignment of the silk fibroins on the basis of the conformation-dependent H chemical shifts of model polypeptides can be determined utilizing H CRAMPS NMR and H- C 2D HETCOR NMR, as described in this section. The chemical shift results of model polypeptides [Ala-Gly] 12 synthesized by Shoji et al. play an important role in determining new structures for silk I and silk II forms, as very recently proposed by Lazo and Downing. ... 

Fig. 10.16. (A) 2D H—HETCOR pulse sequence and (B) 2D HETCOR spectrum for PMA/PVPh = 1/1 cast from methyl ethyl ketone. The interpolymer cross-peak between the carbonyl carbon of PMA (b) and the hydroxyl protons of PVPh (a) is appreciable. (Reprinted with permission from Ref. [26]. 1994 John Wiley, New York.)...

Quantitative evaluation of the 2D-HETCOR spectra are presented in Table 7.3. As expected incubation in r-SBF causes over time dissolution of phases characterised by the distorted states 1 and 2 associated with oxyhydroxyapatite, that is partially dehydroxylated SRO-structured hydroxyapatite as well as dissolution of the thermal decomposition products TTCP and TCP, but to a lesser extent. Concurrently the relative proportion of crystalline well-ordered hydroxyapatite increases from 46 mass% in an as-sprayed coating to 74 mass% in a coating incubated under physiological conditions for 12 weeks. 

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