13C shift

A useful comparison of the 13C shifts for acyclic and cyclic five- and six-membered sulfur compounds has been made86,220, but data on cyclic sulfur compounds of other ring sizes are rather limited. Typically, oxidation of sulfide to a sulfone results in 20-25 ppm... 

Figure 3 shows 13c MAS spectra of acetone-2-13c on various materials. Two isotropic peaks at 231 and 227 ppm were observed for acetone on ZnCl2 powder, and appreciable chemical shift anisotropy was reflected in the sideband patterns at 193 K. The 231 ppm peak was in complete agreement with the shift observed for acetone diffused into ZnY zeolite. A much greater shift, 245 ppm, was observed on AICI3 powder. For comparison, acetone has chemical shifts of 205 ppm in CDCI3 solution, 244 ppm in concentrated H2SO4 and 249 ppm in superacid solutions. The resonance structures 5 for acetone on metal halide salts underscore the similarity of the acetone complex to carbenium ions. The relative contributions of the two canonical forms rationalizes the dependence of the observed isotropic 13c shift on the Lewis acidity of the metal halide. 

An example of the use of chemical shifts to delineate residual secondary structure is given in Figure 3 for the molten globule state of apomyo-globin (Eliezer et al., 1998 Eliezer et al., 2000). Combined use of 13C , H , 13C, and 13CO secondary shifts gives a more precise definition of secondary structure boundaries than use of 13C shifts alone (Eliezer et al., 2000). 

So to a large extent, 1-D 13C NMR interpretation is a case of matching observed singlets to predicted chemical shifts. These predictions can be made by reference to one of the commercially available databases that we ve mentioned, or it can be done the hard way - by a combination of looking up reference spectra of relevant analogues and using tables to predict the shifts of specific parts of your molecule (e.g., aromatic carbons). We have included some useful 13C shift data at the end of the chapter but it is by necessity, very limited. 

Aside from the initial applications of the n, 1- and m,n-ADEQUATE experiments in the elucidation of the structure of 5,6-dihydrolamellarin (3, see Sections 4.1, 5.1 and 6.5), the author is unaware of any reported applications of these experiments in the literature.38 Given the potential ambiguities associated with the two long-range magnetization transfer steps of the latter, coupled with the very low sensitivity and the fact that correlations are observed in Fi at the DQ frequency of the coupled carbons rather than at the 13C shift as in the 1,1- or 1,h-ADEQUATE experiments, it is probably quite likely that the m,n-ADEQUATE experiment will continue to be very sparingly utilized for structure elucidation. 

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

On the other hand, the situation is more complex for the 3-substituted derivatives. In such compounds the substituent can be considered electronically to be located either meta or para to the selenium ring atom (cf. resonance formulas 8 and 9 for a -I -M substituted derivative). However, the substituent-caused shifts in the 3-substituted derivatives indicate that the heteroatom and the substituents are para and not meta related. 77Se chemical shifts are more sensitive than the 13C shifts to changes in electron... 

The 13C shifts are related to TMS the 15N shifts are compared to neat nitromethane. 

In contrast to the 13C shifts, the 13C resonance frequencies are not indicative of residue type, which makes the assignment more demanding in comparison to the 13C , 15N, Hn correlation map. Nevertheless, the HN(CA)CO-TROSY scheme equipped with the selective 13C refocusing pulses during the 13C -13C/ transfer steps is able to provide 13C (i), 15N(i),... 

The chemical structures of five dextrans were partially determined by methylation, and found to be branched molecules having the following types of substitution (a) 6-0 and 3,6-di-O, (b) 6-0, 3-0, and 3,6-di-O, (c) 6-0,3,6-di-O, and 2,3-di-O, (d) 6-0, 4-0, and 3,4-di-O, and (e) 6-0 and 2,3-di-O. At 27° and pH 7 (external, Me4Si standard), the 13C shifts ofO-substituted, non-anomeric carbon atoms were C-2 (76.5), C-3 (81.6), and C-4 (79.5). The C-l resonances were also recorded, and may be used for reference purposes. Some variation of chemical shifts, relative to each other, was observed with changing temperature. (The work serves to emphasize the importance of accurately measuring the temperature of the solution when determining chemical shifts.102)... 

As expected, the performance of this filter over the whole 13C shift range surpasses that of a conventional single or even double filter. The authors have successfully applied this technique to a 2D NOESY spectrum with 13C-filtering in both dimensions, acquired on the 26 kDA complex between a [U-13C,15N]-labeled DNA-binding protein domain and a 15-mer DNA ligand (unlabeled) [24]. 

TABLE 5. Substituent effects on 13C shifts in mono-substituted benzenes19,21-23... 

Correlations with Hantzsch esters) (70) (77BSB267). This latter type of compound is of interest as an effective model for the NAD/NADH coenzyme system, and this work provides detailed analysis of H and 13C NMR of such compounds. 

These conclusions are reinforced by measurement of natural abundance 15N chemical shifts in piperidines and decahydroquinolines (77JA8406,78JA3882,78JA3889). Lack of correlation between 13C shifts of cyclohexanes and 1SN shifts of piperidines bearing the same methyl substituents are attributed to, among other factors, solvent effects and the difference between H-lone pair and H-H interactions. Protonation served to cancel these stereoelec-tronic effects. Correspondence between 1SN shifts in N- and C- methyl substituted piperidines and decahydroquinoline hydrochlorides and the analogous 13C values were, however, generally much closer than for saturated aliphatic amines. 

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