Carbon atoms, chemical shifts effect

Carbon-13 chemical shifts of the a- and (8-carbon atoms of various unsubstituted and 3-substituted thietane oxides and dioxides have been recorded and correlated by the equations S = ay + bx and Sf = ax + by where a and b are parameters characteristic of the sulfoxide or sulfone (y) and the substituent (x)216. The values of the substituent parameters were found to parallel those which determine the effect on the 13C chemical shifts when hydrogen is replaced by a substituent224. 

Analysis of the 13C chemical shifts of C-2 and C-6 in a series of 2-substituted 6-methoxy-3,6-dihydropyran-3-ones (82) allows the assignment of configuration to the isomeric pairs. C-2 is shielded by ca. 5 p.p.m. and C-6 by ca. 2.5 p.p.m. in the trans isomer compared with the corresponding signals of the cis compound (79MI22200). The upheld shift is associated with the 1,3-diaxial disposition of the substituent and the proton on the y-carbon atom. This y-effect has also been noted in the dihydropyran ring system (74MI22200). 

The effect on the coordination chemical shift of varying the phosphine has been studied for two series of trigonal metal-olefin complexes, viz., (CH2 CH2) PtL (46) and (CH2 C(CH3)C02C2H )NiL2 (49). In both cases the chemical shift is found to correlate with the basicity of the phosphine [Table IV (41, 45, 46, 49-52)]. The chemical shift of the unsubstituted olefinic carbon atom (CH2 ) of the ethyl methacrylate complexes is more strongly phosphine-dependent than the substituted olefinic carbon atom, and this effect has been attributed to electron withdrawal from this site by the ethoxycarbonyl substituent. 

Substituents in the aryl ring of 1-arylimidazoles can affect the chemical shifts of the imidazole ring carbon atoms, but these effects are diminished when steric hindrance inhibits coplanarity and hence full interannular delocalization (see CHEC-I). The degree of hindrance can be deduced from a study of the c-2 values, and <5c.3 -(5c-2 can be diagnostic of the amount of hindrance from a 2- or... 

The assignment of thiophene resonances based on chemical shifts is complicated by the fact that the chemical shift of C-2 and C-5 is very similar to that of C-3 and C-4 (125.0 vs. 126.7 ppm). In general, it is found that there is a rough correlation between a substituent s shielding effect on benzene carbon atoms and its effect on the corresponding thiophene ring positions. 

Chemical information obtained using and C NMR is usually obtained on samples in solution (liquid-state NMR) in order to improve resolution. However, C spectra can also be obtained on neat specimens, such as rubber. This is possible as long as there is sufficient molecular motion to average the orientation-dependent variation in chemical shift of chemically identical atoms (chemical shift anisotropy, CSA). Chemical shifts in C NMR spectra span a much wider range than in proton NMR, and therefore the former provides better spectral resolution. However, the Nuclear Overhauser effect (NOE) and other nuclear relaxation processes cause the C absorption intensities to deviate from direct proportionality to the number of carbon atoms. Thus, unless specific techniques are utilized, C NMR spectral intensities using standard liquid-state NMR acquisition methods are not quantitative. 

The H-C(l)-C(2)-H torsion angles of a- and P-D-glucopyranose and of methyl a-D-glucopyranoside in the crystal as well as in solution have been calculated on the basis of double-quantum heteronuclear local field NMR experiments using [l,2- C2]-D-glucopyranose and methyl [l,3- C2]-a-D-glucopyranoside. Mono- and di-fatty acid esters of a-D-glucopyranose have been characterized on the basis of the substituent-induced chemical shift effects on the carbonyl carbon atoms." ... 

Assuming no conformation dependent chemical shift effects to occur and using the chemical shift of orthorhombic polyethylene (33 ppm) [6] we can now calculate the chemical shifts of the methine carbon atoms in the three triads of the solid crystalline E-VOH copolymer, respectively 000 (67 ppm), CX)E (70.4 ppm) and EOE (73.8 ppm) where 000, OOE, EOE are abbreviations for (VOH, VOH, VOH), (VOH, VOH, E) and (E, VOH, E) triads. The chemical shift values presented above are only meant to yield useful assignments of the several methine carbon NMR signals of E-VOH copolymers. These assignments are necessary because Ovenall [5] did not report dependable estimates for all three types of methines sustained by experimental results. We are aware of chemical shift differences between liquids and solids. Moreover, the choice of orthorhombic polyethylene as a basis for the shift calculations is rather arbitrary but this will only cause the same uncertainty in each of the three shifts. Of more importance is the known sensitivity of substituent-induced shifts towards different conformational equilibria. From results obtained by Cantow [7] for different poly (1,2-dimethylbutane) polymers it can be estimated that the uncertainties in our estimations amount to ca. 2 ppm. It is, however, improbable that the order of the three methine carbon signals will be misjudged. 

Table 2.20 presents some data on the NMR spectra of diamagnetic homo- and heteroleptic metal alkoxides. In comparing chemical shifts it should be kept in mind that the 8 values of some metal alkoxides are subject to specific solvent effects. As expected the 8 values of the protons on a-carbon atoms are shifted considerably down-field relative to protons on /5-carbons. Since the H/ C chemical shift is the resultant of several contributing factors, there are no obvious correlations with metal oxidation state, atomic radius, or co-ordination number. However, metal NMR studies have proved to be of considerable importance in indicating the coordination environment of the central metal atom. 

The property measured by the x axis is called chemical shift (think of it as frequency). It is measured in strange units called ppm. The ehemical shift of an atom is related to the density of the electron cloud around that atom. Chemical shift is hard to predict nreciselv. Memorize these NMR ppm ranges ( C). Factors effecting chemical shift are additive. E.g. alkene C attached to a halogen (C=C-Br) could be found above 150 ppm. Type of Carbon Examples (R = H or alkyl) ppm Range... 

Using a multiple linear regression computer program, a set of substituent parameters was calculated for a number of the most commonly occurring groups. The calculated substituent effects allow a prediction of the chemical shifts of the exterior and central carbon atoms of the allene with standard deviations of l.Sand 2.3 ppm, respectively Although most compounds were measured as neat liquids, for a number of compounds duplicatel measurements were obtained in various solvents. 

Making allowance for those effects gives a good correlation between the chemical shifts and the it- and/or tr-electron density of the carbon atom bearing the proton (133, 236,237). 

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