Carbon-13 chemical shifts in substituted

Table 7.53 Carbon-13 Chemical Shifts in Substituted Benzenes 7.104...

Srinivasan, C., Perumal, S., Arumugam, N., and Murugan, R., Linear free-energy relationship in naphthalene system-substituent effects on carbon-13 chemical shifts of substituted naphthylmethyl sulfides, Ind. J. Chem., 25A, 227, 1986. 

Substitution of heteroatoms on carbonium ion centres often has a profound and unexpected effect on the carbon-13 chemical shift which is hardly designed to bolster our confidence in the simplicity of the charge density-chemical shift correlation. A few examples will illustrate the point (in the following structures carbon-13 shifts are given in regular type, proton shifts in italics). 

The dramatic effect of nitrogen substitution on carbon-13 chemical shifts was illustrated at the beginning of this section by the fact that the a/p/ia-carbon resonance of pyridine moves upheld upon protonation at nitrogen. A further demonstration that nitrogen substitution does still more extensive violence to charge density-chemical shift correlations is seen in Figures 19 and 20. Thus, in... 

Carbon-13 chemical shifts of transition metal carbonyls [471, 473] decrease as one proceeds down a given group of the periodic table, as demonstrated for the triad Cr(CO)6, Mo(CO)e, and W(CO)e in Table 4.71. Substitution of CO by cyclopentadienide and other organic ligands deshields the carbonyl carbon nuclei. This is exemplified by iron pentacarbonyl (211 ppm) in comparison to cyclopentadienylirondicarbonyl-dimer, which displays one avaraged carbonyl signal ( 240 ppm) at room temperature [488] due to rapid cis-trans isomerization and intramolecular bridged-terminal carbonyl interconversion ... 

As with other nuclei, theoretical explanations for chemical shift are well developed but are nevertheless unable to predict with any certainty the structural influences on the chemical shift. Thus, there is still widespread reliance on empirical correlations for such problems, which is reflected in the discussion below. Despite the widespread use of fluorine-19 NMR spectroscopy, there are no published, widely applicable additivity rules that enable the chemical shift to be calculated conveniently as they are for carbon-13 spectra. A number of studies have concentrated on the correlation of fluorine chemical shifts in substituted aromatic fluorobenzenes, because of the well defined inductive and resonance... 

A plot of the fluorine-19 chemical shift for para-substituted fluorobenzenes versus the carbon-13 chemical shift for the corresponding substituted benzenes also gives a good bilinear relationship. The smaller slope for the n acceptor substituents is in agreement with canonicals of the form ... 

The chemical shift of the A -methyl group of various 1,4-dimethyl-piperazinium dichlorides has been examined in the presence of a-substituents and the basicities of nitrogens 1 and 4 and the nitrogen 1 inversion rate, in HjO and DjO at 33°, have been determined by analysis of this signal in the case of 1,4-cis-(2,6)-tetramethylpiperazine (1681). Carbon-13 chemical shifts have been measured for a number of substituted nitrosopiperazines (1682), and the population ratio of the ee and ae + ea conformations in 1,4-dichloropiperazine at — 45°C was determined as 3 1 (AG° = 0.5 kcal/mol) (1683). 

The objectives of this work were to verify the assignments of carbon-13 resonances in L-ascorbic acid (I), to use the carbon-13 chemical shifts to assign positions of substitution in derivatives of I, and to determine the conformational preference of I and its sodium salt (II) in aqueous solution. 

Carbon-13 chemical shifts of the acetylenic carbons in 1-substituted hexynes, p.p.m. 

In a continuation of the above study Hammel and Smith (105) examined electric field effects in substituted 9-diketonate complexes. Carbon-13 chemical shifts for metal complexes of acetylacetone (acac), benzoylacetone (bzac) and dibenzoylmethane (dbm) are given in Table LV. 

Adcock, W. and Abeywickrema, A.N., Concerning the origin of substituent-induced fluorine-19 chemical shifts in aliphatic fluorides carbon-13 and fluorine-19 nuclear magnetic resonance study of l-fluoro-4-phenylbicyclo[2,2,2]octanes substituted in the arene ring, J. Org. Chem., 47, 2945, 1982. 

Carbon-13 NMR has been used in the study of thiolate and disub-stituted tetrazolium derivatives.217-219 The chemical shifts of both the substituent and ring carbons show strong sensitivity to substitutents as shown in Table 9 for 130—135.219 Tetrazolium salts have also been studied by nitrogen-15 NMR (Table 10).220 221... 

The NMR and infrared spectra of the derivatized model compounds are useful In establishing the structures and the D.F. of the modified polymers. Careful assignment of all peaks in the 13C-NMR spectra for each of structures 7-13 confirms the regioselectivity of the substitution on the oxyphenyl unit and inertness of the phenyl sulfone units. The chemical shifts of the key carbons for the analysis, those of the oxyphenyl rings, are summarized in Table I. 

Carbon-13 NMR is often a more useful tool than H NMR for the elucidation of heterocyclic structures in which there are few or no ring protons. For symmetrically substituted 1,2,3-thiadiazoles, the carbon adjacent to the nitrogen atom is expected to have a lower field chemical shift than the carbon atom adjacent to the sulfur atom, as exemplified in CHEC-II(1996) <1996CHEC-II(4)289>. Several examples that follow this rule are illustrated in Table 5. There is now a more extensive body of data available and it is possible to more accurately predict the chemical shift of ring carbons. In the case of monosubstituted 1,2,3-thiadiazoles, the substituted carbon usually has a lower field chemical shift than the unsubstituted carbon. 

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