Aromatic compounds carbon-13 chemical shifts

Correlations anticipated in various homonuclear ( H- H) and heteronuclear ( C- H) 2D NMR experiments are conceptualized in Eigure 5.1. A hypothetical model compound (the chemical shifts are not accurate and are for illustrative purposes only) with three aromatic protons and four side-chain protons on its three side-chain carbons is used to illustrate the information available from each experiment. A set of five experiments, in addition to the standard ID proton and carbon spectra, are useful for characterizing any model compound or lignin. The correlation spectroscopy (COSY) experiment correlates directly coupled protons (Figure 5.1a). 

A similar range of values is noted for the ipso carbon chemical shifts of aromatic - S02- compounds as shown in the following tables. 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

Z Arrangement was also ascribed to the isomer absorbing at higher field in the case of the ethyl compounds. CH and CH2 protons near the ring nitrogen are shielded by the aromatic ring in the Z compound. The protons at the ring carbon absorb at lower field (near 5.2 p.p.m.) in the Z compounds than in the E compounds (4.50-4.70 p.p.m.). The chemical shift of this proton may be used for E-Z discrimination in further substances. 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

The isomeric 1- and 2-fluoronaphthalenes have fluorine chemical shifts of -124 and -116 ppm, respectively. A full analysis of the proton and carbon spectra of 1-fluoronaphthalene is given in Scheme 3.56. NMR data for a number of other fluoropolycyclic aromatic compounds are available.7... 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

The pyridine-noreugenin compounds 18, 29, 13, and 32 show characteristic downfield aromatic protons between 8 8.0 and 9.5, and the quaternary nitrogen alkaloids 31 and 32 exhibit a distinct iV-methyl singlet at 8 4.3. The chemical shift of the iV-methyl group in the other alkaloids which contain it is influenced by the substituents on the adjacent carbon atoms. Thus in rohitukine (17), where the adjacent groups are both methylene, the signal is seen at 8 2.21 whereas in N-methylschumannificine (26) (where a CHOH is adjacent) it is seen at 8 2.95 and in (V-methylanhydroschumannificine (28) (where HC=CR3 is adjacent) it is seen as far downfield as 8 3.14. 

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