Aromatic Hydrogen Chemical Shift Correlations

The fact that aromatic hydrogens are deshielded compared to their vinyl cousins is a direct consequence of the magnetic anisotropy of the aromatic it system. Below is a depiction of the toluene molecule showing how the three ti bonds (six 7t electrons) are conjugated, that is, all the p orbitals (one on each sp2 ring carbon) are parallel. This makes all C-C bonds in the ring equivalent, halfway between single and double 

Aromatic hydrogens typically occur in the 8 6.5-8.0 chemical shift range. And just as for vinyl hydrogens, correlation tables of substituent parameters have been developed for aromatic substituents. In this case we will use the chemical 

X ortho U meia Hpara X u onho meta Hpara 

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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... 

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... 

In the benzene series, an approximately linear relationship has been obtained between the chemical shifts of the para-hydrogen in substituted benzenes and Hammett s a-values of the substituents. Attempts have been made, especially by Taft, ° to use the chemical shifts as a quantitative characteristic of the substituent. It is more difficult to correlate the chemical shifts of thiophenes with chemical reactivity data since few quantitative chemical data are available (cf. Section VI,A). Comparing the chemical shifts of the 5-hydrogen in 2-substituted thiophenes and the parahydrogens in substituted benzenes, it is evident that although —I—M-substituents cause similar shifts, large differences are obtained for -j-M-substituents indicating that such substituents may have different effects on the reactivity of the two aromatic systems in question. Differences also... 

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

Abstract-—The hydrogen bonding chemical shift of the —OH proton of methyl alcohol has been separated into two parts, one due to the formation of dimer and the other to all other polymeric species. A molecule which is hydrogen bonded as dimer contributes only about one-third as much as a molecule in a higher polymer. The separation is based on correlation with the —OH stretching infrared data, and is confirmed by the examination of sterically hindered alcohols—both aromatic and aliphatic—which because of bulky substituents, can form dimers but not higher polymers. In these substances, the small H-bond shifts are also correlated with monomer-dimer equilibrium studies of the infrared spectra. 

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