Abboud, JLM; Auhmani, A; Bitar, H; Mouhtadi, ME; Martin, J; Rico, M
The proton-decoupled 13C NMR spectra of benzene, naphthalene, azulene, acenaphthylene, fluoranthene, phenanthrene, and 6,6-pentamethylenefulvene have been obtained in dilute solutions in cyclohexane, triethylamine, di-n-butyl ether, diisopropyl ether, diethyl carbonate, tetrahydrofuran, butyronitrile, γ-butyrolactone, propylene carbonate, dimethyl sulfoxide, benzene, toluene, fluorobenzene, anisole, acetophenone, benzonitrile, and nitrobenzene. It has been found that (1) the chemical shifts (relative to an external reference) of both alternant and nonalternant hydrocarbons are sensitive to solvent dipolarity-polarizability effects. (2) In the case of âselect solventsâ (aliphatic, monofunctional compound with one single dominant bond moment) there is a generally good correlation between the solvent-induced chemical shifts (SICS) and the Ï* scale of solent dipolarity-polarizability. (3) Aromatic solvent induced shifts (ASIS) and specific interactions are significant in aromatic solvents, although dipolarity-polarizability contributions are still very important. (4) With very few exceptions, SICS (relative to cyclohexane solvent) are downfield, and the results are not in favor of the simple reaction field model. (5) For aromatic hydrocarbons, there is a clear proportionality between the SICS and the paramagnetic shifts induced by the stable free-radical 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO). (6) There is no simple relationship between the SICS and the calculated electronic charge distribution of the solute molecules. These results show the important role played by quadrupoles and higher multipoles in determining solvent-solute interactions involving aromatic solutes. They also suggest that these medium effects involve both a âgeneral dielectric effectâ and a more localized noncontinuum action of the peripheral solvent atoms on the peripheral solute atoms. © 1987, American Chemical Society. All rights reserved.