It is well-known that some aromatic anions have the ability to induce viscoelastic transformation in aqueous solutions of cationic surfactants even at added salt concentrations as low as 10-20 mM. This behavior is associated with the formation of an entangled network of elongated micelles. However, the effect of aromatic ring substituents on the anion's ability to promote rapid micelle growth is not well-understood. We have performed ab initio calculations of the carbonyl group rotation barriers in a series of substituted benzoate and naphthoate anions at the MP2/STO-3G level of theory. It was found that aromatic carboxylates, known to be particularly effective in causing sphere-rod transition in cationic micelles, preferably adopt conformations with the COO(-) group in the same plane as the ring(s). This structural preference can be attributed to either intramolecular hydrogen bonding (o-hydroxyl derivatives) or pi-conjugation effects (m- and p-halogenated derivatives). In the former case the barrier to rotation is 40-50 kcal/mol, whereas in the latter case the threshold value is around 3.0 kcal/mol. Propensity for the planar conformation correlates with a greater depth of counterion penetration into the micelle surface, as inferred from NMR experiments, compared to the anions with less hindered carbonyl rotation. This points to favorable hydrophobic interactions between the surfactant methylene groups and the aromatic ring(s) of the anion as a possible explanation for the rapid growth of cationic micelles observed upon addition of certain aromatic carboxylates.

This article reports our experimental and theoretical investigations of fluorine hyperfine coupling constants (hfcc's) in the anion and cation radicals of a number of fluorinated benzenes, naphthalenes, and anthracenes. We have obtained electron spin resonance (ESR) spectra and hfcc's for the electrolytically generated anion radicals of 1,2,3,4-tetrafluoronaphthalene, 1,2,3,4-tetrafluoroanthracene, and 9,10-perfluoroanthraquinone. The experimental values of the hfcc's of these radicals, along with the hfcc's of several cation radicals of fluorinated benzenes and naphthalenes currently available in the literature, have been compared to our theoretical predictions using the UB3LYP density functional method in conjunction with a variety of basis sets. The EPR-III basis set usually gave the best agreement between theory and experiment for the fluorine splittings with an average relative error of 15%. We also find that it is possible to correlate the experimental fluorine hfcc's with the calculated π-and total electron spin populations F on the fluorine atom, the adjacent carbon atom, and the carbon-fluorine bond, thus providing some chemical insight into the origin of the interactions. The best correlation is obtained with a two-parameter equation of the formThe fit to 21 fluorine splittings using the EPR-III basis set and Mulliken π-electron spin populations gives an average error of only 9%. The average error obtained with EPR-II and NBO π-electron spin populations is 8%. Roughly 80% of the fluorine hfcc can be attributed to π-electron spin population on the fluorine atom. Our results indicate that conjugation of the fluorine atom with the ring is the primary source of the unpaired electron density on fluorine and that the often-assumed separability of σ-and π-electrons in aromatic systems is justified in these radicals as well.

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