We have measured the rotational barriers of meta- and para-substituted N-benzyl-2-(2-fluorophenyl)pyridinium bromides in aqueous solution by dynamic NMR as a model system for offset-stacking interactions in proteins. Because the benzyl ring can stack with the 2-fluorophenyl ring in the offset conformation in the ground state, but not the transition state, the rotational barrier reflects the magnitude of the stacking interaction. Only a small (0.1 kcal/mol) change in rotational barrier was found for para substituents relative to hydrogen. A much larger energy difference was found for electronegative meta substituents (up to 0.66 kcal/mol for CF3). Evidence suggests that this is due at least in part to an electrostatic interaction between electron-poor hydrogens on one ring with the electronegative substituents on the other ring.

We have investigated the orientation dependence of the cation-pi interaction between a phenyl ring and a pyridinium ring in the context of a flexible model system in water. Of the four possible positions of the pyridinium nitrogen, ipso, ortho, meta, and para, we found a variation in the interaction energy of about 0.75 kcal mol(-1), with the stacking of the ipso-pyridinium ring providing the strongest interaction. The observed stability is attributed to the maximization of the electrostatic interaction, minimization of rotamers, and possible differences in hydration phenomena arising from alkylation.

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