Unexpected substituent effects in offset pi-pi stacked interactions in water

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.