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.
Rashkin
Mark J
MJ
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA.
Waters
Marcey L
ML
D008958:Models, Molecular; D008968:Molecular Conformation; D011726:Pyridinium Compounds / Q000737:chemistry*; D012996:Solutions; D014867:Water / Q000737:chemistry*
D011726:Pyridinium Compounds; D012996:Solutions; D014867:Water