Inversion vs retention of configuration in gas-phase ammonium ion/alcohol reactions

The potential energy surface for gas-phase reactions of ammonium ion with alcohols was examined by ab initio and DFT methods. Transition structures for inversion (S(N)2) and retention (S(N)F) mechanisms were located for 2-propanol and exo-2-norbornanol. The S(N)F (retention) process was found to be 6-8 kcal/mol higher in energy than the inversion S(N)2 counterpart for the 2-propanol system. The retention process in the 2-exo-norbornanol/ammonium ion system was favored by 1.3 kcal/mol.