Ab initio calculations of negatively charged symmetrical and a unsymme
trical transition states of the S(N)2' reaction indicate that due to e
lectronic seasons and particularly Coulomb interactions the anti trans
ition state is generally more stable than the syn transition state. Th
e experimentally often observed syn preference results mainly from the
interaction with a polar solvent and/or the gegenion, which gives ris
e to higher stabilization of the syn transition state than of the anti
transition state.