The syn-/anti-conformational equilibrium of acetic acid in water studied by the RISM-SCF/MCSCF method

The syn-/anti-conformational equilibrium of acetic acid in water is studied by the RISM-SCF/MCSCF method, a hybridized method of the ab initio quantum chemistry and the statistical mechanics of molecular liquids. The solvent effect on the two conformers are examined in terms of the solvation free en ergy. Significant stabilization due to solvation was observed on the anti-c onformer, while only slight decrease in the free energy is resulted in the syn-conformer. Due to the greater stabilization in the anti-conformer, the energy gap between the two conformers is dramatically reduced in solution; 1.7 kcal/mol in solution compared with 6.9 kcal/mol in gas phase. The chang e in the electron density of acetic acid upon transferring the solute from gas phase into aqueous solution is visualized to understand the polarizatio n effects due to solvent. (C) 1999 Elsevier Science B.V. All rights reserve d.