Ab initio study of the structure of CH3COO− in aqueous solution

Abstract The purpose of this article is to predict the structure of the CH 3 COO − anion in aqueous solution. For this purpose, the following effects of water molecules on the structure of CH 3 COO − in aqueous solution are considered: (A) the effect of the first solvent shell; (B) the effect of water molecules around the O′ of CH 3 COO − (H 2 O′) 6 ; (C) the effect of water molecules around the CH 3 of CH 3 COO − (H 2 O) 6 ; and (D) the effect of water molecules around H′ of CH 3 COO − (HOH′) 6 . In order to estimate these effects, geometry optimization with 6-31++G* basis sets is carried out for CH 3 COO − (H 2 O) n ( n =0,1,2,3,4,5,6), CH 3 COO − (H 2 O) 6 (HOH) m ( m =1,2), CH 3 COO − (HOH) 6 (OH 2 ) m ( m =1,2), and H 2 O…CH 3 COO − (H 2 O) 6 . The vibrational analysis for CH 3 COO − (H 2 O) n ( n =0,1,2,3,4,5,6) with 6-31++G* was performed at the optimized structures to confirm that all vibrational frequencies are real. The effect of the first solvent shell is significant. The other effects are predicted to be small. Thus, the structure of CH 3 COO − in aqueous solution is nearly the same as the structure of CH 3 COO − in CH 3 COO − (H 2 O) 6 . The C–C bond length of CH 3 COO − in aqueous solution is predicted to be smaller than the C–C bond length of CH 3 COO − in the gas phase by 0.023 A.