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

Abstract The purpose of this article is to predict the structure of the CH3COO− anion in aqueous solution. For this purpose, the following effects of water molecules on the structure of CH3COO− in aqueous solution are considered: (A) the effect of the first solvent shell; (B) the effect of water molecules around the O′ of CH3COO−(H2O′)6; (C) the effect of water molecules around the CH3 of CH3COO−(H2O)6; and (D) the effect of water molecules around H′ of CH3COO−(HOH′)6. In order to estimate these effects, geometry optimization with 6-31++G* basis sets is carried out for CH3COO−(H2O)n (n=0,1,2,3,4,5,6), CH3COO−(H2O)6(HOH)m (m=1,2), CH3COO−(HOH)6(OH2)m (m=1,2), and H2O…CH3COO−(H2O)6. The vibrational analysis for CH3COO−(H2O)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 CH3COO− in aqueous solution is nearly the same as the structure of CH3COO− in CH3COO−(H2O)6. The C–C bond length of CH3COO− in aqueous solution is predicted to be smaller than the C–C bond length of CH3COO− in the gas phase by 0.023 A.