The syn-anti equilibrium for the -COOH group reinvestigated. Theoretical conformation analysis for acetic acid in the gas phase and in solution

The B97D/aug-cc-pvtz optimized molecular geometry of the monomeric acetic acid with a syn -COOH group and the degree of dimerization agree well with the experimental data. The anti -COOH conformation was estimated at a relative free energy of 4.22 and 4.85 kcal/mol at the B97D/CBS and CCSD(T)/CBS (complete basis set limit) levels, respectively. The relative free energy for the anti conformer decreases by about 2 and 3 kcal/mol in infinitely dilute chloroform and water solutions, respectively, as calculated by means of the IEF-PCM continuum dielectric solvent approximation. Modeling very dilute and 0.1 M aqueous solutions in Monte Carlo simulations, the syn conformer has been predicted to be present at populations of 83% and 98%, respectively. Possible syn-anti transformation paths have been proposed by considering a neutral and a protonated acetic acid tetrahydrate complex. The syn conformer forms a stably maintained, doubly-hydrogen-bonded dimer in chloroform. In-water calculations for an about 0.2 M model are sensitive to the applied atomic charges. Ultimately, an association degree of 96% has been obtained for a dimer with one hydrogen bond. (C) 2013 Elsevier B.V. All rights reserved.