Structure and hydration property of acetone and 3-pentaone in the neutral and cationic state were investigated by using molecular dynamics (MD) and free energy calculations. The force field parameters of stretching vibration, angle bending, and partial charges of each molecule in the neutral and cationic state were developed by using density functional theory (DFT) calculations with B3LYP method and 6-31+G** basis set. The optimized structures by using these force field parameters in gas phase were compared with the experimental data and AMBER force fields parameters (parm99). From the results, the optimized structure in the neutral state of acetone was in good agreement with the experimental data. The evaluated hydration free energy in the neutral state of acetone was closed to the experimental data, while that of 3-pentaone was little bit larger than the experimental data. The ionization effect of ketone molecule on the hydration free energies was found to be significant in both molecules
