Computation of the acetone ultraviolet spectrum in gas phase and in aqueous solution by a mixed discrete/continuum model

The ultraviolet spectrum of acetone in vacuum and in aqueous solution has been computed by different quantum mechanical methods coupled to the polarizable continuum model (PCM) for describing bulk solvent effects. The results in vacuo show that the time-dependent density functional theory (TDDFT) approach using the PBEO functional reproduces quite well the result obtained at the CASPT2 level. Supermolecule computations confirm that water molecules belonging to the first shell of polar groups (here the carbonyl oxygen) must be explicitly included in the quantum mechanical treatment, whereas the effect of other solvent molecules (which is far from being negligible) can be reliably described by the PCM. Moreover, statistical averaging effects have been taken into account by performing canonical molecular dynamics (MD) simulations followed by TDDFT quantum mechanical computations on representative clusters of increasing dimensions immersed in a polarizable continuum. The results show that the combined MD/DFT/PCM approach is reliable and effective, although the performances of the force field used in the MD simulations must be further investigated.