Structure and electronic properties of hydrated mesityl oxide: a sequential quantum mechanics/molecular mechanics approach

The hydration of mesityl oxide (MOx) was investigated through a sequential quantum mechanics/molecular mechanics approach. Emphasis was placed on the analysis of the role played by water in the MOx syn–anti equilibrium and the electronic absorption spectrum. Results for the structure of the MOx–water solution, free energy of solvation and polarization effects are also reported. Our main conclusion was that in gas-phase and in low-polarity solvents, the MOx exists dominantly in syn-form and in aqueous solution in anti-form. This conclusion was supported by Gibbs free energy calculations in gas phase and in-water by quantum mechanical calculations with polarizable continuum model and thermodynamic perturbation theory in Monte Carlo simulations using a polarized MOx model. The consideration of the in-water polarization of the MOx is very important to correctly describe the solute–solvent electrostatic interaction. Our best estimate for the shift of the π–π^* transition energy of MOx, when it changes from gas-phase to water solvent, shows a red-shift of −2,520 ± 90 cm⁻¹, which is only 110 cm⁻¹ (0.014 eV) below the experimental extrapolation of −2,410 ± 90 cm⁻¹. This red-shift of around −2,500 cm⁻¹ can be divided in two distinct and opposite contributions. One contribution is related to the syn → anti conformational change leading to a blue-shift of ~1,700 cm⁻¹. Other contribution is the solvent effect on the electronic structure of the MOx leading to a red-shift of around −4,200 cm⁻¹. Additionally, this red-shift caused by the solvent effect on the electronic structure can by composed by approximately 60 % due to the electrostatic bulk effect, 10 % due to the explicit inclusion of the hydrogen-bonded water molecules and 30 % due to the explicit inclusion of the nearest water molecules.