Solvent effects on the internal rotation of neutral and protonated glyoxal

Torsional energy profiles for internal rotation of the CHO group in glyoxal and protonated glyoxal have been studied both in gas phase and aqueous solution at the MP2/6-31G(d) level. The effect of solvation on the basic features of the internal rotation profiles has been accounted for by using a discrete-continuum supermolecule Polarizable Continuum Model (PCM) representation of the solvent. For protonated glyoxal, a more detailed analysis has been conducted, studying the whole rotational potential energy surface generated as a function of the C-C bond rotation and proton cis-trans isomerism coordinates. Additional single point iterative MRCI calculations were performed using the CIPSI program to assess the quality of the one-configuration representation. The effect of solvation on the structural parameters and relative energies is thoroughly analyzed for each of the systems under study, with particular attention addressed to correctly ascribe the role of explicit solute-solvent interactions together with macroscopic solvent effects. In order to predict free energies in solution at 298 K, zero point energy and thermal corrections have also been calculated at the same level of theory for all the stationary points here characterized.