Hybrid QM/MM studies on energetics of malonaldehyde in condensed phase

Energetics of eight enol isomers of a malonaldehyde (MA) at the ground state in CCl₄ solvent environment have been investigated using a hybrid quantum mechanical (QM)/molecular mechanics (MM) method. It is found that relative energies of the isomers slightly change due to interactions between MA and surrounding atoms. In an isolated environment all eight isomers have stable planar structure. On the other hand, most of the isomers have nonplanar structure in CCl₄ whose interactions with the solute molecules are, however, weak. Mainly, structural changes are found in the H? O? C?C dihedral angle ?_{H? O? C?C}, i.e., a hydrogen atom that is connected directly with an oxygen atom, is located at a nonplanar position, and other atoms remain almost planar. Vertical excitation energies of low‐lying excited states at the resultant optimized structure of each isomer are evaluated. The vertical excitation energies in CCl₄ are almost the same as those in the isolated environment, but some changes were found in the triplet excitation states. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004