Excited State Intramolecular Proton Transfer in Anionic Analogues of Malonaldehyde

The transfer of a proton in malonaldehyde takes place within an intramolecular H-bond involving a five-membered ring. This process is compared via ab initio methods with the transfer in analogous systems in which the size of the ring is altered to four and to six and in which the system bears an overall negative charge. In addition to the ground state, calculations are applied to the singlet and triplet ππ* states, as well as to 1nπ* and 3nπ*. The barriers to proton transfer are found to correlate strongly with various geometric and energetic markers of the strength of the H-bond. The H-bond is weakened by n → π* excitation, particularly for the neutral molecule, resulting in a higher transfer barrier. In the case of the two anions, excitation to 3ππ* strengthens the H-bond, while the result is more ambiguous for the 1ππ* state. This trend is reversed in malonaldehyde where the singlet is strengthened by the excitation and the triplet weakened. Some of these patterns are traced directly to the nature of t...