Excited state intramolecular proton transfer in anionic analogues of malonaldehyde

The transfer of a proton in malonaldehyde takes place within an intramolecular II-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 foul 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 pi pi* states, as well as to (1)n pi* and (3)n pi*. The barriers to proton transfer are found to correlate strongly with various geometric and energetic markers of the strength of the II-bond. The II-bond is weakened by n --> pi* excitation, particularly for the neutral molecule, resulting in a higher transfer barrier. In the case of the two anions, excitation to (3) pi pi* strengthens the II-bond, while the result is more ambiguous for the (1) pi pi* 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 the pertinent orbitals and the density shifts arising from the excitation.