EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER IN ANIONIC ANALOGS OF MALONALDEHYDE

The transfer of a proton in malonaldehyde takes place within an intram olecular II-bond involving a five-membered ring. This process is compa red via ab initio methods with the transfer in analogous systems in wh ich the size of the ring is altered to foul and to six and in which th e system bears an overall negative charge. In addition to the ground s tate, calculations are applied to the singlet and triplet pi pi state s, as well as to (1)n pi and (3)n pi*. The barriers to proton transfe r 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 i n 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 ambig uous for the (1) pi pi state. This trend is reversed in malonaldehyde where the singlet is strengthened by the excitation and the triplet w eakened. Some of these patterns are traced directly to the nature of t he pertinent orbitals and the density shifts arising from the excitati on.