PROTON-TRANSFER IN THE GROUND AND 1ST EXCITED TRIPLET-STATES OF MALONALDEHYDE

The intramolecular proton-transfer process in malonaldehyde is studied by ab initio methods in its ground electronic state and in its excited pi --> pi* triplet state. The transfer barrier is considerably higher in T1, which is attributed in part to the virtual disappearance of the hydrogen-bonding interaction present in the ground state. The weakening of this H bond in the excited state is associated with a less acidic proton-donor OH group and less basic acceptor oxygen. The pi --> pi* excitation adds antibonding character to the C=O bond, which causes it to elongate. The ensuing reduction in its bond dipole helps to weaken the intramolecular H bond.