Theoretical studies of excited state proton transfer in small model systems

Ab initio calculations that address the problem of excited-state proton tra nsfer across an intramolecular hydrogen bond are reviewed. Small molecules, such as malononaldehyde, containing such a H-bond are first examined. This work reveals that in comparison to the ground state, the H-bond is strengt hened and the transfer barrier reduced in the(1)pi pi* state; opposite tren ds are noted in the triplet pi pi* as well as n pi* states. Replacement of the H-bonding O atoms of malonaldehyde by N has only a small effect upon th ese results, as does enlargement or reduction of the malonaldehyde ring, co upled with anionic charge. The transfer barrier is linearly related to the equilibrium length of the H-bond in the various states of each system. Atta chment of a phenyl ring to malonaldehyde introduces a fundamental asymmetry into the proton transfer potential, as the enol and keto tautomers are ine quivalent. Whereas the enol is more stable in the ground and n pi* states, a reversal occurs in the pi pi* states, which may be understood on the basi s of the level of aromaticity within the phenyl ring. Nonetheless, when thi s asymmetry is accounted for, the phenyl ring affects the intrinsic barrier to proton transfer in the smaller malonaldehyde by a surprisingly small am ount. Because of the high transfer barriers in the n pi* states, coupled wi th low barriers to bond rotation, rotamerization is likely to dominate over proton transfer in these states. This behavior contrasts sharply with the pi pi* states, where proton transfer is far more likely than bond rotations . While it is clear that inclusion of electron correlation is essential to a quantitative reproduction of the proton-transfer process in excited state s, the most accurate yet affordable method by which to include correlation remains an open question.