Theoretical Studies of Excited State Proton Transfer in Small Model Systems

Ab initio calculations that address the problem of excited-state proton transfer across an intramolecular hydrogen bond are reviewed. Small molecules, such as malonaldehyde, containing such a H-bond are first examined. This work reveals that in comparison to the ground state, the H-bond is strengthened and the transfer barrier reduced in the1ππ* state; opposite trends are noted in the triplet ππ* as well as nπ* states. Replacement of the H-bonding O atoms of malonaldehyde by N has only a small effect upon these results, as does enlargement or reduction of the malonaldehyde ring, coupled with anionic charge. The transfer barrier is linearly related to the equilibrium length of the H-bond in the various states of each system. Attachment of a phenyl ring to malonaldehyde introduces a fundamental asymmetry into the proton transfer potential, as the enol and keto tautomers are inequivalent. Whereas the enol is more stable in the ground and nπ* states, a reversal occurs in the ππ* states, which may be understood on the basis of the level of aromaticity within the phenyl ring. Nonetheless, when this asymmetry is accounted for, the phenyl ring affects the intrinsic barrier to proton transfer in the smaller malonaldehyde by a surprisingly small amount. Because of the high transfer barriers in the nπ* states, coupled with low barriers to bond rotation, rotamerization is likely to dominate over proton transfer in these states. This behavior contrasts sharply with the ππ* 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 states, the most accurate yet affordable method by which to include correlation remains an open question.