The proton transfer from one oxygen atom to the other within the intramolecular H-bond in a molecule like o-hydroxybenzaldehyde (oHBA) would be precluded by a prior rotational isomerism that breaks this H-bond. The likelihood of such rotamerization in the ground and several excited electronic states is investigated by ab initio calculations at the CIS and MP2 levels with a 6-31+G** basis set. In the ground state, the energetics of proton transfer and rotamerization are competitive with one another; both processes are endothermic and must surmount an energy barrier. Excitation to the singlet or triplet ðð* states presents a situation where tautomerization to the keto is exothermic, with a small barrier. In contrast, rotamerization is endothermic with high intervening barriers, so excited-state proton transfer is favored. The opposite situation is encountered in the nð* states, where rotations of the hydroxyl and carbonyl groups are facile and lead energetically downhill, in contrast to the high barriers opposing endothermic tautomerization. The computations provide insights into the fundamental causes for the discrepancies between the behaviors of the ðð* and nð* states.
- proton transfer,