Abstract
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 ππ* 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 nπ* states, where rotations of the hydroxyl and carbonyl groups are facile and lead energetically downhill, in contrast to the high barriers op...
Published Version
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