COMPETITION BETWEEN ROTAMERIZATION AND PROTON-TRANSFER IN O-HYDROXYBENZALDEHYDE

The proton transfer from one oxygen atom to the other within the intra molecular 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 excit ed 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 ener gy barrier. Excitation to the singlet or triplet pi pi states present s a situation where tautomerization to the keto is exothermic, with a small barrier. In contrast. rotamerization is endothermic with high in tervening barriers, so excited-state proton transfer is favored. The o pposite situation is encountered in the n pi states, where rotations of the hydroxyl and carbonyl groups are facile and lead energetically downhill, in contrast to the high barriers opposing endothermic tautom erization. The computations provide insights into the fundamental caus es for the discrepancies between the behaviors of the pi pi and n pi* states.