Effects of intramolecular hydrogen bonding on the excited state dynamics of phenol chromophores

The theoretical prediction and experimental confirmation of the 1πσ* excited state of phenol which is repulsive along the O–H bond has a large impact on the interpretation of phenol and tyrosine photochemistry. In this work, we demonstrate that this excited state changes significantly if the OH functional group is involved in the formation of an intramolecular hydrogen bond in the ground state. We investigate the excited state dynamics of 2-, 3-, and 4-hydroxyacetophenone (HAP) separately in a molecular beam at 193 nm using multimass ion imaging techniques. H atom elimination from the repulsive excited state and Norrish type I reactions are the major dissociation channels of 3-HAP and 4-HAP which do not have intramolecular hydrogen bonding. However, the H atom elimination channel is completely quenched for 2-HAP which shows intramolecular hydrogen bonding. In addition, the ground state and the excited state potential energy surfaces (PESs) of HAP, 2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride, and 2-hydroxybenzamide are investigated using ab initio calculations. The results also show that the excited state potential along the O–H bond distance of the hydroxyl group changes significantly for molecules with intramolecular hydrogen bonding. The changes include: (a) the repulsive potential energy surface becomes an attractive potential near the ground state equilibrium geometry, (b) the conical intersection between the first and the second excited states along the O–H bond moves to a much higher energy level, and (c) the conical intersection between the repulsive excited state and the ground state along the O–H bond distance disappears. The results suggest that the interpretation of the photochemistry for molecules with a phenol chromophore must take these effects into consideration.