Local and Remote Conformational Switching in 2-Fluoro-4-Hydroxy Benzoic Acid

In this work, 2-F-4-OH benzoic acid was isolated in Ar matrices and conformational changes were induced by near-IR irradiating the sample. Upon deposition, three conformers could be observed in the matrix, denoted as A1, A2, and D1, respectively. A1 and A2 are trans carboxylic acids, i.e., there is an intramolecular H bond between the H and the carbonyl O atoms in the COOH group, whereas D1 is a cis carboxylic acid with an intramolecular H bond between the F atom and the H atom in the COOH group, which otherwise has the same structure as A1. The difference between A1 and A2 is in the orientation of the carbonyl O atom with regard to the F atom, i.e., whether they are on the opposite or on the same side of the molecule, respectively. All three conformers have their H atom in their 4-OH group, facing the opposite direction with regard to the F atom. The stretching overtones of the 4-OH and the carboxylic OH groups were selectively excited in the case of each conformer. Unlike A2, which did not show any response to irradiation, A1 could be converted to the higher energy form D1. The D1 conformer spontaneously converts back to A1 via tunneling; however, the conversion rate could be significantly increased by selectively exciting the OH vibrational overtones of D1. Quantum efficiencies have been determined for the ‘local’ or ‘remote’ excitations, i.e., when the carboxylic OH or the 4-OH group is excited in order to induce the rotamerization of the carboxylic OH group. Both ‘local’ and ‘remote’ conformational switching are induced by the same type of vibration, which allows for a direct comparison of how much energy is lost by energy dissipation during the two processes. The experimental findings indicate that the ‘local’ excitation is only marginally more efficient than the ‘remote’ one.