Monitoring Protonation and migration with UV Spectroscopy – Looking at Solvation effect of the mobile proton on 4-aminobenzoic acid in water clustersMonitoring Protonation and migration with UV Spectroscopy – Looking at Solvation effect of the mobile proton on 4-aminobenzoic acid in water clusters*

We utilize cryogenic ion mass spectrometry along with UV action spectroscopy to monitor the effect of protonation on the electronic states of the molecule 4-aminobenzoic acid (4-ABA), both free in the gas phase and within small water clusters. Previous research has concluded that the proton would prefer the so called “O-protomer”, with the condensed phase showing heavy implications of a preferred “N-protomer”, both being determined by ultraviolet spectroscopy.¹ We employ this spectroscopy of the electronic transitions of the N and O protomer to observe the effect of hydration on the charge localization and the electronic structure. Hydration shows a large blue shift in the main identifying feature of the O-Protomer, starting around 340nm for bare 4-ABA and showing stability around 300nm at 4-(ABA)(H₂O)₅. When the 6^th water is added, the cross section of the absorption is dramatically reduced and is further blue shifted than before, possibly indicating a change in the protonation site.

Phys. Chem. Chem. Phys., 2017,19, 17434-17440