Laser Induced Water-Cluster-Mediated Long Range Proton Transfer in Microhydrated 4-Aminobenzoic Acid

**Abstract:**

We investigate water-mediated proton transfer between two distinct protonation sites in 4-aminobenzoic acid (4ABA) using spectroscopy. This study focuses on H⁺-4ABA·(H₂O)₆ cluster ions confined within a temperature-controlled ion trap. Protonation is observed at both the acid (O-protomer) and amine (N-protomer) groups, which exhibit markedly different ultraviolet (UV) absorption bands. Proton transfer rates between these protomers are measured using a selective excitation scheme. Size-selected ions in a cryogenic ion trap are excited in the infrared with a 5 ns pulsed laser, and the proton location is monitored by the time-dependent UV absorption yield at 300 nm, characteristic of the O-protomer. The proton transfer is initiated from well-defined hydration structures, raising the system to ~3000–3300 cm⁻¹—above the barrier for spontaneous, water-mediated transfer between the two protonation sites. This cooperative solvent-mediated process occurs at an energy below the dissociation threshold of a water molecule, ensuring the system remains in a microcanonical ensemble throughout the large-amplitude proton transfer dynamics.