Protonated neurotransmitters in the gas-phase: clusters of 2-aminoethanol with phenol

Abstract

Clusters of phenol and ethanolamine (2-aminoethanol) have been studied using a combination of spectroscopic techniques (mass-selected resonant two-photon ionisation (R2PI), two-colour ultraviolet hole-burning, and infrared ion-dip spectroscopy) and ab initio computation. Three distinct 1∶1 clusters have been identified and assigned to two alternative types of hydrogen-bonded structure. The phenol molecule either binds (as a proton donor) to the alcoholic oxygen of ethanolamine, leaving the intramolecular hydrogen bond of ethanolamine intact, or it inserts across the intramolecular OH → N bond. Insertion provokes a change in the configuration of the ethanolamine molecule; the gGg′ conformation, which is energetically unfavourable and not populated in the bare molecule, presents an ideal geometry to accommodate phenol in a 1∶1 cluster. The time-of-flight mass spectra of all three 1∶1 clusters indicate the formation of protonated ethanolamine following ultraviolet excitation. While this may occur within the neutral excited state or within the cation, the distinct asymmetry in the time-of-flight profiles coupled with ab initio computation of the preferred cationic cluster structures, provides evidence for proton transfer in the ionic state, a conclusion in agreement with the relative proton affinities of ethanolamine and the phenoxy radical.