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

Clusters of phenol and ethanolamine (2-aminoethanol) have been studied using 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 proton donor) to the alcoholic oxygen of ethanolamine, leaving the intramolecular hydrogen bond of ethanolamine intact, or it inserts across the intra molecular OH --> N bond. Insertion provokes 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 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, conclusion in agreement with the relative proton affinities of ethanolamine and the phenoxy radical.