Protonation of aromatic molecules: competition between ring and oxygen protonation of phenol (Ph) revealed by IR spectra of PhH+–Arn

The protonation sites of phenol are investigated by IR photodissociation spectroscopy of protonated phenol complexed with one and two Ar ligands, PhH+–Arn (n=1,2), in the vicinity of the O–H stretch fundamentals. The complexes are produced in a supersonic expansion of Ph, H2, and Ar combined with electron impact ionization. The vibrational analysis of the IR spectra unambiguously reveals the presence of at least two PhH+ isomers in the expansion: protonation occurs at oxygen and at the aromatic ring (in para and/or ortho position). This observation represents the first spectroscopic evidence for the existence of several PhH+ isomers in the gas phase. The Ar ligands in PhH+–Arn prefer hydrogen bonding over other binding sites. Quantum chemical calculations support the interpretation of the experimental data and provide further insight into the relative stability of various PhH+ isomers and their complexes with Ar.