Ionization-induced π → H site-switching in phenol-CH4 complexes studied using IR dip spectroscopy.

IR spectra of phenol-CH4 complexes generated in a supersonic expansion were measured before and after photoionization. The IR spectrum before ionization shows the free OH stretching vibration (v(OH)) and the structure of neutral phenol-CH4 in the electronic ground state (S-0) is assigned to a p-bound geometry, in which the CH4 ligand is located above the phenol ring. The IR spectrum after ionization to the cationic ground state (D-0) exhibits a red shifted v(OH) band assigned to a hydrogen-bonded cationic structure, in which the CH4 ligand binds to the phenolic OH group. In contrast to phenol-Ar/Kr, the observed ionization-induced pi -> H migration has unity yield for CH4. This difference is attributed to intracluster vibrational energy redistribution processes.