IR spectra of phenol+–(O2)n cation clusters (n = 1–4): Hydrogen bonding versus stacking interactions

IR photodissociation spectra of phenol–(O2)n cations, Ph+–(O2)n, are recorded in the O–H stretch range (νOH). Two isomers of Ph+–O2 are identified by their characteristic ΔνOH shifts upon complexation, namely the H-bonded global minimum featuring a nearly linear O–H–O hydrogen bond (ΔνOH = −86 cm−1) and the less stable π-bonded local minimum (ΔνOH = +8 cm−1). The diradical character of O2 has little influence on the Ph+–O2 interaction. IR spectra of larger Ph+–(O2)n clusters with n = 2–4 display a single νOH band with small incremental blue shifts from νOH of H-bonded Ph+–O2 (ΔνOH < 12 cm−1), indicating that these clusters have a single H-bond and (n−1) π-bonds. Quantum chemical calculations support the interpretation of the experimental data.