Microsolvation of the Phenol Cation (Ph+) in Nonpolar Environments:  Infrared Spectra of Ph+−L n (L = He, Ne, Ar, N2, CH4)

Infrared photodissociation spectra of several phenol−L n cation clusters (Ph+−L n ; L = He, Ne, Ar, N2, CH4) are recorded in the vicinity of the O−H stretch vibration (ν1) of bare Ph+. The Ph+−L n complexes are produced in an electron impact (EI) ion source, which generates predominantly the most stable isomer of each cluster ion. The spectra of all dimers (n = 1) show strong ν1 transitions (at 3537, 3534, 3464, 3365, 3365 cm-1 for L = He, Ne, Ar, N2, CH4), which are attributed to proton-bound structures based upon the complexation-induced redshifts, Δν1. A linear correlation between Δν1 and the proton affinity of L is observed. In the case of Ph+−Ar, a weak transition at 3536 cm-1 is assigned to the ν1 band of the less stable π-bound isomer. The analysis of photofragmentation branching ratios and systematic frequency shifts in the spectra of larger Ph+−L n clusters (n ≤ 2 for CH4, n ≤ 5 for Ar, n ≤ 7 for N2) provide information about the microsolvation process of Ph+ in nonpolar environments. The ν1 transitions of the most stable isomers display small incremental blueshifts with respect to the dimer transitions, suggesting that further solvation causes little destabilization of the intermolecular proton bond to the first ligand. In the case of the Ph+−(N2) n complexes, the existence of two isomers is observed in the size range n = 5−7. For several Ph+−L n clusters, the most stable cation structures produced in the EI source differ considerably from the geometries observed by resonant enhanced multiphoton ionization (REMPI) of the corresponding neutral precursors. The limitations of REMPI techniques (arising from the Franck-Condon principle) for the generation and spectroscopic characterization of cluster cations are discussed.