Size dependence of intracluster proton transfer of phenol-(H2O)(n) (n=1-4) cations

The intracluster proton transfer reaction occurring in the hydrogen-bonded cluster cations of phenol (C6H5OH) with water has been studied with trapped ion photodissociation (TIP) spectroscopy. Electronic spectra of [C6H5OH-(H2O)(n)](+) (n = 1-4) were observed in the region 380-400 nm, which reveals the chromophore of the cluster ions. It was found that the cluster ions of n less than or equal to 2 have the phenol ion (C6H5OH+) chromophore, while the n greater than or equal to 3 cluster ions exhibit the phenoxy radical (C6H5O) chromophore, demonstrating that n = 3 is the critical size for intracluster proton transfer. Chromophore switching, occurring in the cluster ions of phenol with other solvent molecules as well as water, was discussed on the basis of the proton affinity of the solvent species. It was concluded that the intracluster proton transfer is a ''push and pull'' reaction determined only by the difference in the proton affinity between the phenoxy radical and the solvent molecules.