Probing hydrogen bonding in benzene–(water)nclusters using resonant ion–dip IR spectroscopy

Abstract

Resonant ion–dip infrared spectroscopy (RIDIRS) is used to record clustersize-specific spectra of C6H6–(H2O)n(BWn) clusters with n= 1–7 in the C—H and O—H stretch regions (2900–4000 cm–1). The Fermi triad characteristic of benzene in the C—H stretch region is virtually unchanged by addition of as many as seven water molecules. By contrast, the O—H stretch spectra show a dramatic dependence on cluster size. In C6H6–H2O, the O—H stretch transitions are complicated by additional structure due to internal rotation of H2O in the complex. In C6H6–(H2O)2, the O—H stretch spectrum closely mimics that of the free water dimer. For the n= 3–5 clusters, the spectra can be divided roughly into three categories, due to free O—H (3715 cm–1), π H-bonded O—H (3650 cm–1), and single donor O—H stretches, consistent with a BWn structure composed of a cyclic Wn cluster in which one of the free O—H groups is used in π hydrogen bonding with benzene. At n= 6, the spectra show distinct new transitions in the 3500–3600 cm–1 region. By comparing with the predictions of ab initio calculations of Wn, these new transitions are assigned to double donor O—H stretches associated with the formation of a more compact, ice-like structure in W6. The transformation to double-donating waters continues to develop in BW7, where additional transitions in the double-donor O—H stretch region appear at the expense of free O—H transition intensity.