Infrared spectroscopic studies on hydrogen-bonded water networks in gas phase clusters

Size-selective infrared (IR) spectroscopy of gas phase water-containing clusters is performed to probe microscopic natures of hydrogen-bonded water networks. Size-selective observation is extended to the size range of a few tens to hundreds of molecules to bridge the gap between simple but unique networks in small-sized water clusters and those in bulk water. IR spectra of two types of large water-containing clusters, phenol-(H₂O)_n (n < ∼50) and H⁺(H₂O) _n (n ≤ 221), are measured in the OH stretching vibrational region. Clear size dependence of the observed spectra is interpreted in terms of coordination numbers of water molecules, ring sizes of hydrogen bond networks and interior crystallisation of the clusters. Structural features of hydrogen bond networks seen in bulk water are confirmed in these large-sized clusters. For more detailed analyses of spectra of H⁺(H₂O)_n, the inert gas 'tagging' technique is employed. Tuning of internal energy and isomer distribution of H⁺(H₂O)_n is achieved by a choice of inert gas species. An effective approach to spectral isomer separation is proposed on the basis of unexpected inert gas dependence of isomer distribution. Inert gas tagging is also applied to probe hydrogen bond network motifs of large-sized H⁺(H₂O)_n in the size range of n = 20-50. A particular success of the tagging is demonstrated on the n = 22 cluster, which has been known as an anti-magic number cluster. An important application of gas phase water clusters is simplification of a complicated phenomenon in liquid water and such model clusters allow us to extract the physical essence of the phenomenon. Radiation chemistry of water (ionisation and following processes) is studied by IR spectroscopy of water cluster radical cations, (H₂O)_n⁺. The H₃O⁺-OH ion-radical contact pair formation upon ionisation of water is confirmed in n ≤ 4. Separation of the ion-radical pair by hydration processes is observed in n ≥ 5. This result shows the intrinsic instability of the H₃O⁺-OH ion-radical contact pair in water networks, and implies higher mobility of the OH radical due to its release from the charged site.