Structural trends of ionized water networks: Infrared spectroscopy of water cluster radical cations (H₂O)_n⁺ (n = 3-11)

The nature of water networks exposed to ionizing radiation is important in various radiation-related chemistry and biology. To understand structural evolution of ionized water networks at the molecular level, we report here infrared spectra of water cluster radical cations (H₂O)_n⁺ (n = 3 - 11) in the gas phase. Spectral features of free OH stretch modes are quite similar to those of protonated water clusters H⁺(H₂O)_n, of which the hydrogen-bond network structures have been revealed. In addition, we observed an extra band attributed to the stretch of an OH radical in (H₂O)_n⁺. These results indicate that nominal (H₂O)_n⁺ should be regarded as H⁺(H₂O)_n-1(OH) motifs having similar network shapes to those of H⁺(H₂O)_n. We also analyzed hydrogen-bonded OH stretch bands and found that hydrogenbond strength is a key factor to determine the position of the OH radical relative to the protonated site (H₃O⁺/H₅O₂⁺). Because an OH radical is a weaker hydrogen bond acceptor than water, the first solvation shell of the protonated site is preferentially filled with water. As a result, the OH radical is separated from the protonated (charged) site by at least one water molecule in n≥ 5 clusters. This result shows the instability of the H₃O⁺-OH ion-radical contact pair in water networks, and implies the higher mobility of the OH radical due to its release from the charged site. Observed structural preferences are confirmed both in cold and warm cluster ion sources.