Experimental determination of structural motifs of interference-free water undecamer cluster (H2O)11.

Structural characterization of archetypal water clusters is essential for exploring the nature of aqueous hydrogen-bonding interactions that are responsible for the properties of water. While spectroscopic measurement of interference-free neutral water clusters has been proven to be challenging due to the difficulty in size selection, recent studies have successfully measured the infrared spectra of small water clusters (H2O)n (n = 2-10). Thus far, experimental evidence for structural motifs of larger water clusters (H2O)n (n ≥ 11) without environmental perturbation such as an ultraviolet-chromophore label, a messenger tag, or a host matrix has been lacking. Here utilizing the recently-developed size-specific infrared spectroscopy apparatus with a tunable vacuum ultraviolet free electron laser (VUV-FEL) and quantum-chemical studies, we have provided experimental evidence to characterize the structure of interference-free neutral water undecamer (H2O)11. Distinct OH stretching bands provide the evidence for the three lowest-energy isomer families denoted as 515, 43'4, and 55'1 structural motifs. The 515 structure is found to be the dominant one, which features a "5 + 1 + 5" assembling of two stacked 5-membered rings with an additional H2O on the side. Formation mechanism of these three structural motifs is proposed based on calculated energetics. This work provides crucial insights into the microscopic development of hydrogen-bonding water networks and advances our capabilities toward size-dependence study of a diverse range of neutral hydrated clusters for exploring the stepwise mechanisms of solvation processes such as salt dissolution and acid dissociation.