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.