Zeolites that prefer alkynes: Alkenes such as ethylene and propene must be separated from alkynes before they can be converted in polymers. Drawbacks in current methods, such as hydrogenation of alkynes producing unwanted alkanes, has spurred interest in sorption separation methods. Zeolites have generally been inefficient, given the similar sizes and volatilities of the molecules. Chai et al. incorporated atomically dispersed divalent transition metal cations into faujasite zeolite and found that the nickel-containing analog efficiently removed alkynes from olefins through chemoselective binding at open nickel(II) sites. At ambient conditions in the presence of water and carbon dioxide, the zeolites retained separation selectivities of 100 and 92, respectively, for acetylene over ethylene and propyne over propylene for 10 adsorption-desorption cycles. Science , this issue p. 1002

Infrared spectroscopic study of neutral water clusters is crucial to understanding of the hydrogen-bonding networks in liquid water and ice. Here we report infrared spectra of size-selected neutral water clusters, (H₂O)_n (n = 3−6), in the OH stretching vibration region, based on threshold photoionization using a tunable vacuum ultraviolet free-electron laser. Distinct OH stretch vibrational fundamentals observed in the 3,500−3,600-cm⁻¹ region of (H₂O)₅ provide unique spectral signatures for the formation of a noncyclic pentamer, which coexists with the global-minimum cyclic structure previously identified in the gas phase. The main features of infrared spectra of the pentamer and hexamer, (H₂O)_n (n = 5 and 6), span the entire OH stretching band of liquid water, suggesting that they start to exhibit the richness and diversity of hydrogen-bonding networks in bulk water. Significance Spectroscopic investigation of neutral water clusters is critical in understanding the structures and properties of condensed-phase water. Because of the difficulty for size selection of neutral clusters in general, infrared spectroscopic study of structural evolution of confinement-free, neutral water clusters has been a grand challenge. In this work, we exploit recently developed, hybrid instruments that integrate infrared spectroscopy with a tunable vacuum ultraviolet free-electron laser to capture structural evolution of water clusters. Striking spectral change in the OH stretch region is observed from water tetramer to penta- and hexamer, due to appearance of three-dimensional hydrogen-bonding networks. The technique has the potential to obtain the infrared spectra of size-selected neutral clusters and to access their rich structural landscape.

The water octamer with its cubic structure consisting of six four-membered rings presents an excellent cluster system for unraveling the cooperative interactions driven by subtle changes in the hydrogen-bonding topology. Despite prediction of many distinct structures, it has not been possible to extract the structural information encoded in their vibrational spectra because this requires size-selectivity of the neutral clusters with sufficient resolution to identify the contributions of the different isomeric forms. Here we report the size-specific infrared spectra of the isolated cold, neutral water octamer using a scheme based on threshold photoionization using a tunable vacuum ultraviolet free electron laser. A plethora of sharp vibrational bands features are observed. Theoretical analysis of these patterns reveals the coexistence of five cubic isomers, including two with chirality. The relative energies of these structures are found to reflect topology-dependent, delocalized multi-center hydrogen-bonding interactions. These results demonstrate that even with a common structural motif, the degree of cooperativity among the hydrogen-bonding network creates a hierarchy of distinct species. The implications of these results on possible metastable forms of ice are speculated.