NON-CYCLIC ISOMERS OF (H$_{2}$O)$_{4}$ IN HELIUM NANODROPLETS: INFRARED SPECTROSCOPY AND AB INITIO CALCULATIONS

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2011

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Ohio State University

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Water clusters are assembled via the sequential pick-up of water molecules by helium nanodroplets. Unlike previous infrared spectroscopy experiments of water clusters in helium droplets, \bf{287}, 293 (2000).} one or two Neon atoms are added to the droplets prior to water pick-up. The upstream pick-up of a Neon atom results in several new bands in the infrared spectrum in addition to the bands that correspond to the water monomer, dimer and larger \textit{cyclic} water complexes. The new spectral features are determined to be signatures of a (H$_{2}$O)$_{4}$ cluster on the basis of the pick-up cell pressure dependence of the band intensities. A dc electric field is applied to the laser droplet beam interaction region, and these clusters are determined to be polar with permanent dipole moments between 2 and 3 Debye. On the basis of comparisons to CCSD(T) anharmonic frequency calculations, the new bands are assigned to OH stretch vibrations of a non-cyclic 3+1 cluster, which corresponds to a water molecule hydrogen bonded to a trimer ring. The presence of the Neon atom substantially affects the barrier to ring insertion of the fourth water molecule into a preformed cyclic trimer complex. In contrast, no new bands corresponding to open (non-cyclic) trimers or pentamers are observed.

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Author Institution: Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556; CHEMICAL AND MATERIALS SCIENCES DIVISION, PACIFIC NORTHWEST NATIONAL LABORATORY, 906 BATTELLE BOULEVARD, MS K1-83, RICHLAND, WASHINGTON 99352

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