DOI: 10.1126/science.287.5451.295 , 295 (2000); 287Science et al.Marco Martina,Potentials in Interneuron DendritesDistal Initiation and Active Propagation of Action www.sciencemag.org (this information is current as of January 5, 2007 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/287/5451/295version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/287/5451/295#otherarticles, 15 of which can be accessed for free: cites 39 articlesThis article 108 article(s) on the ISI Web of Science. cited byThis article has been http://www.sciencemag.org/cgi/content/full/287/5451/295#otherarticles 56 articles hosted by HighWire Press; see: cited byThis article has been http://www.sciencemag.org/cgi/collection/neuroscienceNeuroscience : subject collectionsThis article appears in the following http://www.sciencemag.org/help/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. c 2006 by the American Association for the Advancement of Science; all rights reserved. The title SCIENCE is a CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on January 5, 2007 www.sciencemag.orgDownloaded frombased on harmonic frequency calculations. Al-though these cyclic clusters all have multipleO–H vibrational bands, the most intense onesfor each cluster size (the ring stretches) are closein frequency and thus merge into a single band,which is particularly intense. Indeed, ab initiocalculations show that the intensities of the O–Hmodes associated with less symmetric struc-tures, in which the molecules are inequivalent,are much smaller than for the cyclic structures(26). Analogous effects were observed in thecase of the linear chains of HCN (27), where themost redshifted C–H stretches have the highestintensity. The application of a large electric fieldhas no influence on this band, which furthersupports the cyclic structure. In contrast with thepolar chains of HCN reported previously (28),for which an electric field markedly sharpensand intensifies the spectrum, the equilibriumgeometry for the cyclic water hexamer is non-polar and no such effect is expected. The cyclictetramer is also nonpolar, and although the cy-clic trimer and pentamer have polar equilibriumstructures, vibrational averaging yields an effec-tive dipole moment of zero (29).Having established that the water mole-cules insert into a preexisting hydrogen-bonded ring of smaller size, the continuationof this growth pattern naturally leads to theformation of a cyclic hexamer. The path be-tween this hexamer and the cage will involvea great deal of hydrogen bond rearrangement,which we expect will be difficult in liquidhelium (see Fig. 1). Apparently, there is notenough energy available to the system toreach the three-dimensional cage. Severalother local minima lie lower in energy thanthe cyclic hexamer (26 ). Thus, we have notsimply formed the next higher energy isomerof the hexamer but rather have used thisgrowth process to steer the system kineticallyto this specific structural isomer.Despite considerable effort, we found noevidence for the cyclic heptamer in the heli-um spectra. At the present time, we are un-sure whether this means that the cyclic hep-tamer is not stabilized by the helium or if thefrequency shift between this species and thecyclic hexamer is simply too small to permitus to resolve the corresponding peaks. Wenote that there is a weak band further to thered (indicated by the question mark in Fig. 1),which could be due to either a small amountof cage hexamer that has managed to rear-range or the heptamer in a cage form. Furtherab initio calculations on both the cyclic andcage forms of the heptamer will be helpful indetermining what happens with the largerwater clusters grown in helium.These experiments show that growth in liq-uid helium can provide access to different struc-tures than those obtained from gas-phase nucle-ation, allowing us to explore at least some of therich structural landscape that has been identifiedby theoretical calculations. The cyclic waterhexamer is the smallest possible ice-like cluster,and its detailed study should provide importantinsights into the properties of bulk ice. Molec-ular dynamics studies show that this cyclicmotif is also important in liquid water. Thepresent study suggests a class of experimentsthat, when combined with the correspondingtheoretical calculations, could shed light on thisinteresting hydrogen bond ring insertion pro-cess. Because a chainlike trimer has also beenshown to be a local minimum on the corre-sponding surface (20), we are hopeful that withfurther improvements to the sensitivity of theapparatus, water chains might also be observed.References and Notes1. K. Liu et al., Nature 381, 501 (1996).2. K. Liu, M. G. Brown, R. J. Saykally, J. Phys. Chem. A101, 8995 (1997).3. N. Pugliano and R. J. Saykally, Science 257, 1937(1992).4. J. D. Cruzan et al., Science 271, 59 (1996).5. U. Buck, I. Ettischer, M. Melzer, V. Buch, J. Sadlej, Phys.Rev. Lett. 80, 2578 (1998), and references therein.6. J. Brudermann et al., J. Chem. Phys. 110, 10649(1999), and references therein.7. C. J. Tsai and K. D. Jordan, Chem. Phys. Lett. 213, 181(1993).8. J. Kim, D. Majumdar, H. M. Lee, K. S. Kim, J. Chem.Phys. 110, 9128 (1999), and references therein.9. S. S. Xantheas and T. H. Dunning Jr., J. Chem. Phys.99, 8774 (1993).10. M. R. Viant et al., J. Chem. Phys. 110, 4369 (1999).11. A. C. Belch and S. A. Rice, J. Chem. 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