{"68567":{"#nid":"68567","#data":{"type":"news","title":"Scientists Discover Dielectron Charging of Water Nano-droplets","body":[{"value":"\u003Cp\u003EScientists have discovered fundamental steps of charging of\nnano-sized water droplets and unveiled the long-sought-after mechanism of\nhydrogen emission from irradiated water. Working together at the Georgia\nInstitute of Technology and Tel Aviv University, scientists have discovered\nwhen the number of water molecules in a cluster exceeds 83, two excess\nelectrons may attach to it \u2014\nforming dielectrons \u2014 making it\na doubly negatively charged nano droplet. Furthermore, the scientists found\nexperimental and theoretical evidence that in droplets comprised of 105\nmolecules or more, the excess dielectrons participate in a water-splitting\nprocess resulting in the liberation of molecular hydrogen and formation of two\nsolvated hydroxide anions.\u0026nbsp; The\nresults appear in the June 30 issue of \u003Cem\u003Ethe \u003C\/em\u003E\u003Cem\u003EJournal of Physical Chemistry A\u003C\/em\u003E.\u003C\/p\u003E\n\n\u003Cp\u003EIt has been known since the early 1980s that while single\nelectrons may attach to small water clusters containing as few as two molecules,\nonly much larger clusters may attach more than single electrons. Size-selected,\nmultiple-electron, negatively-charged water clusters have not been observed \u2014 until now.\u003C\/p\u003E\n\n\u003Cp\u003EUnderstanding the nature of excess electrons in water has captured\nthe attention of scientists for more than half a century, and the hydrated\nelectrons are known to appear as important reagents in charge-induced aqueous\nreactions and molecular biological processes.\u0026nbsp; Moreover, since the discovery in the early 1960s that the\nexposure of water to ionizing radiation causes the emission of gaseous molecular\nhydrogen, scientists have been puzzled by the mechanism underlying this\nprocess.\u0026nbsp; After all, the bonds in\nthe water molecules that hold the hydrogen atoms to the oxygen atoms are very\nstrong. The dielectron hydrogen-evolution\u0026nbsp;\n(DEHE) reaction, which produces hydrogen gas and hydroxide anions, may\nplay a role in radiation-induced reactions with oxidized DNA that have been\nshown to underlie mutagenesis, cancer and other diseases.\u003C\/p\u003E\n\n\u003Cp\u003E\u201cThe attachment of multiple electrons\nto water droplets is controlled by a fine balancing act between the forces that\nbind the electrons to the polar water molecules and the strong repulsion\nbetween the negatively charged electrons,\u201d said Uzi Landman, Regents\u2019 and\nInstitute Professor of Physics, F.E. Callaway Chair and director of the Center\nfor Computational Materials Science (CCMS) at Georgia Tech.\u003C\/p\u003E\n\n\u003Cp\u003E\u201cAdditionally, the binding of an\nelectron to the cluster disturbs the equilibrium arrangements between the\nhydrogen-bonded water molecules and this too has to be counterbalanced by the\nattractive binding forces.\u0026nbsp; To\ncalculate the pattern and strength of single and two-electron charging of\nnano-size water droplets, we developed and employed first-principles quantum mechanical\nmolecular dynamics simulations that go well beyond any ones that have been used\nin this field,\u201d he added.\u0026nbsp;\u003C\/p\u003E\n\n\u003Cp\u003EInvestigations on controlled size-selected clusters allow\nexplorations of intrinsic properties of finite-sized material aggregates, as\nwell as probing of the size-dependent evolution of materials properties from\nthe molecular nano-scale to the condensed phase regime.\u003C\/p\u003E\n\n\u003Cp\u003EIn the 1980s Landman, together\nwith senior research scientists in the CCMS Robert Barnett, the late Charles\nCleveland and Joshua Jortner, professor of chemistry at Tel Aviv University,\ndiscovered that there are two ways that single excess electrons can attach to\nwater clusters \u2013 one in which they bind to the surface of the water droplet,\nand the other where they localize in a cavity in the interior of the droplet,\nas in the case of bulk water. Subsequently, Landman, Barnett and graduate\nstudent Harri-Pekka Kaukonen reported in 1992 on theoretical investigations\nconcerning the attachment of two excess electrons to water clusters. They\npredicted that such double charging would occur only for sufficiently large nano-droplets.\nThey also commented on the possible hydrogen evolution reaction. No other work\non dielectron charging of water droplets has followed since.\u003C\/p\u003E\n\n\u003Cp\u003EThat is until recently, when Landman, now one of the world leaders in the area of cluster and nano\nscience, and Barnett teamed up with Ori Chesnovsky, professor of\nchemistry, and research associate Rina Giniger at Tel\nAviv University, in a joint project aimed at understanding the process\nof dielectron charging of water clusters and the mechanism of the ensuing\nreaction - which has not been observed previously in experiments on water\ndroplets. Using large-scale, state-of-the-art\nfirst-principles dynamic simulations, developed at the CCMS, with all valence\nand excess electrons treated quantum mechanically and equipped with a newly\nconstructed high-resolution time-of-flight mass spectrometer, the researchers\nunveiled the intricate physical processes that govern the fundamental dielectron\ncharging processes of microscopic water droplets and the detailed mechanism of\nthe water-splitting reaction induced by double charging.\u003C\/p\u003E\n\n\u003Cp\u003EThe mass\nspectrometric measurements, performed at Tel Aviv, revealed that singly charged\nclusters were formed in the size range of six to more than a couple of hundred\nwater molecules. However, for clusters containing more than a critical size of\n83 molecules, doubly charged clusters with two attached excess electrons were\ndetected for the first time. Most significantly, for clusters with 105 or more\nwater molecules, the mass spectra provided direct evidence for the loss of a\nsingle hydrogen molecule from the doubly charged clusters.\u003C\/p\u003E\n\n\u003Cp\u003EThe theoretical\nanalysis demonstrated two dominant attachment modes of dielectrons to water\nclusters. The first is a surface mode (SS\u2019), where the two repelling electrons\nreside in antipodal sites on the surface of the cluster (see the two wave\nfunctions, depicted in green and blue, in Figure 1). The second is another\nattachment mode with both electrons occupying a wave function localized in a\nhydration cavity in the interior of the cluster \u2014 the so-called II binding mode\n(see wave function depicted in pink in Figure 2). While both dielectron\nattachment modes may be found for clusters with 105 molecules and larger ones,\nonly the SS\u2019 mode is stable for doubly charged smaller clusters.\u003C\/p\u003E\n\n\u003Cp\u003E\u201cMoreover, starting\nfrom the II, internal cavity attachment mode in a cluster comprised of 105\nwater molecules, our quantum dynamical simulations showed that the concerted\napproach of two protons from two neighboring water molecules located on the\nfirst shell of the internal hydration cavity, leads, in association with the\ncavity-localized excess dielectron (see Figure 2), to the formation of a\nhydrogen molecule. The two remnant hydroxide anions diffuse away via a sequence\nof proton shuttle processes, ultimately solvating near the surface region of\nthe cluster, while the hydrogen molecule evaporates,\u201d said Landman.\u003C\/p\u003E\n\n\u003Cp\u003E\u201cWhat\u2019s more, in\naddition to uncovering the microscopic reaction pathway, the mechanism which we\ndiscovered requires initial proximity of the two reacting water molecules and\nthe excess dielectron. This can happen only for the II internal cavity\nattachment mode. Consequently, the theory predicts, in agreement with the\nexperiments, that the reaction would be impeded in clusters with less than 105\nmolecules where the II mode is energetically highly improbable. Now, that\u2019s a\nnice consistency check on the theory,\u201d he added.\u003C\/p\u003E\n\n\u003Cp\u003EAs for future plans,\nLandman remarked, \u201cWhile I believe that our work sets methodological and\nconceptual benchmarks for studies in this area, there is a lot left to be done.\nFor example, while our calculated values for the excess single electron\ndetachment energies are found to be in quantitative agreement with\nphotoelectron measurements in a broad range of water cluster sizes \u2014 containing\nfrom 15 to 105 molecules \u2014 providing a consistent interpretation of these\nmeasurements, we would like to obtain experimental data on excess dielectron\ndetachment energies to compare with our predicted values,\u201d he said.\u003C\/p\u003E\n\n\u003Cp\u003E\u201cAdditionally, we\nwould like to know more about the effects of preparation conditions on the\nproperties of multiply charged water clusters. We also need to understand the\ntemperature dependence of the dielectron attachment modes, the influence of\nmetal impurities, and possibly get data from time-resolved measurements. The understanding\nthat we gained in this experiment about charge-induced water splitting may\nguide our research into artificial photosynthetic systems, as well as the\nmechanisms of certain bio-molecular processes and perhaps some atmospheric phenomena.\u201d\u003C\/p\u003E\n\n\u003Cp\u003E\u201cYou know,\u201d he added. \u201cWe started\nworking on excess electrons in water clusters quite early, in the 1980s \u2014 close\nto 25 years ago. If we are to make future progress in this area, it will have\nto happen faster than that.\u201d\u003C\/p\u003E\n\n\u003Cp\u003E\u003Cem\u003EThis research was funded by the U.S. Office of Basic Energy Sciences and the Israel Science Foundation.\u003C\/em\u003E\u003C\/p\u003E\n\n\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003EScientists have discovered fundamental steps of charging of nano-sized\nwater droplets and unveiled the long-sought-after mechanism of hydrogen\nemission from irradiated water.\u0026nbsp;\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Scientists, led by Uzi Landman, discover how hydrogen disappears from irradiated water."}],"uid":"27310","created_gmt":"2011-06-27 14:10:12","changed_gmt":"2016-10-08 03:09:40","author":"David Terraso","boilerplate_text":"","field_publication":"","field_article_url":"","dateline":{"date":"2011-06-27T00:00:00-04:00","iso_date":"2011-06-27T00:00:00-04:00","tz":"America\/New_York"},"extras":[],"hg_media":{"68560":{"id":"68560","type":"image","title":"Surface Attachment Mode","body":null,"created":"1449177185","gmt_created":"2015-12-03 21:13:05","changed":"1475894594","gmt_changed":"2016-10-08 02:43:14","alt":"Surface Attachment Mode","file":{"fid":"192608","name":"pr_figure_1_ss___surface_attachment_hr_landman.jpg","image_path":"\/sites\/default\/files\/images\/pr_figure_1_ss___surface_attachment_hr_landman_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/pr_figure_1_ss___surface_attachment_hr_landman_0.jpg","mime":"image\/jpeg","size":258966,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/pr_figure_1_ss___surface_attachment_hr_landman_0.jpg?itok=zRdYFPhm"}},"68561":{"id":"68561","type":"image","title":"Internal Attachment Mode","body":null,"created":"1449177185","gmt_created":"2015-12-03 21:13:05","changed":"1475894594","gmt_changed":"2016-10-08 02:43:14","alt":"Internal Attachment Mode","file":{"fid":"192609","name":"pr_figure_2__ii_interior_correct_figure__june_24_landman_hr.jpeg","image_path":"\/sites\/default\/files\/images\/pr_figure_2__ii_interior_correct_figure__june_24_landman_hr_0.jpeg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/pr_figure_2__ii_interior_correct_figure__june_24_landman_hr_0.jpeg","mime":"image\/jpeg","size":218775,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/pr_figure_2__ii_interior_correct_figure__june_24_landman_hr_0.jpeg?itok=zYG5d7IE"}}},"media_ids":["68560","68561"],"groups":[{"id":"1183","name":"Home"}],"categories":[{"id":"149","name":"Nanotechnology and Nanoscience"},{"id":"150","name":"Physics and Physical Sciences"}],"keywords":[{"id":"4896","name":"College of Sciences"},{"id":"7619","name":"hydrogen"},{"id":"382","name":"nanoscience"},{"id":"166937","name":"School of Physics"},{"id":"9180","name":"Uzi Landman"}],"core_research_areas":[],"news_room_topics":[],"event_categories":[],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cp\u003E\u003Cstrong\u003EGeorgia Tech Media Relations\u003C\/strong\u003E\u003Cbr \/\u003ELaura Diamond\u003Cbr \/\u003E\u003Ca href=\u0022mailto:laura.diamond@comm.gatech.edu\u0022\u003Elaura.diamond@comm.gatech.edu\u003C\/a\u003E\u003Cbr \/\u003E404-894-6016\u003Cbr \/\u003EJason Maderer\u003Cbr \/\u003E\u003Ca href=\u0022mailto:maderer@gatech.edu\u0022\u003Emaderer@gatech.edu\u003C\/a\u003E\u003Cbr \/\u003E404-660-2926\u003C\/p\u003E","format":"limited_html"}],"email":["mattnagel@gatech.edu"],"slides":[],"orientation":[],"userdata":""}}}