He then assembled a dodecahedron containing a hydrogen bond where the donor was also a double acceptor, and the acceptor was also a double donor – the maximum possible cooperativity. 2 ‘With this huge variety of possible hydrogen bond placements, you can definitely find clusters such as fit your needs,’ says Kananenka. The range of possible arrangements and the spectrum of possible hydrogen bonds they create were enumerated computationally by Sherwin Singer of Ohio State University and his collaborators in the early 2000s. The cluster is something of a chemical physics celebrity, closely related to clathrates, approximating water molecules at a surface, and sometimes occupied by an oxonium ion to make a stable magic number cluster. He turned to the (H 2O) 20 dodecahedron, where each molecule is hydrogen bonded to three neighbours in any one of 30,026 possible arrangements. ‘We decided to use these ideas of cooperativity to essentially engineer or build that cluster that we really need for our classification.’ As strong as it gets ‘We learned that there’s a lot of overlap.’ While devising more reliable definitions, Skinner needed a wide selection of bonds to classify, but the hydrogen bonds in most water clusters are relatively weak. ‘You see that this weak hydrogen bond is actually as strong as that strong hydrogen bond, in different water clusters,’ he explains.
When two water molecules join together how to#
Kananenka had been exploring how to categorise hydrogen bonds in water with his collaborator James Skinner from the University of Chicago, and concluded that existing classifications didn’t carry well from system to system because of collective effects such as these. Source: © Alexei Kananenka/University of DelawareĪ schematic of a hydrogen bonding network, showing the cooperative effects around the strong hydrogen bond (green acceptor left, donor right) ‘It is not a property of a single water molecule, it is a property of the collective behaviour of water molecules,’ she says. Collective effects like these cause macroscopic phenomena in condensed phases, like water’s self-dissociation. ‘You can think of the electronic density of a molecular cluster like a totality: it adapts to the phenomena happening on one side of the bond or the other,’ comments Margarita Bernal–Uruchurtu, an expert on molecular interactions and physical chemistry in aqueous systems from the Autonomous University of the State of Morelos in Mexico. The inverse holds true for the acceptor at the other side of the bond. When a functional group acts as a hydrogen bond donor, it gains electron density, and becomes a better acceptor for other hydrogen bonds. Hydrogen bonds can be affected by their neighbours. ‘It makes liquid water denser than ice, it links two DNA strands together, and it also drives protein folding among other factors,’ says Alexei Kananenka, from the University of Delaware, who performed the new research. That simple arrangement drives a wide variety of chemical phenomena. 1 The study provides an extreme benchmark for hydrogen bonds in water, but also hints at more subtle effects, such as how enzymes perturb bonds inside their substrates.Ī hydrogen bond forms when an electron-poor hydrogen atom attached to an electronegative atom, a donor, reaches across space to an electron-rich acceptor atom, which transfers electrons back in return. Computational chemists in the US have modelled the strongest possible hydrogen bond for a neutral water cluster, by manipulating how adjacent bonds cooperate.
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