Evidence of a more covalent nature and more possible partners than previously thought leads to a long-needed reclassification
The world authority on chemical nomenclature is preparing to scrap the familiar hydrogen bond definition, in light of recent evidence about its true nature. The International Union of Pure and Applied Chemistry’s (Iupac’s) Physical and Biophysical Chemistry Division has now published its proposal for the revised definition, and the chemical community has until the end of March 2011 to respond. Barring significant objections, it will be adopted shortly thereafter.
Two key factors are motivating the bonding interaction’s redefinition. One is a shift in the traditional view that a hydrogen bond is a purely electrostatic attraction between dipoles or charges on a hydrogen and another, electronegative, atom. Elangannan Arunan, who co-chairs the Iupac group assigned to categorise hydrogen bonds and other intermolecular interactions, highlights that there is a variety of evidence, including nuclear magnetic resonance data, that some electron density is shared between them. ’This shows that the hydrogen bond has a covalent nature,’ he says.
The current classification also concentrates on fluorine, oxygen and nitrogen, which Arunan says research has been shown as too simplistic. ’Most existing definitions insist hydrogen is connected to the most electronegative atoms,’ he tells Chemistry World. ’That is far from complete now.’ Arunan points out that Richard Nelmes at the University of Edinburgh, UK discovered that solid hydrogen sulfide has a hydrogen bonding structure resembling ice. ’That’s what really shocked chemists,’ he says.
Arunan’s own research, at the Indian Institute of Science, Bangalore, has demonstrated that hydrogen sulfide molecules can hydrogen bond with ethylene. He also notes that methane, noble gases like krypton, and unpaired-electron radical species have been seen as hydrogen’s bonding partner.
A 14 member group including hydrogen bonding textbook authors like Utah State University’s Steve Scheiner and Arunan’s Bangalore colleague Gautam Desiraju has been developing the latest classification since 2004. Arunan hopes that their combined contribution will help minimise any controversy. ’Hydrogen bonding is not string theory or gravitons, which no-one has ever seen yet,’ he emphasises. ’There are plenty of experimental and theoretical results available.’
Slawomir Grabowski, a hydrogen bonding expert at the University of the Basque Country in Spain underlines that Nobel laureate Linus Pauling originated the best-used current definition in 1960. ’The change of definition is needed,’ he tells Chemistry World, ’but because of the broad spectrum of interactions which exist in nature I am slightly sceptical it’s possible to propose a definition that satisfies the whole community of chemists.’