Sticking tough with new hydrogen bonds.

Hydrogen, the simplest element, can still spring a few surprises.

Researchers have now identified a previously unnoticed type of bonding between two hydrogen atoms belonging to different molecules. "This is a new type of intermolecular interaction," says chemist Robert H. Crabtree of Yale University. Crabtree and his collaborators describe their discovery in the July Accounts of Chemical Research.

The existence of the conventional hydrogen bond has long been recognized. In a water molecule, for example, a hydrogen bond links a hydrogen atom of one water molecule and the oxygen atom of another molecule. This bond forms because, within a water molecule, a strong covalent bond ties each of the two hydrogen atoms to the oxygen atom, leaving the hydrogen atom positively charged and the oxygen atom negatively charged.

Crabtree and his coworkers have found that in certain molecules, hydrogen atoms can have different charges. In amine borane (H3BNH3), for instance, the hydrogens linked to the boron atom have a negative charge, while those linked to the nitrogen atom have a positive charge. Thus, some of the hydrogens of one molecule are attracted to those of another molecule.

The researchers call this type of interaction a dihydrogen bond. The presence of such bonding can strongly influence a substance's melting point and other physical characteristics. Amine borane melts at 104#161#C, whereas ethane (H3CCH3), which has an analogous structure but features no dihydrogen bonding, melts at -181#161#C.

"Dihydrogen bonding leads to strong adhesion," Crabtree says.

Robert H. Morris of the University of Toronto in Ontario has also identified molecules with dihydrogen bonding. It's likely that there are other cases in which these forces exist, but they haven't yet been recognized, he says.