New type of hydrogen bond discovered
There’s a new chemical bond in town, covalent and ionic bond, and it loves to shake things up. It has taken decades to nail down, but researchers in Canada have finally identified a new chemical bond, which they’re calling a ‘vibrational bond’. This vibrational bond seems to break the law of chemistry that states if you increase the temperature, the rate of reaction will speed up. The phenomenon was first suggested over 30 years ago, but no evidence existed to support it, until now. Recent work with exotic isotopomers has been the key to finally explaining this peculiar interaction, whose qualities defy traditional chemical explanation.
An entirely new class of hydrogen bond that forms between a boron–hydrogen group and the aromatic, π-electron system of a benzene ring has been discovered. The non-classical B–H…π bond can be seen in the gas phase locking together diborane and benzene with a strength comparable to the hydrogen bonds that hold water dimers together.
Dieter Cremer and Wenli Zou of the computational and theoretical chemistry group at Southern Methodist University, Dallas in the US, worked with coordination chemists from Nanjing University in China to investigate the theory of non-classical hydrogen bonds that might form between a B–H group and organic structures and to demonstrate one such system experimentally.
Non-covalent bonds between aromatic rings and hydrogen joined to either a carbon, nitrogen or oxygen atom are common and critical in molecular biology. These relatively weak electrostatic bonds are the currency of countless bio molecular interactions between proteins, nucleic acids and the milieu of molecules on which life relies. Theoreticians and experimentalists have looked closely at many of these bonds and modeled the likes of benzene–haloalkane, benzene–ammonia and benzene–water complexes. Such bonds in, for example, carborane systems have hinted at a new approach to drug design in which a B–H group interacts with a proton donor in a target bio molecule. However, an interaction between a B–H and an aromatic ring had not been observed until now.
The team points out that given the usual trend with such systems one might expect boron’s electropositive character to make any interaction between a B–H compound and an aromatic ring repulsive rather than attractive. Of course, the bonding in diborane is itself rather unusual with its two-electron, but three-centre motif. The team suspected that the B–H group that might otherwise be repelled by a benzene ring would, in the case of diborane, be stabilised because the bonding hydrogen atom would have a residual positive charge.
The team’s quantum chemical calculations suggest that just such a bond should exist and that the interaction would be electrostatic. They generated the non-classical bond between diborane and an aryl group in a phosphine derivative of iridium-dimercapto-carborane complex. X-ray diffraction revealed the distance between the hydrogen atom and the π ring system to be between 2.40 and 2.76Å.
Meanwhile at Aarhus University, chemistry Professor Jeppe Olsen is as well surprised by the results of the new study, which show that the implausible type of hydrogen bond can occur between a phosphorus atom and a hydrogen atom-something no one thought was possible. Nevertheless, a team from the University of Copenhagen’s Department of Chemistry with Professor Henrik Kjærgaard in charge has managed to become the first to bond positively charged phosphorus atoms with positively charged hydrogen ones.
Useful for medicine research
According to the Professor Henrik Kjærgaard, the discovery is important to our understanding of how molecules behave and bond with each other. Hydrogen bonds determine, among other things, how our genome – through DNA strings, gets it distinctive twisted shape, and the bonds also help determine how the body’s vital proteins fold together.
“If, for instance, we want to make models of how proteins fold, it’s important to know which hydrogen bonds are at play, otherwise we can’t obtain correct computer models. You use this kind of computer model if you want to test how the body reacts to new kinds of medicine,” Kjærgaard explains.
Olsen also believes that the new discovery may be significant when it comes to our understanding of how phosphorous behaves in our atmosphere and various networks in nature. “This may well be a new piece in our understanding of the atmosphere,” he concludes.
- American Chemical Society
- University of Copenhagen