The name’s Bond, vibrational bond. First suggested thirty years ago, this new type of bonding has recently become a confirmed reality, as described in a paper from the University of British Columbia, published in Angewandte Chemie International Edition.
The phenomenon has been established as a means of explaining unusual in the chemistry of some isotopes – atoms of the same element, but with a different number of neutrons. Muonium (Mu) is an exotic atom with an antimuon nucleus orbited by one electron, making it considerably lighter than hydrogen, though fairly similar in its chemistry. When reacted with bromine, the rate was found to decrease with rising temperature, seemingly disobeying the laws of thermodynamics. This was not observed, however, with chlorine or fluorine.
Classically, it is thought that a bond will form if there is a net reduction in the potential energy of the system. However, modern quantum chemistry has been able to show that in this case, with a sufficiently large decrease in the vibrational zero point energy, the system can be stabilised, despite not fulfilling the classical criteria.
Donald Fleming, the lead author, told Scientific American that he though bromine and muonium might have formed a brief molecular structure via a kind of “vibrational” bond, the muonium ‘ping-ponging’ between two bromines. As the temperature increased, the bond becomes stronger, delaying the further reaction of this intermediate.
Commenting on the work, David Clary at Oxford University says: “The study provides rigorous theoretical evidence for vibrational bonding and suggests that isotopic substitution can have a dramatic effect on the ability of molecules to form bound states.”