Vibrational bond existence confirmed

Chemistry students around the globe are pretty familiar with ionic, covalent, hydrogen, and van der Waals bonds, but a study has demonstrated the existence of one more: vibrational bonding. 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. A description of the work was published in the journal Angewandte Chemie International Edition, with Donald Fleming from the University of British Columbia as lead author. 

Elements are defined by the number of protons in each atom’s nucleus, though the number of neutrons can vary. These variations are called isotopes, and the respective difference in the atom’s mass influences a number of attributes, including bonding. The vast majority of elements have a number of stable and naturally-occurring isotopes (tin, for example, has 10), but exotic versions can be created in the lab. Beyond merely altering the number of neutrons, other subatomic particles can be added. 

Muonium (Mu) is an exotic atom with an antimuon nucleus orbited by one electron, making it considerably lighter than the hydrogen isotope protium (1H), though they have similar chemical attributes. Muonium was involved in a 1989 experiment when vibrational bonding was first unwittingly observed. Reactions between muonium and bromine curiously decreased as temperature increased; the opposite of what normally happens. This did not happen when muonium reacted with chlorine or fluorine, however, so they were at a loss about what happened, and the technology needed to experiment further did not exist yet.

Though the muonium and bromine were expected to interact through van der Waals bonds, the authors of the new paper attempt to explain the odd relationship through the hypothetical vibrational bonding. Muonium, the three naturally-occurring hydrogen isotopes (1H, 2H, 3H), and a heavy hydrogen isotope were each reacted with bromine, with the researchers investigating the quantum mechanics of the interactions.

“The lightest isotopomer, BrMuBr, with Mu the muonium atom, alone exhibits vibrational bonding in accord with its possible observation in a recent experiment on the Mu + Br2 reaction,” the authors write in the paper. "Accordingly, BrMuBr is stabilized at the saddle point of the potential energy surface due to a net decrease in vibrational zero point energy that overcompensates the increase in potential energy.” 

Essentially, when the super lightweight muonium is between two heavier bromines, it transitions between the two very rapidly, vibrating and bonding together the BrMuBr structure. This force lowers the total energy and explains why reactions would slow, even when temperatures increase. Future studies may explain if this bonding is limited to muonium and bromine, or can be seen with other elements with similar differences in mass.

"Our calculations on BrMuBr are the first clear evidence for the existence of this new type of bonding," co-author Jörn Manz from Freie Universität Berlin told phys.org. "In addition, they are the first indication that isotopic substitution can change the nature of chemical bonding in a profound manner. The different isotopomers of the radical we have studied and compared here have completely different structures, symmetries, and, most importantly, energetics and mechanisms of chemical bonding."