Breaking bond

The name’s bond. Vibrational bond. Move over ionic, covalent and metallic attachments, a new bond is here. Arguably the first chemical bond to be discovered in nearly 100 years (the last, metallic bond, was discovered between 1913 and 1935), the vibrational bond will expand the conceptual boundaries of chemistry. However, in the last decade, chemical science has welcomed the arrival of quite a few new ‘bonds’ — the first-ever ties between a super-heavy element (Seaborgium) and a carbon atom; a new sub-type of covalent bond, the phi bond; and bonds forged in extreme magnetic fields found in proximity to white dwarf stars, currently very difficult to simulate on Earth. Yet, the discovery of the newest bond, theoretically proposed in the ’80s, has caused a flurry of excitement in the scientific world. Its application in explaining how isotopes react and their reaction rates, which defy a basic law of thermodynamics, proves that not all is known about the fundamental nature of the particle.

“Binding chemical atoms together to form molecules is like conceiving a child: There aren’t that many ways to do it,” wrote science writer Adrian Cho on new chemical bonds in Science magazine. As students, we learned about the basic bonds that explained how atoms combined to form molecules, how life itself is held together by invisible forces, some strong, some weak, some electrically charged, some especially stable. Even Helium, the smallest, most inert of noble gases has finally been found to exhibit some form of chemical bonding. We learned of ionic bonds, in which electrons are taken, not shared; covalent bonds, where electrons are shared equally or unequally. Remember those diagrams that took months of practice to get right, where we’d split atomic numbers of elements into K,L,M,N shells and s,p,d,f sub-shells, dotting sodium atoms with electrons and sketching lines across to chlorine atoms to indicate bonding? Atoms of most elements follow the octet rule, elbowing their way to gain or lose electrons from their valence (last) shells to achieve stable configurations just like that of the noble gases. So, Sodium (atomic number 11) readily gives away its lone electron to gluttonous Chlorine (17) to form NaCl, common salt. Carbon, with four electrons in its valence shell, has no option but to share them with four hydrogen or two oxygen atoms to form CH4 or CO2. The former transfer is ionic, the latter, covalent. In the case of the vibrational bond, observed in the reaction between Bromine (which belongs to the most reactive group in the periodic table, the Halogens) and Muonium (call it a Hydrogen isotope, but it’s really an exotic sub-atomic particle), the lightweight Mu atoms prefer to oscillate between two Br atoms “like a Ping Pong ball bouncing between two bowling balls,” says Donald Fleming, a chemist at University of British Columbia involved with the experiment.

Like all good scientific problems, the vibrational bond has been under study for more than three decades. In 1989, at a nuclear accelerator in Vancouver, chemists — Fleming included — observing the reaction between Bromine and Muonium recorded something unusual. According to the Arrhenius equation, most chemical reactions go faster with every 10-degree Celsius rise in temperature, that is, the speed of a reaction is directly proportional to temperature increase. In this case, the reaction slowed down with increase in temperature. Researchers thought this was evidence of an interim bond, where the hydrogen isotope would hold the bromine atoms together briefly for a few milli-seconds, reduce the energy level and, therefore, increase the speed of the reaction. Chemists then didn’t have the equipment to study a fleeting chemical reaction. The vibrational bond remained a theoretical possibility.

As Fleming and researchers Jörn Manz (FU Berlin), Kazuma Sato and Toshiyuki Takayanagi (Saitama University, Japan) waited for advances in nuclear accelerators that would finally allow them to track the tenuous energy changes in the BrMuBr reaction, a century earlier, Linus Pauling too had to wait for X-ray crystallography to develop before he could pioneer research into chemical bonding. As an undergraduate student at Oregon State University in 1917, Pauling, who would go on to become one of the founders of quantum chemistry and win two Nobel prizes, was still being taught the ‘Hook and eye’ model of atomic attachment, first developed by Greek philosopher Democritus in 410 BC! The teaching of this archaic method apparently drove Pauling to write the 1939 textbook On the Nature of the Chemical Bond , now a definitive chemistry text. The science of chemical bonding has taken a few but astonishing leaps since Leucippus’s elementary atomic theory in 450 BC. By the mid-20th century, the valence bond theory and ideas of quantum chemistry were in place. We knew how atoms moved, interacted and dissociated.

Fleming, whose research interests lie in the field of nuclear and particular muon chemistry, had authored the 1989 paper on vibrational bonding as a theoretical possibility. He is also the lead author of the December 2014 paper, published in Angewandte Chemie International Edition, confirming the same. In their experiment in a nuclear accelerator at Rutherford Appleton Laboratory, UK, the bond between Bromine and Muonium didn’t exhibit the necessary characteristics to be called either covalent or a van der Waals bond. It was brought about by a peculiarity of particle physics, where three atoms were temporarily bound together. While Protium, Deuterium and Tritium are naturally occurring isotopes of Hydrogen, Muonium was synthesised in the lab. Chemically, it is much like a hydrogen isotope, but nine times lighter. The chemical reaction to form the BrHBr radical was the same for all the isotopes, except Muonium; the BrMuBr radical seemed to be held by a precarious vibrational bond. This is the first confirmation that isotopes of an element can potentially change the nature of chemical bonding. However, the vibrational bond has been observed only in the case of this reaction; further research will determine if heavy and light elements react this way. So don’t expect it to make it to the textbooks yet.

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