Extreme isotope effects: Changing bonds

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  • Published: Nov 15, 2014
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Extreme isotope effects: Changing bonds

Make or break

Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution (Credit: Wiley/Angew Chem)

The effects of different isotopes present in a molecule can affect the making and breaking of chemical bonds during the course of a chemical reaction. However, work published in Angewandte Chemie uses theoretical calculations and other techniques to demonstrate that, in extreme cases, isotopic substitution can even cause a fundamental change in the type of chemical bond.

Isotopes are atoms of the same element, same atomic number, but a different number of neutrons and so a different atomic mass. This mass difference can be manifest in various ways, such as a different reaction rate for reagents made with a higher ratio of one isotope to the others. This kinetic isotope effect is exploited in studying reaction mechanisms where the isotope ratio is artificially and deliberately adjusted at a specific point in a molecules, swapping out hydrogen for deuterium, for instance, to increase the magnitude of any effect and to home in on the specific sites in a molecule that are most directly involved in the making and breaking of bonds. And, of course the isotope effect is also patently manifest in, for example, infrared spectra.

Massive difference

Now, Donald Fleming of TRIUMF and Department of Chemistry University of British Columbia, Jörn Manz of Shanxi University, Taiyuan, China and Freie Universität, Berlin, Germany and Kazuma Sato and Toshiyuki Takayanagi of Saitama University, Japan, have demonstrated how isotopic substitution can lead to fundamental changes in the type of chemical bond present in a compound. This could disrupt theoretical models and our understanding of some reaction mechanisms.

The isotope is most pronounced when the difference between isotope masses is largest as is the case with hydrogen and deuterium and tritium. Given that the mass differences change the frequency of vibration of bonds holding those atoms, this is perhaps no surprise. The isotope effect can also be demonstrated using artificial isotopes synthesized in the laboratory, including those of exotic atoms containing non-atomic elementary particles such as muonium (symbol Mu) which is comprised of an electron and a nucleus consisting of an antimuon. The chemistry of muonium is akin to the hydrogen isotope, but because its mass is nine times smaller than hydrogen, the isotope effect is large. Indeed, there is a whole branch of chemistry called "muonium chemistry", typically for studies of reactions of the type H + AB ---> HA + B, where AB denotes many different molecules, from diatomics till organic molecules. In muonium chemistry, H is replaced by Mu, with the advantage that Mu is an isotope of H, but experimentally, one typically has just a single Mu atom instead of myriads of H atoms, so that one  can trace the mechanisms of Mu more easily than those of H.

Vibrational bonds

The team has now carried out novel quantum mechanical ab initio calculations for the reaction between hydrogen bromide and bromine atoms to form the BrHBr radical. They were particularly interested in comparing the hydrogen isotopes H (hydrogen), D (deuterium), T (tritium) and Mu, as well as a heavy exotic isotope. “We found that the four heavy isotopomers [isotopic isomers] behave essentially the same way,” explains Manz. “In contrast, the lightest isotopomer, BrMuBr, is held together by a completely different kind of chemical bond.”

The team further explains that hydrogen dibromide, BrHBr, and its heavy analogues can adopt either a linear or a bent form. In the bent form, both bromine atoms are bound to each other. In the linear form, the two bromine atoms are bonded through the H atom, which is in close proximity to one of the pair of bromine atoms but sits a substantial distance away from the other. The molecule is held together through van der Waals bonds - the non-covalent and non-ionic forces between molecules that results from attractive short-term charge transfers.

On the other hand, the team says, BrMuBr is bound together by completely different type of bond - a vibrational bond - that was first proposed by theoretical chemists in 1982. Intriguingly, it is the motion of the molecular fragments that hold the atoms together in which the muonium vibrates in a transition state between the two bromine atoms. “Our calculations on BrMuBr are the first clear evidence for the existence of this new type of bonding,” adds Manz. “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.”

"For the next steps, understanding this system allows us to search for others," Manz told Spectroscopynow, pointing out that Takayanagi has already found the next example. "Ultimately, I believe that this type of vibrational bonding may exist not only in systems with artificial isotopes, but also in natural ones," Manz adds. 

Related Links

Angew Chem Int Edn 2014, online: "Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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